[PATCH v2 12/16] Always put inferiors in their own terminal/session [gdb/9425, gdb/14559]

[Pedro Alves <pedro@palves.net>]

Currently, on GNU/Linux, it is not possible to interrupt with Ctrl-C
programs that block or ignore SIGINT, with e.g., sigprocmask or
signal(SIGINT, SIG_IGN).  You type Ctrl-C, but nothing happens.
Similarly, if a program uses sigwait to wait for SIGINT, and the
program receives a SIGINT, the SIGINT is _not_ intercepted by ptrace,
it goes straight to the inferior.  These problems have been known for
years, and recorded in gdb/9425, gdb/14559.

This is a consequence of how GDB implements interrupting programs with
Ctrl-C -- when GDB spawns a new process, it makes the process use the
same terminal as GDB, and then makes the process's process group be
the foreground process group in GDB's terminal.  This means that when
the process is running in the foreground, after e.g. "continue", when
the user types Ctrl-C, the kernel sends a SIGINT to the foreground
process group, which is the inferior.  GDB then intercepts the SIGINT
via ptrace, the same way it intercepts any other signal, stops all the
other threads of the inferior if in all-stop, and presents the
"Program received SIGINT" stop to the user.

This patch paves the way to address gdb/9425, gdb/14559, by turning
Ctrl-C handling around such that the SIGINT always reaches GDB first,
not the inferior.  That is done by making GDB put inferiors in their
own terminal/session created by GDB.  I.e., GDB creates a
pseudo-terminal master/slave pair, makes the inferior run with the
slave as its terminal, and pumps output/input on the master end.
Because the inferior is run with its own session/terminal, GDB is free
to remain as the foreground process in its own terminal, which means
that the Ctrl-C SIGINT always reaches GDB first, instead of reaching
the inferior first, and then GDB reacting to the ptrace-intercepted
SIGINT.  Because GDB gets the SIGINT first, GDB is then free to
handle it by interrupting the program any way it sees fit.  A
following patch will then make GDB interrupt the program with SIGSTOP
instead of SIGINT, which always works even if the inferior
blocks/ignores SIGINT -- SIGSTOP can't be ignored.  (In the future GDB
may even switch to PTRACE_INTERRUPT, though that's a project of its
own.)

Having the inferior in its own terminal also means that GDB is in
control of when inferior output is flushed to the screen.  When
debugging with the CLI, this means that inferior output is now never
interpersed with GDB's output in an unreadable fashion.  This will
also allow fixing the problem of inferior output really messing up the
screen in the TUI, forcing users to Ctrl-L to refresh the screen.
This patch does not address the TUI part, but it shouldn't be too hard
-- I wrote a quick&dirty prototype patch doing that a few years back,
so I know it works.

Implementation wise, here's what is happening:

 - when GDB spawns an inferior, unless the user asked otherwise with
   "tty /dev/tty", GDB creates a pty pair, and makes the slave end the
   inferior's terminal.  Note that starting an inferior on a given
   terminal already exists, given the "tty" command.  GDB records the
   master and slave ends of the pty.

 - GDB registers that new terminal's master end on the event loop.
   When the master is written to, it means the inferior has written
   some output on its terminal.  The event loop wakes up and GDB
   flushes the inferior output to its own terminal / to the screen.

 - When target_terminal state is switched to "inferior", with
   target_tarminal::inferiors(), GDB registers the stdin file
   descriptor on the event loop with a callback that forwards input
   typed on GDB's terminal to the inferior's tty.

 - Similarly, when GDB receives a SIGWINCH signal, meaning GDB's
   terminal was resized, GDB resizes the inferior's terminal too.

 - GDB puts the inferior in its own session, and there's a "session
   leader" process between GDB and the inferior.  The latter is
   because session leaders have special properties, one of which is,
   if they exit, all progresses in the foreground process group in the
   session get a SIGHUP.  If the spawned inferior was the session
   leader itself, if you were debugging an inferior that forks and
   follow to the child, if the parent (the session leader) exits, then
   the child would get a SIGHUP.  Forking twice when launching an
   inferior, and making the first child be the session leader, and the
   second child the inferior avoids that problem.

 - When the inferior exits or is killed, GDB sends a SIGHUP to the
   session leader, waits for the leader to exit and then destroys the
   terminal.  The session leader's SIGHUP handler makes the session
   leader pgrp be the foreground process group and then exits.  This
   sequence is important comparing to just closing the terminal and
   letting the session leader terminate due to the SIGHUP the kernel
   sends, because when the session leader exits, all processes in the
   foreground process group get a SIGHUP, meaning that if the detached
   process was still in the foreground, it would get a SIGHUP, and
   likely die.

 - The gdb.multi/multi-term-settings.exp was adjusted to test for
   shared and not-shared terminal/session.  Without the change, we get
   failures:

    FAIL: gdb.multi/multi-term-settings.exp: inf1_how=run: inf2_how=run: continue (expected SIGTTOU)
    FAIL: gdb.multi/multi-term-settings.exp: inf1_how=run: inf2_how=run: stop with control-c (Quit)

Tested on GNU/Linux native, gdbserver and gdbserver + "maint target
set-non-stop on".  Also build-tested tested on mingw32-w64, Solaris
11, and OpenBSD.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	PR gdb/9425
	PR gdb/14559
	* fork-child.c (child_has_managed_tty_hook): New.
	* inf-ptrace.c (inf_ptrace_me): If we created a managed tty, raise
	SIGSTOP.
	(inf_ptrace_handle_session_leader_fork): New.
	(inf_ptrace_target::create_inferior): Pass it down as
	handle_session_leader_fork callback.
	* inf-ptrace.h (inf_ptrace_target) <handle_session_leader_fork>:
	New virtual method.
	* inferior.h (child_terminal_on_sigwinch): Declare.
	* inflow.c: Include "gdbsupport/event-loop.h",
	"gdbsupport/refcounted-object.h", "gdbsupport/gdb_wait.h",
	"gdbsupport/managed-tty.h".
	(USES_FORK_CHILD): Define, and wrap fork-child.c-related code with
	it.
	(struct run_terminal_info): New.
	(struct terminal_info) <run_terminal>: Now a run_terminal_info.
	<process_group>: Default to -1.
	<save_from_tty>: New method.
	(sigint_ours): Update comments.
	(inferior_thisrun_terminal_pty_fd): New.
	(input_fd_redirected): New.
	(sharing_input_terminal): Adjust.
	(gdb_tcgetattr, gdb_tcsetattr, make_raw, class scoped_raw_termios)
	(child_terminal_flush_from_to, child_terminal_flush_stdout)
	(inferior_stdout_event_handler, inferior_stdin_event_handler): New.
	(child_terminal_inferior): Handle inferiors with gdb-managed ttys.
	(child_terminal_save_inferior): Handle inferiors with gdb-managed
	ttys.  Use save_from_tty.
	(child_terminal_ours_1): Handle inferiors with gdb-managed ttys.
	(terminal_info::~terminal_info): Use delete instead of xfree.
	(child_terminal_on_sigwinc): New.
	(inflow_inferior_exit): Release terminal created by GDB.
	(copy_terminal_info): Assert there's no run_terminal yet in TO
	yet.  Incref run_terminal after copying.
	(child_terminal_info): Handle inferiors with gdb-managed ttys.
	(new_tty_prefork): Allocate pseudo-terminal.
	(created_managed_tty): New.
	(new_tty): Remove __GO32__ and _WIN32 #ifdefs, not needed given
	USES_FORK_CHILD.
	(new_tty_postfork): Handle inferiors with gdb-managed ttys.
	(show_debug_managed_tty): New.
	(_initialize_inflow): Register "set/show debug managed-tty".
	* linux-nat.c (waitpid_sigstop, waitpid_fork)
	(linux_nat_target::handle_session_leader_fork): New.
	* linux-nat.h (linux_nat_target) <handle_session_leader_fork>:
	Declare override.
	* nat/fork-inferior.c: Include
	"gdbsupport/scoped_ignore_sigttou.h", "gdbsupport/managed-tty.h",
	<sys/types.h> and <sys/wait.h>.
	(session_leader_hup): New.
	(fork_inferior): Add handle_session_leader_fork parameter.  If the
	inferior has a gdb-managed tty, don't use vfork, and fork twice,
	with the first fork becoming the session leader.  Call
	handle_session_leader_fork.
	* nat/fork-inferior.h (fork_inferior): Add
	handle_session_leader_fork parameter and update comment.
	(child_has_managed_tty_hook): Declare.
	* terminal.h (created_managed_tty, child_gdb_owns_session):
	Declare.
	* tui/tui-win.c: Include "inferior.h".
	(tui_async_resize_screen): Call child_terminal_on_sigwinch.

gdbsupport/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	PR gdb/9425
	PR gdb/14559
	* Makefile.am (libgdbsupport_a_SOURCES): Add managed-tty.cc.
	* Makefile.in: Regenerate.
	* managed-tty.cc: New.
	* managed-tty.h: New.

gdbserver/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	PR gdb/9425
	PR gdb/14559
	* fork-child.cc (child_has_managed_tty_hook): New.

gdb/testsuite/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	PR gdb/9425
	PR gdb/14559
	* gdb.multi/multi-term-settings.exp (create_inferior): Document
	"run-session", "run-share" and "run-tty" instead of "run" and
	"tty".  Adjust to handle "run-session" vs "run-share".
	(coretest): Adjust to handle "run-session" vs "run-share".
	(how_modes): Use "run-session", "run-share" and "run-tty" instead
	of "run" and "tty".

Change-Id: I2569e189294044891e68a66401b381e4b999b19c
---
 gdb/fork-child.c                              |   8 +
 gdb/inf-ptrace.c                              |  23 +-
 gdb/inf-ptrace.h                              |   8 +
 gdb/inferior.h                                |   4 +
 gdb/inflow.c                                  | 699 +++++++++++++++---
 gdb/linux-nat.c                               |  76 ++
 gdb/linux-nat.h                               |   2 +
 gdb/nat/fork-inferior.c                       | 118 ++-
 gdb/nat/fork-inferior.h                       |  23 +-
 gdb/terminal.h                                |   3 +
 .../gdb.multi/multi-term-settings.exp         |  50 +-
 gdb/tui/tui-win.c                             |   3 +
 gdbserver/fork-child.cc                       |   8 +
 gdbsupport/Makefile.am                        |   1 +
 gdbsupport/Makefile.in                        |  15 +-
 gdbsupport/managed-tty.cc                     |  22 +
 gdbsupport/managed-tty.h                      |  42 ++
 17 files changed, 987 insertions(+), 118 deletions(-)
 create mode 100644 gdbsupport/managed-tty.cc
 create mode 100644 gdbsupport/managed-tty.h

diff --git a/gdb/fork-child.c b/gdb/fork-child.c
index 41135b53f9b..10395718ed9 100644
--- a/gdb/fork-child.c
+++ b/gdb/fork-child.c
@@ -73,6 +73,14 @@ prefork_hook ()
 
 /* See nat/fork-inferior.h.  */
 
+bool
+child_has_managed_tty_hook ()
+{
+  return created_managed_tty ();
+}
+
+/* See nat/fork-inferior.h.  */
+
 void
 postfork_hook (pid_t pid)
 {
diff --git a/gdb/inf-ptrace.c b/gdb/inf-ptrace.c
index afa38de6ef7..5064cb39e65 100644
--- a/gdb/inf-ptrace.c
+++ b/gdb/inf-ptrace.c
@@ -61,6 +61,26 @@ inf_ptrace_me (void)
   /* "Trace me, Dr. Memory!"  */
   if (ptrace (PT_TRACE_ME, 0, (PTRACE_TYPE_ARG3) 0, 0) < 0)
     trace_start_error_with_name ("ptrace");
+  if (created_managed_tty ())
+    {
+      /* We're about to fork again, so that this child remains as
+	 session leader, and the grandchild becomes the real inferior.
+	 Let GDB grab control of this child, and enable tracing the
+	 grandchild fork.  */
+      raise (SIGSTOP);
+    }
+}
+
+/* fork_inferior handle_session_leader_fork hook.  Dispatches to
+   inf_ptrace_target.  */
+
+static pid_t
+inf_ptrace_handle_session_leader_fork (pid_t sl_pid)
+{
+  auto *proc_target = current_inferior ()->process_target ();
+  auto *ptrace_targ = static_cast<inf_ptrace_target *> (proc_target);
+
+  return ptrace_targ->handle_session_leader_fork (sl_pid);
 }
 
 /* Start a new inferior Unix child process.  EXEC_FILE is the file to
@@ -88,7 +108,8 @@ inf_ptrace_target::create_inferior (const char *exec_file,
     }
 
   pid_t pid = fork_inferior (exec_file, allargs, env, inf_ptrace_me, NULL,
-			     NULL, NULL, NULL);
+			     NULL, NULL, NULL,
+			     inf_ptrace_handle_session_leader_fork);
 
   ptid_t ptid (pid);
   /* We have something that executes now.  We'll be running through
diff --git a/gdb/inf-ptrace.h b/gdb/inf-ptrace.h
index 8aded9b60db..f5291ea0504 100644
--- a/gdb/inf-ptrace.h
+++ b/gdb/inf-ptrace.h
@@ -44,6 +44,14 @@ struct inf_ptrace_target : public inf_child_target
   void create_inferior (const char *, const std::string &,
 			char **, int) override;
 
+  /* Targets that support putting the inferior in its own gdb-managed
+     terminal must override this method.  */
+  virtual pid_t handle_session_leader_fork (pid_t sl_pid)
+  {
+    gdb_assert_not_reached ("handle_session_leader_fork called");
+    return -1;
+  }
+
   void mourn_inferior () override;
 
   bool thread_alive (ptid_t ptid) override;
diff --git a/gdb/inferior.h b/gdb/inferior.h
index f61b5889e85..b75c587e397 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -182,6 +182,10 @@ extern void child_pass_ctrlc (struct target_ops *self);
 
 extern void child_interrupt (struct target_ops *self);
 
+/* Called when we get a SIGWINCH, to manage the sizes of inferior
+   terminals created by GDB.  */
+extern void child_terminal_on_sigwinch ();
+
 /* From fork-child.c */
 
 /* Helper function to call STARTUP_INFERIOR with PID and NUM_TRAPS.
diff --git a/gdb/inflow.c b/gdb/inflow.c
index 0a9df6054b0..35a51b26fb5 100644
--- a/gdb/inflow.c
+++ b/gdb/inflow.c
@@ -35,6 +35,10 @@
 #endif
 #include "gdbsupport/job-control.h"
 #include "gdbsupport/scoped_ignore_sigttou.h"
+#include "gdbsupport/event-loop.h"
+#include "gdbsupport/refcounted-object.h"
+#include "gdbsupport/gdb_wait.h"
+#include "gdbsupport/managed-tty.h"
 
 #ifdef HAVE_SYS_IOCTL_H
 #include <sys/ioctl.h>
@@ -44,6 +48,14 @@
 #define O_NOCTTY 0
 #endif
 
+/* Defined as 1 if the native target uses fork-child.c to spawn
+   processes.  */
+#if !defined(__GO32__) && !defined(_WIN32)
+# define USES_FORK_CHILD 1
+#else
+# define USES_FORK_CHILD 0
+#endif
+
 static void pass_signal (int);
 
 static void child_terminal_ours_1 (target_terminal_state);
@@ -52,6 +64,25 @@ static void child_terminal_ours_1 (target_terminal_state);
 
 static struct serial *stdin_serial;
 
+/* "run terminal" terminal info.  This is info about the terminal we
+   give to the inferior when it is started.  This is refcounted
+   because it is potentially shared between multiple inferiors -- a
+   fork child is associated with the same terminal as its parent.  */
+
+struct run_terminal_info : public refcounted_object
+{
+  /* The name of the tty (from the `tty' command) that we gave to the
+     inferior when it was started.  */
+  gdb::unique_xmalloc_ptr<char> ttyname;
+
+  /* The file descriptor of the master end of the pty created for the
+     inferior.  -1 if no terminal was created by GDB.  */
+  int pty_fd = -1;
+
+  /* The PID of the terminal's session leader.  */
+  pid_t session_leader = -1;
+};
+
 /* Terminal related info we need to keep track of.  Each inferior
    holds an instance of this structure --- we save it whenever the
    corresponding inferior stops, and restore it to the terminal when
@@ -63,9 +94,10 @@ struct terminal_info
 
   terminal_info &operator= (const terminal_info &) = default;
 
-  /* The name of the tty (from the `tty' command) that we gave to the
-     inferior when it was started.  */
-  char *run_terminal = nullptr;
+  /* Info about the tty that we gave to the inferior when it was
+     started.  This is potentially shared between multiple
+     inferiors.  */
+  run_terminal_info *run_terminal = nullptr;
 
   /* TTY state.  We save it whenever the inferior stops, and restore
      it when it resumes in the foreground.  */
@@ -85,11 +117,26 @@ struct terminal_info
      inf2's pgrp in the foreground instead of inf1's (which would be
      problematic since it would be left stopped: Ctrl-C wouldn't work,
      for example).  */
-  pid_t process_group = 0;
+  pid_t process_group = -1;
 #endif
 
   /* fcntl flags.  Saved and restored just like ttystate.  */
   int tflags = 0;
+
+  /* Save terminal settings from TTY_SERIAL.  */
+  void save_from_tty (serial *tty_serial)
+  {
+    xfree (this->ttystate);
+    this->ttystate = serial_get_tty_state (tty_serial);
+
+#ifdef HAVE_TERMIOS_H
+    this->process_group = tcgetpgrp (tty_serial->fd);
+#endif
+
+#ifdef F_GETFL
+    this->tflags = fcntl (tty_serial->fd, F_GETFL, 0);
+#endif
+  }
 };
 
 /* Our own tty state, which we restore every time we need to deal with
@@ -108,9 +155,10 @@ static serial_ttystate initial_gdb_ttystate;
 
 static struct terminal_info *get_inflow_inferior_data (struct inferior *);
 
-/* While the inferior is running, we want SIGINT and SIGQUIT to go to the
-   inferior only.  If we have job control, that takes care of it.  If not,
-   we save our handlers in these two variables and set SIGINT and SIGQUIT
+/* While the inferior is running, and the inferior is sharing the same
+   terminal as GDB, we want SIGINT and SIGQUIT to go to the inferior
+   only.  If we have job control, that takes care of it.  If not, we
+   save our handlers in these two variables and set SIGINT and SIGQUIT
    to SIG_IGN.  */
 
 static sighandler_t sigint_ours;
@@ -118,6 +166,8 @@ static sighandler_t sigint_ours;
 static sighandler_t sigquit_ours;
 #endif
 
+#if USES_FORK_CHILD
+
 /* The name of the tty (from the `tty' command) that we're giving to
    the inferior when starting it up.  This is only (and should only
    be) used as a transient global by new_tty_prefork,
@@ -125,6 +175,18 @@ static sighandler_t sigquit_ours;
    fork_inferior, while forking a new child.  */
 static const char *inferior_thisrun_terminal;
 
+#if GDB_MANAGED_TERMINALS
+
+/* The file descriptor of the master end of the pty that we're giving
+   to the inferior when starting it up, iff we created the terminal
+   ourselves.  This is set by new_tty_prefork, and like
+   INFERIOR_THISRUN_TERMINAL, is transient.  */
+static int inferior_thisrun_terminal_pty_fd = -1;
+
+#endif /* GDB_MANAGED_TERMINALS */
+
+#endif /* USES_FORK_CHILD */
+
 /* Track who owns GDB's terminal (is it GDB or some inferior?).  While
    target_terminal::is_ours() etc. tracks the core's intention and is
    independent of the target backend, this tracks the actual state of
@@ -137,6 +199,11 @@ static const char *inferior_thisrun_terminal;
    different terminal).  */
 static target_terminal_state gdb_tty_state = target_terminal_state::is_ours;
 
+/* True if stdin is redirected.  As long as this is true, any input
+   typed in GDB's terminal is forwarded to the foreground inferior's
+   gdb-managed terminal.  See inferior_stdin_event_handler.  */
+static bool input_fd_redirected = false;
+
 /* See terminal.h.  */
 
 void
@@ -309,7 +376,7 @@ sharing_input_terminal (inferior *inf)
 	 positive we just end up trying to save/restore terminal
 	 settings when we didn't need to or we actually can't.  */
       if (tinfo->run_terminal != NULL)
-	res = is_gdb_terminal (tinfo->run_terminal);
+	res = is_gdb_terminal (tinfo->run_terminal->ttyname.get ());
 
       /* If we still can't determine, assume yes.  */
       if (res == TRIBOOL_UNKNOWN)
@@ -319,6 +386,199 @@ sharing_input_terminal (inferior *inf)
   return res == TRIBOOL_TRUE;
 }
 
+#if GDB_MANAGED_TERMINALS
+
+/* Wrappers around tcgetattr/tcsetattr to log errors.  We don't throw
+   on error instead because an error here is most likely caused by
+   stdin having been closed (e.g., GDB lost its terminal), and we may
+   be called while handling/printing exceptions.  E.g., from
+   target_target::ours_for_output() before printing an exception
+   message.  */
+
+static int
+gdb_tcgetattr (int fd, struct termios *termios)
+{
+  if (tcgetattr (fd, termios) != 0)
+    {
+      managed_tty_debug_printf (_("tcgetattr(fd=%d) failed: %d (%s)\n"),
+				fd, errno, safe_strerror (errno));
+      return -1;
+    }
+
+  return 0;
+}
+
+/* See gdb_tcgetattr.  */
+
+static int
+gdb_tcsetattr (int fd, int optional_actions, struct termios *termios)
+{
+  if (tcsetattr (fd, optional_actions, termios) != 0)
+    {
+      managed_tty_debug_printf (_("tcsetattr(fd=%d) failed: %d (%s)\n"),
+				fd, errno, safe_strerror (errno));
+      return -1;
+    }
+
+  return 0;
+}
+
+/* Disable echo, canonical mode, and \r\n -> \n translation.  Leave
+   ISIG, since we want to grab Ctrl-C before the inferior sees it.  If
+   CLEAR_OFLAG is true, also clear the output modes, otherwise, leave
+   them unmodified.  */
+
+static void
+make_raw (struct termios *termios, bool clear_oflag)
+{
+  termios->c_iflag &= ~(INLCR | IGNCR | ICRNL);
+  if (clear_oflag)
+    termios->c_oflag = 0;
+  termios->c_lflag &= ~(ECHO | ICANON);
+  termios->c_cflag &= ~CSIZE;
+  termios->c_cflag |= CLOCAL | CS8;
+  termios->c_cc[VMIN] = 0;
+  termios->c_cc[VTIME] = 0;
+}
+
+/* RAII class to temporarily set the terminal to raw mode, with
+   `oflag` cleared.  See make_raw.  */
+
+class scoped_raw_termios
+{
+public:
+  scoped_raw_termios ()
+  {
+    if (gdb_tcgetattr (STDIN_FILENO, &m_saved_termios) == 0)
+      {
+	m_saved_termios_p = true;
+
+	struct termios raw_termios = m_saved_termios;
+	make_raw (&raw_termios, true);
+	gdb_tcsetattr (STDIN_FILENO, TCSADRAIN, &raw_termios);
+      }
+  }
+
+  ~scoped_raw_termios ()
+  {
+    if (m_saved_termios_p)
+      gdb_tcsetattr (STDIN_FILENO, TCSADRAIN, &m_saved_termios);
+  }
+
+private:
+  /* The saved termios data.  */
+  struct termios m_saved_termios;
+
+  /* True iff M_SAVED_TERMIOS is valid.  */
+  bool m_saved_termios_p;
+};
+
+/* Flush input/output from READ_FD to WRITE_FD.  WHAT is used for
+   logging purposes.  */
+
+static void
+child_terminal_flush_from_to (int read_fd, int write_fd, const char *what)
+{
+  char buf[1024];
+
+  gdb::optional<scoped_raw_termios> save_termios;
+  save_termios.emplace ();
+
+  while (1)
+    {
+      int r = read (read_fd, buf, sizeof (buf));
+      if (r <= 0)
+	{
+	  /* Restore terminal state before printing debug output or
+	     warnings.  */
+	  save_termios.reset ();
+
+	  if (r == 0)
+	    ;
+	  else if (r == -1 && errno == EAGAIN)
+	    ;
+	  else if (r == -1 && errno == EIO)
+	    {
+	      managed_tty_debug_printf (_("%s: bad read: closed?\n"),
+					what);
+	    }
+	  else
+	    {
+	      /* Unexpected.  */
+	      warning (_("%s: bad read: %d: (%d) %s"), what, r,
+		       errno, safe_strerror (errno));
+	    }
+	  return;
+	}
+
+      const char *p = buf;
+
+      while (r > 0)
+	{
+	  int w = write (write_fd, p, r);
+	  if (w == -1 && errno == EAGAIN)
+	    continue;
+	  else if (w <= 0)
+	    {
+	      int err = errno;
+
+	      /* Restore terminal state before printing the
+		 warning.  */
+	      save_termios.reset ();
+
+	      warning (_("%s: bad write: %d: (%d) %s"), what, r,
+		       err, safe_strerror (err));
+	      return;
+	    }
+
+	  r -= w;
+	  p += w;
+	}
+    }
+}
+
+/* Flush inferior terminal output to GDB's stdout.  Used when the
+   inferior is associated with a terminal created and managed by
+   GDB.  */
+
+static void
+child_terminal_flush_stdout (run_terminal_info *run_terminal)
+{
+  gdb_assert (run_terminal->pty_fd != -1);
+  child_terminal_flush_from_to (run_terminal->pty_fd, STDOUT_FILENO,
+				"stdout");
+}
+
+/* Event handler associated with the inferior's terminal pty.  Used
+   when the inferior is associated with a terminal created and managed
+   by GDB.  Whenever the inferior writes to its terminal, the event
+   loop calls this handler, which then flushes inferior terminal
+   output to GDB's stdout.  */
+
+static void
+inferior_stdout_event_handler (int error, gdb_client_data client_data)
+{
+  run_terminal_info *run_terminal = (run_terminal_info *) client_data;
+  child_terminal_flush_stdout (run_terminal);
+}
+
+/* Event handler associated with stdin.  Used when the inferior is
+   associated with a terminal created and managed by GDB.  Whenever
+   the user types on GDB's terminal, the event loop calls this
+   handler, which then flushes user input to the inferior's terminal
+   input.  */
+
+static void
+inferior_stdin_event_handler (int error, gdb_client_data client_data)
+{
+  run_terminal_info *run_terminal = (run_terminal_info *) client_data;
+  gdb_assert (run_terminal->pty_fd != -1);
+  child_terminal_flush_from_to (STDIN_FILENO, run_terminal->pty_fd,
+				"stdin");
+}
+
+#endif /* GDB_MANAGED_TERMINALS */
+
 /* Put the inferior's terminal settings into effect.  This is
    preparation for starting or resuming the inferior.  */
 
@@ -338,63 +598,98 @@ child_terminal_inferior (struct target_ops *self)
   terminal_info *tinfo = get_inflow_inferior_data (inf);
 
   if (gdb_has_a_terminal ()
-      && tinfo->ttystate != NULL
-      && sharing_input_terminal (inf))
+      && tinfo->ttystate != nullptr
+      && ((tinfo->run_terminal != nullptr
+	   && tinfo->run_terminal->pty_fd != -1)
+	  || sharing_input_terminal (inf)))
     {
-      int result;
+      if (!job_control)
+	{
+	  sigint_ours = signal (SIGINT, SIG_IGN);
+#ifdef SIGQUIT
+	  sigquit_ours = signal (SIGQUIT, SIG_IGN);
+#endif
+	}
 
       /* Ignore SIGTTOU since it will happen when we try to set the
 	 terminal's state (if gdb_tty_state is currently
 	 ours_for_output).  */
       scoped_ignore_sigttou ignore_sigttou;
 
-#ifdef F_GETFL
-      result = fcntl (0, F_SETFL, tinfo->tflags);
-      OOPSY ("fcntl F_SETFL");
-#endif
+#if GDB_MANAGED_TERMINALS
+      if (tinfo->run_terminal != nullptr
+	  && tinfo->run_terminal->pty_fd != -1)
+	{
+	  /* Set stdin to raw (see make_raw) so we can later marshal
+	     unadulterated input to the inferior's terminal, but leave
+	     the output flags intact.  Importantly, we don't want to
+	     disable \n -> \r\n translation on output, mainly to avoid
+	     the staircase effect in debug logging all over the code
+	     base while terminal_inferior is in effect.  */
+	  struct termios termios;
+
+	  if (gdb_tcgetattr (STDIN_FILENO, &termios) == 0)
+	    {
+	      make_raw (&termios, false);
+	      gdb_tcsetattr (STDIN_FILENO, TCSADRAIN, &termios);
+	    }
 
-      result = serial_set_tty_state (stdin_serial, tinfo->ttystate);
-      OOPSY ("setting tty state");
+	  /* Register our stdin-forwarder handler in the event
+	     loop.  */
+	  add_file_handler (0, inferior_stdin_event_handler,
+			    tinfo->run_terminal,
+			    string_printf ("stdin-forward-%d", inf->num),
+			    true);
 
-      if (!job_control)
+	  input_fd_redirected = true;
+	}
+      else
+#endif /* GDB_MANAGED_TERMINALS */
 	{
-	  sigint_ours = signal (SIGINT, SIG_IGN);
-#ifdef SIGQUIT
-	  sigquit_ours = signal (SIGQUIT, SIG_IGN);
+	  int result;
+
+#ifdef F_GETFL
+	  result = fcntl (0, F_SETFL, tinfo->tflags);
+	  OOPSY ("fcntl F_SETFL");
 #endif
-	}
 
-      if (job_control)
-	{
+	  result = serial_set_tty_state (stdin_serial, tinfo->ttystate);
+	  OOPSY ("setting tty state");
+
+	  if (job_control)
+	    {
 #ifdef HAVE_TERMIOS_H
-	  /* If we can't tell the inferior's actual process group,
-	     then restore whatever was the foreground pgrp the last
-	     time the inferior was running.  See also comments
-	     describing terminal_state::process_group.  */
+	      /* If we can't tell the inferior's actual process group,
+		 then restore whatever was the foreground pgrp the
+		 last time the inferior was running.  See also
+		 comments describing
+		 terminal_state::process_group.  */
 #ifdef HAVE_GETPGID
-	  result = tcsetpgrp (0, getpgid (inf->pid));
+	      result = tcsetpgrp (0, getpgid (inf->pid));
 #else
-	  result = tcsetpgrp (0, tinfo->process_group);
+	      result = tcsetpgrp (0, tinfo->process_group);
 #endif
-	  if (result == -1)
-	    {
+	      if (result == -1)
+		{
 #if 0
-	      /* This fails if either GDB has no controlling terminal,
-		 e.g., running under 'setsid(1)', or if the inferior
-		 is not attached to GDB's controlling terminal.  E.g.,
-		 if it called setsid to create a new session or used
-		 the TIOCNOTTY ioctl, or simply if we've attached to a
-		 process running on another terminal and we couldn't
-		 tell whether it was sharing GDB's terminal (and so
-		 assumed yes).  */
-	      fprintf_unfiltered
-		(gdb_stderr,
-		 "[tcsetpgrp failed in child_terminal_inferior: %s]\n",
-		 safe_strerror (errno));
+		  /* This fails if either GDB has no controlling
+		     terminal, e.g., running under 'setsid(1)', or if
+		     the inferior is not attached to GDB's controlling
+		     terminal.  E.g., if it called setsid to create a
+		     new session or used the TIOCNOTTY ioctl, or
+		     simply if we've attached to a process running on
+		     another terminal and we couldn't tell whether it
+		     was sharing GDB's terminal (and so assumed
+		     yes).  */
+		  fprintf_unfiltered
+		    (gdb_stderr,
+		     "[tcsetpgrp failed in child_terminal_inferior: %s]\n",
+		     safe_strerror (errno));
 #endif
-	    }
+		}
 #endif
-	}
+	    }
+	  }
 
       gdb_tty_state = target_terminal_state::is_inferior;
     }
@@ -444,21 +739,27 @@ child_terminal_save_inferior (struct target_ops *self)
   inferior *inf = current_inferior ();
   terminal_info *tinfo = get_inflow_inferior_data (inf);
 
+#if GDB_MANAGED_TERMINALS
+  run_terminal_info *run_terminal = tinfo->run_terminal;
+  if (run_terminal != nullptr && run_terminal->pty_fd != -1)
+    {
+      /* The inferior has its own terminal, so there are no settings
+	 to save.  However, do flush inferior output -- usually we'll
+	 be grabbing the terminal in reaction to an inferior stop, and
+	 it's only logical to print inferior output before we announce
+	 the stop, since the inferior printed it before it
+	 stopped.  */
+      child_terminal_flush_stdout (run_terminal);
+      return;
+    }
+#endif /* GDB_MANAGED_TERMINALS */
+
   /* No need to save/restore if the inferior is not sharing GDB's
      tty.  */
   if (!sharing_input_terminal (inf))
     return;
 
-  xfree (tinfo->ttystate);
-  tinfo->ttystate = serial_get_tty_state (stdin_serial);
-
-#ifdef HAVE_TERMIOS_H
-  tinfo->process_group = tcgetpgrp (0);
-#endif
-
-#ifdef F_GETFL
-  tinfo->tflags = fcntl (0, F_GETFL, 0);
-#endif
+  tinfo->save_from_tty (stdin_serial);
 }
 
 /* Switch terminal state to DESIRED_STATE, either is_ours, or
@@ -481,27 +782,37 @@ child_terminal_ours_1 (target_terminal_state desired_state)
 	 terminal's pgrp.  */
       scoped_ignore_sigttou ignore_sigttou;
 
-      /* Set tty state to our_ttystate.  */
       serial_set_tty_state (stdin_serial, our_terminal_info.ttystate);
 
-      /* If we only want output, then leave the inferior's pgrp in the
-	 foreground, so that Ctrl-C/Ctrl-Z reach the inferior
-	 directly.  */
+      /* If we only want output, then:
+	  - if the inferior is sharing GDB's session, leave the
+	    inferior's pgrp in the foreground, so that Ctrl-C/Ctrl-Z
+	    reach the inferior directly.
+	  - if the inferior has its own session, leave stdin
+            forwarding to the inferior.  */
       if (job_control && desired_state == target_terminal_state::is_ours)
 	{
+	  if (input_fd_redirected)
+	    {
+	      delete_file_handler (0);
+	      input_fd_redirected = false;
+	    }
+	  else
+	    {
 #ifdef HAVE_TERMIOS_H
-	  result = tcsetpgrp (0, our_terminal_info.process_group);
+	      result = tcsetpgrp (0, our_terminal_info.process_group);
 #if 0
-	  /* This fails on Ultrix with EINVAL if you run the testsuite
-	     in the background with nohup, and then log out.  GDB never
-	     used to check for an error here, so perhaps there are other
-	     such situations as well.  */
-	  if (result == -1)
-	    fprintf_unfiltered (gdb_stderr,
-				"[tcsetpgrp failed in child_terminal_ours: %s]\n",
-				safe_strerror (errno));
+	      /* This fails on Ultrix with EINVAL if you run the
+		 testsuite in the background with nohup, and then log
+		 out.  GDB never used to check for an error here, so
+		 perhaps there are other such situations as well.  */
+	      if (result == -1)
+		fprintf_unfiltered (gdb_stderr,
+				    "[tcsetpgrp failed in child_terminal_ours: %s]\n",
+				    safe_strerror (errno));
 #endif
 #endif /* termios */
+	    }
 	}
 
       if (!job_control && desired_state == target_terminal_state::is_ours)
@@ -608,7 +919,7 @@ static const struct inferior_key<terminal_info> inflow_inferior_data;
 
 terminal_info::~terminal_info ()
 {
-  xfree (run_terminal);
+  delete run_terminal;
   xfree (ttystate);
 }
 
@@ -627,17 +938,108 @@ get_inflow_inferior_data (struct inferior *inf)
   return info;
 }
 
-/* This is a "inferior_exit" observer.  Releases the TERMINAL_INFO member
-   of the inferior structure.  This field is private to inflow.c, and
-   its type is opaque to the rest of GDB.  PID is the target pid of
-   the inferior that is about to be removed from the inferior
-   list.  */
+#ifdef TIOCGWINSZ
+
+/* See inferior.h.  */
+
+void
+child_terminal_on_sigwinch ()
+{
+  struct winsize size;
+
+  if (ioctl (0, TIOCGWINSZ, &size) == -1)
+    return;
+
+  /* For each inferior that is connected to a terminal that we
+     created, resize the inferior's terminal to match GDB's.  */
+  for (inferior *inf : all_inferiors ())
+    {
+      terminal_info *info = inflow_inferior_data.get (inf);
+      if (info != nullptr
+	  && info->run_terminal != nullptr
+	  && info->run_terminal->pty_fd != -1)
+	ioctl (info->run_terminal->pty_fd, TIOCSWINSZ, &size);
+    }
+}
+
+#endif
+
+/* This is an "inferior_exit" observer.  Releases the TERMINAL_INFO
+   member of the inferior structure.  This field is private to
+   inflow.c, and its type is opaque to the rest of GDB.  If INF's
+   terminal is a GDB-managed terminal, then destroy it.  */
 
 static void
 inflow_inferior_exit (struct inferior *inf)
 {
   inf->terminal_state = target_terminal_state::is_ours;
-  inflow_inferior_data.clear (inf);
+
+  terminal_info *info = inflow_inferior_data.get (inf);
+  if (info != nullptr)
+    {
+      /* Release the terminal created by GDB, if there's one.  This
+	 closes the session leader process.  */
+      if (info->run_terminal != nullptr)
+	{
+	  run_terminal_info *run_terminal = info->run_terminal;
+
+	  /* The terminal may be used by other processes (e.g., if the
+	     inferior forked).  Only actually destroy the terminal if
+	     the refcount reaches 0.  */
+	  run_terminal->decref ();
+	  if (run_terminal->refcount () == 0)
+	    {
+#if GDB_MANAGED_TERMINALS
+	      if (run_terminal->pty_fd != -1)
+		{
+		  /* Flush any pending output and close the pty.  */
+		  delete_file_handler (run_terminal->pty_fd);
+		  child_terminal_flush_stdout (run_terminal);
+
+		  /* Explicitly send a SIGHUP instead of just closing
+		     the terminal and letting the kernel send it,
+		     because we want the session leader to have a
+		     chance to put itself in the foreground, so that
+		     its children, if any (e.g., we're detaching),
+		     don't get a SIGHUP too.  */
+		  kill (run_terminal->session_leader, SIGHUP);
+
+		  /* The session leader should exit in reaction to
+		     SIGHUP.  */
+		  managed_tty_debug_printf (_("reaping session leader "
+					      "for inf %d (sid=%d)\n"),
+					    inf->num,
+					    (int) run_terminal->session_leader);
+
+		  int status;
+		  int res = waitpid (run_terminal->session_leader,
+				     &status, 0);
+		  if (res == -1)
+		    warning (_("unexpected waitstatus "
+			       "reaping session leader for inf %d (sid=%d): "
+			       "res=-1, errno=%d (%s)"),
+			     inf->num, (int) run_terminal->session_leader,
+			     errno, safe_strerror (errno));
+		  else if (res != run_terminal->session_leader
+			   || !WIFEXITED (status)
+			   || WEXITSTATUS (status) != 0)
+		    warning (_("unexpected waitstatus "
+			       "reaping session leader for inf %d (sid=%d): "
+			       "res=%d, status=0x%x"),
+			     inf->num, (int) run_terminal->session_leader,
+			     res, status);
+
+		  /* We can now close the terminal.  */
+		  close (run_terminal->pty_fd);
+		}
+#endif /* GDB_MANAGED_TERMINALS */
+	      delete run_terminal;
+	    }
+	  info->run_terminal = nullptr;
+	}
+
+      inflow_inferior_data.clear (inf);
+    }
 }
 
 void
@@ -648,14 +1050,12 @@ copy_terminal_info (struct inferior *to, struct inferior *from)
   tinfo_to = get_inflow_inferior_data (to);
   tinfo_from = get_inflow_inferior_data (from);
 
-  xfree (tinfo_to->run_terminal);
+  gdb_assert (tinfo_to->run_terminal == nullptr);
   xfree (tinfo_to->ttystate);
 
   *tinfo_to = *tinfo_from;
-
-  if (tinfo_from->run_terminal)
-    tinfo_to->run_terminal
-      = xstrdup (tinfo_from->run_terminal);
+  if (tinfo_from->run_terminal != nullptr)
+    tinfo_from->run_terminal->incref ();
 
   if (tinfo_from->ttystate)
     tinfo_to->ttystate
@@ -702,6 +1102,25 @@ child_terminal_info (struct target_ops *self, const char *args, int from_tty)
   inf = current_inferior ();
   tinfo = get_inflow_inferior_data (inf);
 
+  /* child_terminal_save_inferior doesn't bother with saving terminal
+     settings if the inferior isn't sharing the terminal with GDB, so
+     refresh them now.  Note that if the inferior _is_ sharing a
+     terminal with GDB, then we must not refresh settings now, as that
+     would be reading GDB's terminal settings, not the inferiors.  */
+
+  serial *term_serial
+    = (tinfo->run_terminal->pty_fd != -1
+       ? serial_fdopen (tinfo->run_terminal->pty_fd)
+       : stdin_serial);
+  SCOPE_EXIT
+    {
+      if (term_serial != stdin_serial)
+	serial_un_fdopen (term_serial);
+    };
+
+  if (!sharing_input_terminal (inf))
+    tinfo->save_from_tty (term_serial);
+
   printf_filtered (_("Inferior's terminal status "
 		     "(currently saved by GDB):\n"));
 
@@ -765,9 +1184,12 @@ child_terminal_info (struct target_ops *self, const char *args, int from_tty)
   printf_filtered ("Process group = %d\n", (int) tinfo->process_group);
 #endif
 
-  serial_print_tty_state (stdin_serial, tinfo->ttystate, gdb_stdout);
+  serial_print_tty_state (term_serial, tinfo->ttystate, gdb_stdout);
 }
 \f
+
+#if USES_FORK_CHILD
+
 /* NEW_TTY_PREFORK is called before forking a new child process,
    so we can record the state of ttys in the child to be formed.
    TTYNAME is null if we are to share the terminal with gdb;
@@ -780,12 +1202,56 @@ child_terminal_info (struct target_ops *self, const char *args, int from_tty)
 void
 new_tty_prefork (const char *ttyname)
 {
-  /* Save the name for later, for determining whether we and the child
-     are sharing a tty.  */
-  inferior_thisrun_terminal = ttyname;
+  /* Save the name and fd for later, for determining whether we and
+     the child are sharing a tty.  */
+
+  if (ttyname != nullptr)
+    {
+      inferior_thisrun_terminal = ttyname;
+      inferior_thisrun_terminal_pty_fd = -1;
+    }
+#if GDB_MANAGED_TERMINALS
+  else
+    {
+      /* Open an unused pty master device.  */
+      int pty_fd = posix_openpt (O_RDWR | O_NONBLOCK | O_CLOEXEC | O_NOCTTY);
+      if (pty_fd == -1)
+	perror_with_name ("posix_openpt");
+
+      /* Grant access to the slave tty.  */
+      if (grantpt (pty_fd) == -1)
+	{
+	  int err = errno;
+	  close (pty_fd);
+	  errno = err;
+	  perror_with_name ("grantpt");
+	}
+
+      /* Unlock the pty master/slave pair.  */
+      if (unlockpt (pty_fd) == -1)
+	{
+	  close (pty_fd);
+	  perror_with_name ("unlockpt");
+	}
+
+      inferior_thisrun_terminal = ptsname (pty_fd);
+      inferior_thisrun_terminal_pty_fd = pty_fd;
+
+      if (initial_gdb_ttystate != nullptr)
+	{
+	  serial *pty_fd_serial = serial_fdopen (pty_fd);
+
+	  int result = serial_set_tty_state (pty_fd_serial,
+					     initial_gdb_ttystate);
+	  gdb_assert (result != -1);
+	  OOPSY ("setting tty state");
+
+	  serial_un_fdopen (pty_fd_serial);
+	}
+    }
+#endif
 }
 
-#if !defined(__GO32__) && !defined(_WIN32)
 /* If RESULT, assumed to be the return value from a system call, is
    negative, print the error message indicated by errno and exit.
    MSG should identify the operation that failed.  */
@@ -798,7 +1264,14 @@ check_syscall (const char *msg, int result)
       _exit (1);
     }
 }
-#endif
+
+/* See terminal.h.  */
+
+bool
+created_managed_tty ()
+{
+  return inferior_thisrun_terminal_pty_fd != -1;
+}
 
 void
 new_tty ()
@@ -806,7 +1279,6 @@ new_tty ()
   if (inferior_thisrun_terminal == nullptr
       || is_gdb_terminal (inferior_thisrun_terminal))
     return;
-#if !defined(__GO32__) && !defined(_WIN32)
   int tty;
 
 #ifdef TIOCNOTTY
@@ -855,12 +1327,13 @@ new_tty ()
 
   if (tty > 2)
     close (tty);
-#endif /* !go32 && !win32 */
 }
 
 /* NEW_TTY_POSTFORK is called after forking a new child process, and
    adding it to the inferior table, to store the TTYNAME being used by
-   the child, or null if it sharing the terminal with gdb.  */
+   the child, or null if it sharing the terminal with gdb.  If the
+   child is using a terminal created by GDB, the corresponding pty
+   master fd is stored.  */
 
 void
 new_tty_postfork (void)
@@ -870,15 +1343,38 @@ new_tty_postfork (void)
 
   struct inferior *inf = current_inferior ();
   struct terminal_info *tinfo = get_inflow_inferior_data (inf);
+  auto *run_terminal = new run_terminal_info ();
+  tinfo->run_terminal = run_terminal;
+  run_terminal->incref ();
 
   if (inferior_thisrun_terminal != nullptr)
-    tinfo->run_terminal = xstrdup (inferior_thisrun_terminal);
+    {
+      run_terminal->ttyname = make_unique_xstrdup (inferior_thisrun_terminal);
+      run_terminal->pty_fd = inferior_thisrun_terminal_pty_fd;
+      if (run_terminal->pty_fd != -1)
+	{
+	  run_terminal->session_leader = getsid (inf->pid);
+	  gdb_assert (run_terminal->session_leader != -1);
+	}
+
+      if (run_terminal->pty_fd != -1)
+	{
+	  add_file_handler (run_terminal->pty_fd,
+			    inferior_stdout_event_handler, run_terminal,
+			    string_printf ("pty_fd-%s",
+					   run_terminal->ttyname.get ()),
+			    true);
+	}
+    }
   else
-    tinfo->run_terminal = xstrdup ("/dev/tty");
+    run_terminal->ttyname = make_unique_xstrdup ("/dev/tty");
 
-  inferior_thisrun_terminal = NULL;
+  inferior_thisrun_terminal = nullptr;
+  inferior_thisrun_terminal_pty_fd = -1;
 }
 
+#endif /* USES_FORK_CHILD */
+
 \f
 /* Call set_sigint_trap when you need to pass a signal on to an attached
    process when handling SIGINT.  */
@@ -961,6 +1457,16 @@ initialize_stdin_serial (void)
   stdin_serial = serial_fdopen (0);
 }
 
+/* "show" callback for "set debug managed-tty".  */
+
+static void
+show_debug_managed_tty (ui_file *file, int from_tty,
+			cmd_list_element *c, const char *value)
+{
+  fprintf_filtered (file, _("Debugging of GDB-managed terminals is %s.\n"),
+		    value);
+}
+
 void _initialize_inflow ();
 void
 _initialize_inflow ()
@@ -968,6 +1474,13 @@ _initialize_inflow ()
   add_info ("terminal", info_terminal_command,
 	    _("Print inferior's saved terminal status."));
 
+  add_setshow_boolean_cmd
+    ("managed-tty", class_maintenance, &debug_managed_tty,
+     _("Set debugging of GDB-managed terminals."),
+     _("Show debugging of GDB-managed terminals."),
+     _("When non-zero, GDB-managed terminals specific debugging is enabled."),
+     nullptr, show_debug_managed_tty, &setdebuglist, &showdebuglist);
+
   /* OK, figure out whether we have job control.  */
   have_job_control ();
 
diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 34a2aee41d7..d31bcd98e84 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -1075,6 +1075,82 @@ linux_nat_post_attach_wait (ptid_t ptid, int *signalled)
   return status;
 }
 
+/* Wait for a SIGSTOP out of PID.  */
+
+static void
+waitpid_sigstop (pid_t pid)
+{
+  int status;
+  int res = waitpid (pid, &status, 0);
+  if (res == -1)
+    perror_with_name (_("waiting for child"));
+  else if (res != pid)
+    error (_("wait returned unexpected PID %d"), res);
+  else if (!WIFSTOPPED (status) || WSTOPSIG (status) != SIGSTOP)
+    error (_("wait returned unexpected status 0x%x"), status);
+}
+
+/* Wait for a fork event out of PID.  */
+
+static void
+waitpid_fork (pid_t pid)
+{
+  int status;
+  int res = waitpid (pid, &status, 0);
+  if (res == -1)
+    perror_with_name (_("waiting for child"));
+  else if (res != pid)
+    error (_("wait returned unexpected PID %d"), res);
+  else if (!WIFSTOPPED (status))
+    error (_("wait returned unexpected status 0x%x"), status);
+  else
+    {
+      int event = linux_ptrace_get_extended_event (status);
+      if (event != PTRACE_EVENT_FORK)
+	error (_("wait returned unexpected status 0x%x"), status);
+    }
+}
+
+pid_t
+linux_nat_target::handle_session_leader_fork (pid_t sl_pid)
+{
+  /* The first fork child is the session leader.  In turn its fork
+     child (i.e., GDB's granchild) is the inferior we want to debug.
+     Enable tracefork in order to trace the grandchild, and get its
+     pid.  */
+
+  waitpid_sigstop (sl_pid);
+
+  linux_enable_event_reporting (sl_pid, PTRACE_O_TRACEFORK);
+  ptrace (PTRACE_CONT, sl_pid, (PTRACE_TYPE_ARG3) 1, 0);
+
+  /* We should see a fork event now, for the second fork.  */
+  waitpid_fork (sl_pid);
+
+  /* Extract the grandchild's pid.  This is the final inferior
+     process.  */
+  unsigned long inf_pid;
+
+  if (ptrace (PTRACE_GETEVENTMSG, sl_pid, 0, &inf_pid) == -1)
+    perror_with_name (_("getting event message"));
+
+  /* The new child has a pending SIGSTOP.  We can't affect it until it
+     hits the SIGSTOP, but we're already attached.  */
+  waitpid_sigstop (inf_pid);
+
+  /* We don't need to continue debugging the session leader.  It's
+     simpler to just detach from it.  */
+  ptrace (PTRACE_DETACH, sl_pid, (PTRACE_TYPE_ARG3) 1, 0);
+
+  /* Resume the grandchild / inferior.  Disable event reporting to
+     avoid confusing startup_inferior with extra events as the
+     inferior goes through the shell.  */
+  linux_disable_event_reporting (inf_pid);
+  ptrace (PTRACE_CONT, inf_pid, (PTRACE_TYPE_ARG3) 1, 0);
+
+  return inf_pid;
+}
+
 void
 linux_nat_target::create_inferior (const char *exec_file,
 				   const std::string &allargs,
diff --git a/gdb/linux-nat.h b/gdb/linux-nat.h
index 5426a5c6900..1f050165adb 100644
--- a/gdb/linux-nat.h
+++ b/gdb/linux-nat.h
@@ -40,6 +40,8 @@ class linux_nat_target : public inf_ptrace_target
   void create_inferior (const char *, const std::string &,
 			char **, int) override;
 
+  pid_t handle_session_leader_fork (pid_t sl_pid) override;
+
   void attach (const char *, int) override;
 
   void detach (inferior *, int) override;
diff --git a/gdb/nat/fork-inferior.c b/gdb/nat/fork-inferior.c
index 7d56250c979..f140242ace5 100644
--- a/gdb/nat/fork-inferior.c
+++ b/gdb/nat/fork-inferior.c
@@ -27,7 +27,11 @@
 #include "gdbsupport/pathstuff.h"
 #include "gdbsupport/signals-state-save-restore.h"
 #include "gdbsupport/gdb_tilde_expand.h"
+#include "gdbsupport/scoped_ignore_sigttou.h"
+#include "gdbsupport/managed-tty.h"
 #include <vector>
+#include <sys/types.h>
+#include <sys/wait.h>
 
 extern char **environ;
 
@@ -81,6 +85,33 @@ class execv_argv
   std::string m_storage;
 };
 
+#if GDB_MANAGED_TERMINALS
+
+/* SIGHUP handler for the session leader processes.  GDB sends this
+   explicitly, though it'll also be called if GDB crashes and the
+   terminal is abruptly closed.  */
+
+static void
+session_leader_hup (int sig)
+{
+  scoped_ignore_sigttou ignore_sigttou;
+
+  /* We put the inferior (a child of the session leader) in the
+     foreground, so by default, on detach, if we did nothing else, the
+     inferior would get a SIGHUP when the terminal is closed by GDB.
+     That SIGHUP would likely kill the inferior.  To avoid it, we put
+     the session leader in the foreground before the terminal is
+     closed.  Only processes in the foreground process group get the
+     automatic SIGHUP, so the detached process doesn't get it.  */
+  int res = tcsetpgrp (0, getpid ());
+  if (res == -1)
+    trace_start_error (_("tcsetpgrp failed in session leader\n"));
+
+  _exit (0);
+}
+
+#endif
+
 /* Create argument vector for straight call to execvp.  Breaks up
    ALLARGS into an argument vector suitable for passing to execvp and
    stores it in M_ARGV.  E.g., on "run a b c d" this routine would get
@@ -271,7 +302,8 @@ fork_inferior (const char *exec_file_arg, const std::string &allargs,
 	       void (*pre_trace_fun) (),
 	       const char *shell_file_arg,
 	       void (*exec_fun)(const char *file, char * const *argv,
-				char * const *env))
+				char * const *env),
+	       pid_t (*handle_session_leader_fork) (pid_t sl_pid))
 {
   pid_t pid;
   /* Set debug_fork then attach to the child while it sleeps, to debug.  */
@@ -354,7 +386,7 @@ fork_inferior (const char *exec_file_arg, const std::string &allargs,
      actually be a call to fork(2) due to the fact that autoconf will
      ``#define vfork fork'' on certain platforms.  */
 #if !(defined(__UCLIBC__) && defined(HAS_NOMMU))
-  if (pre_trace_fun || debug_fork)
+  if (pre_trace_fun || debug_fork || child_has_managed_tty_hook ())
     pid = fork ();
   else
 #endif
@@ -405,6 +437,85 @@ fork_inferior (const char *exec_file_arg, const std::string &allargs,
 
       restore_original_signals_state ();
 
+      /* Fork again so that the resulting inferior process is not the
+	 session leader.  This makes it possible for the inferior to
+	 exit without killing its own children.  If we instead let the
+	 inferior process be the session leader, when it exits, it'd
+	 cause a SIGHUP to be sent to all processes in its session
+	 (i.e., it's children).  The code is disabled on no-MMU
+	 machines because those can't do fork, only vfork.  In theory
+	 we could make this work with vfork by making the session
+	 leader process exec a helper process, probably gdb itself in
+	 a special mode (e.g., something like
+	   exec /proc/self/exe --session-leader-for PID
+      */
+#if GDB_MANAGED_TERMINALS
+      if (child_has_managed_tty_hook ())
+	{
+	  /* Fork again, to make sure the inferior is not the new
+	     session's leader.  */
+	  pid_t child2 = fork ();
+	  if (child2 != 0)
+	    {
+	      /* This is the parent / session leader process.  It just
+		 stays around until GDB closes the terminal.  */
+
+	      /* Gracefully handle SIGHUP.  */
+	      signal (SIGHUP, session_leader_hup);
+
+	      managed_tty_debug_printf
+		(_("session-leader (sid=%d): waiting for child pid=%d exit\n"),
+		 (int) getpid (), (int) child2);
+
+	      /* Reap the child/inferior exit status.  */
+	      int status;
+	      int res = waitpid (child2, &status, 0);
+
+	      managed_tty_debug_printf (_("session-leader (sid=%d): "
+					  "wait for child pid=%d returned: "
+					  "res=%d, waitstatus=0x%x\n"),
+					(int) getpid (), child2, res, status);
+
+	      if (res == -1)
+		warning (_("session-leader (sid=%d): unexpected waitstatus "
+			   "reaping child pid=%d: "
+			   "res=-1, errno=%d (%s)"),
+			 (int) getpid (), child2, errno, safe_strerror (errno));
+	      else if (res != child2)
+		warning (_("session-leader (sid=%d): unexpected waitstatus "
+			   "reaping child pid=%d: "
+			   "res=%d, status=0x%x"),
+			 (int) getpid (), child2, res, status);
+
+	      /* Don't exit yet.  While our direct child is gone,
+		 there may still be grandchildren attached to our
+		 session.  We'll exit when our parent (GDB) closes the
+		 pty, killing us with SIGHUP.  */
+	      while (1)
+		pause ();
+	    }
+	  else
+	    {
+	      /* This is the child / final inferior process.  */
+
+	      int res;
+
+	      /* Run the inferior in its own process group, and make
+		 it the session's foreground pgrp.  */
+
+	      res = gdb_setpgid ();
+	      if (res == -1)
+		trace_start_error (_("setpgid failed in grandchild"));
+
+	      scoped_ignore_sigttou ignore_sigttou;
+
+	      res = tcsetpgrp (0, getpid ());
+	      if (res == -1)
+		trace_start_error (_("tcsetpgrp failed in grandchild\n"));
+	    }
+	}
+#endif /* GDB_MANAGED_TERMINALS */
+
       /* There is no execlpe call, so we have to set the environment
 	 for our child in the global variable.  If we've vforked, this
 	 clobbers the parent, but environ is restored a few lines down
@@ -434,6 +545,9 @@ fork_inferior (const char *exec_file_arg, const std::string &allargs,
   /* Restore our environment in case a vforked child clob'd it.  */
   environ = save_our_env;
 
+  if (child_has_managed_tty_hook () && handle_session_leader_fork != nullptr)
+    pid = handle_session_leader_fork (pid);
+
   postfork_hook (pid);
 
   /* Now that we have a child process, make it our target, and
diff --git a/gdb/nat/fork-inferior.h b/gdb/nat/fork-inferior.h
index f45ec9047c8..c0fb2beb5f3 100644
--- a/gdb/nat/fork-inferior.h
+++ b/gdb/nat/fork-inferior.h
@@ -31,12 +31,15 @@ struct process_stratum_target;
    implementations.  */
 #define START_INFERIOR_TRAPS_EXPECTED 1
 
-/* Start an inferior Unix child process and sets inferior_ptid to its
-   pid.  EXEC_FILE is the file to run.  ALLARGS is a string containing
-   the arguments to the program.  ENV is the environment vector to
-   pass.  SHELL_FILE is the shell file, or NULL if we should pick
-   one.  EXEC_FUN is the exec(2) function to use, or NULL for the default
-   one.  */
+/* Start an inferior Unix child process and return its pid.  EXEC_FILE_ARG
+   is the file to run.  ALLARGS is a string containing the arguments
+   to the program.  ENV is the environment vector to pass.
+   SHELL_FILE_ARG is the shell file, or NULL if we should pick one.
+   EXEC_FUN is the exec(2) function to use, or NULL for the default
+   one.  HANDLE_SESSION_LEADER_FORK is the function to use to debug
+   the session leader process across the double-fork and extract the final
+   inferior PID if the inferior is using a gdb-managed terminal, or
+   NULL otherwise.  */
 
 /* This function is NOT reentrant.  Some of the variables have been
    made static to ensure that they survive the vfork call.  */
@@ -48,7 +51,9 @@ extern pid_t fork_inferior (const char *exec_file_arg,
 			    const char *shell_file_arg,
 			    void (*exec_fun) (const char *file,
 					      char * const *argv,
-					      char * const *env));
+					      char * const *env),
+			    pid_t (*handle_session_leader_fork) (pid_t sl_pid)
+			      = nullptr);
 
 /* Accept NTRAPS traps from the inferior.
 
@@ -70,6 +75,10 @@ extern void postfork_hook (pid_t pid);
    place.  This function is mainly used by fork_inferior.  */
 extern void postfork_child_hook ();
 
+/* True if the inferior child has a terminal created and managed by
+   GDB or GDBserver.  */
+extern bool child_has_managed_tty_hook ();
+
 /* Flush both stdout and stderr.  This function needs to be
    implemented differently on GDB and GDBserver.  */
 extern void gdb_flush_out_err ();
diff --git a/gdb/terminal.h b/gdb/terminal.h
index e1f642b8466..f8ef29bc2a9 100644
--- a/gdb/terminal.h
+++ b/gdb/terminal.h
@@ -27,6 +27,9 @@ extern void new_tty (void);
 
 extern void new_tty_postfork (void);
 
+/* Returns true if new_tty_prefork created a GDB-managed terminal.  */
+extern bool created_managed_tty ();
+
 extern void copy_terminal_info (struct inferior *to, struct inferior *from);
 
 /* Exchange the terminal info and state between inferiors A and B.  */
diff --git a/gdb/testsuite/gdb.multi/multi-term-settings.exp b/gdb/testsuite/gdb.multi/multi-term-settings.exp
index dcc6f2ece0f..5275391a73c 100644
--- a/gdb/testsuite/gdb.multi/multi-term-settings.exp
+++ b/gdb/testsuite/gdb.multi/multi-term-settings.exp
@@ -48,9 +48,18 @@ set attach_pid2 [spawn_id_get_pid $attach_spawn_id2]
 # Create inferior WHICH_INF.  INF_HOW is how to create it.  This can
 # be one of:
 #
-#  - "run" - for "run" command.
-#  - "tty" - for "run" + "tty TTY".
-#  - "attach" - for attaching to an existing process.
+#  - "run-session" - for "run" command, GDB puts inferior in its own
+#                    session and gives its own controlling terminal,
+#                    on systems that support it (e.g., GNU/Linux) (the
+#                    default).
+#
+#  - "run-share"   - for "run" command, GDB and inferior share terminal
+#                    ("set inferior-tty /dev/tty").
+#
+#  - "run-tty"     - for "run" + "tty TTY".
+#
+#  - "attach"      - for attaching to an existing process.
+
 proc create_inferior {which_inf inf_how} {
     global binfile
 
@@ -69,12 +78,22 @@ proc create_inferior {which_inf inf_how} {
 	}
     }
 
-    if {$inf_how == "run"} {
+    if {$inf_how == "run-session"} {
+	# Clear inferior-tty back to the default, in case we're
+	# running the testsuite with "set inferior-tty /dev/tty"
+	# forced.
+	gdb_test_no_output "set inferior-tty"
+	if [my_runto_main] {
+	    global inferior_spawn_id
+	    return $inferior_spawn_id
+	}
+    } elseif {$inf_how == "run-share"} {
+	gdb_test_no_output "set inferior-tty /dev/tty"
 	if [my_runto_main] {
 	    global inferior_spawn_id
 	    return $inferior_spawn_id
 	}
-    } elseif {$inf_how == "tty"} {
+    } elseif {$inf_how == "run-tty"} {
 	# Create the new PTY for the inferior.
 	spawn -pty
 	set inf_spawn_id $spawn_id
@@ -161,8 +180,21 @@ proc coretest {inf1_how inf2_how} {
     # "run" makes each inferior be a process group leader.  When we
     # run both inferiors in GDB's terminal/session, only one can end
     # up as the terminal's foreground process group, so it's expected
-    # that the other receives a SIGTTOU.
-    set expect_ttou [expr {$inf1_how == "run" && $inf2_how == "run"}]
+    # that the other receives a SIGTTOU.  Similarly, if we run one
+    # inferior in GDB's terminal, and another in its own GDB-created
+    # terminal/session, then if we resume the latter first, GDB won't
+    # put the former in the foreground either (since GDB must stay in
+    # the foreground to forward input and catch Ctrl-C).
+
+    # Only the native target knows how to put the inferior in its own
+    # terminal/session.
+    set can_per_inf_session [gdb_is_target_native]
+
+    set expect_ttou [expr {(($inf1_how == "run-share"
+			     || (!$can_per_inf_session
+				 && $inf1_how == "run-session"))
+			    && ($inf2_how == "run-share"
+				|| $inf2_how == "run-session"))}]
 
     global gdb_prompt
 
@@ -256,11 +288,11 @@ proc coretest {inf1_how inf2_how} {
     }
 }
 
-set how_modes {"run" "attach" "tty"}
+set how_modes {"run-session" "run-share" "run-tty" "attach"}
 
 foreach_with_prefix inf1_how $how_modes {
     foreach_with_prefix inf2_how $how_modes {
-	if {($inf1_how == "tty" || $inf2_how == "tty")
+	if {($inf1_how == "run-tty" || $inf2_how == "run-tty")
 	    && [target_info gdb_protocol] == "extended-remote"} {
 	    # No way to specify the inferior's tty in the remote
 	    # protocol.
diff --git a/gdb/tui/tui-win.c b/gdb/tui/tui-win.c
index 4e75a66a00e..88a5f1c1d05 100644
--- a/gdb/tui/tui-win.c
+++ b/gdb/tui/tui-win.c
@@ -36,6 +36,7 @@
 #include "gdbsupport/event-loop.h"
 #include "gdbcmd.h"
 #include "async-event.h"
+#include "inferior.h"
 
 #include "tui/tui.h"
 #include "tui/tui-io.h"
@@ -565,6 +566,8 @@ tui_async_resize_screen (gdb_client_data arg)
 	}
       tui_redisplay_readline ();
     }
+
+  child_terminal_on_sigwinch ();
 }
 #endif
 
diff --git a/gdbserver/fork-child.cc b/gdbserver/fork-child.cc
index 9678133243d..23eff9fbf45 100644
--- a/gdbserver/fork-child.cc
+++ b/gdbserver/fork-child.cc
@@ -58,6 +58,14 @@ prefork_hook ()
 
 /* See nat/fork-inferior.h.  */
 
+bool
+child_has_managed_tty_hook ()
+{
+  return false;
+}
+
+/* See nat/fork-inferior.h.  */
+
 void
 postfork_hook (pid_t pid)
 {
diff --git a/gdbsupport/Makefile.am b/gdbsupport/Makefile.am
index 6d4678c8c9b..ccc7d1e5c51 100644
--- a/gdbsupport/Makefile.am
+++ b/gdbsupport/Makefile.am
@@ -56,6 +56,7 @@ libgdbsupport_a_SOURCES = \
     gdb_wait.cc \
     gdb_vecs.cc \
     job-control.cc \
+    managed-tty.cc \
     netstuff.cc \
     new-op.cc \
     pathstuff.cc \
diff --git a/gdbsupport/Makefile.in b/gdbsupport/Makefile.in
index d7f2d4914b0..ed9385381c5 100644
--- a/gdbsupport/Makefile.in
+++ b/gdbsupport/Makefile.in
@@ -153,12 +153,13 @@ am_libgdbsupport_a_OBJECTS = agent.$(OBJEXT) btrace-common.$(OBJEXT) \
 	fileio.$(OBJEXT) filestuff.$(OBJEXT) format.$(OBJEXT) \
 	gdb-dlfcn.$(OBJEXT) gdb_tilde_expand.$(OBJEXT) \
 	gdb_wait.$(OBJEXT) gdb_vecs.$(OBJEXT) job-control.$(OBJEXT) \
-	netstuff.$(OBJEXT) new-op.$(OBJEXT) pathstuff.$(OBJEXT) \
-	print-utils.$(OBJEXT) ptid.$(OBJEXT) rsp-low.$(OBJEXT) \
-	run-time-clock.$(OBJEXT) safe-strerror.$(OBJEXT) \
-	scoped_mmap.$(OBJEXT) search.$(OBJEXT) signals.$(OBJEXT) \
-	signals-state-save-restore.$(OBJEXT) tdesc.$(OBJEXT) \
-	thread-pool.$(OBJEXT) xml-utils.$(OBJEXT) $(am__objects_1)
+	managed-tty.$(OBJEXT) netstuff.$(OBJEXT) new-op.$(OBJEXT) \
+	pathstuff.$(OBJEXT) print-utils.$(OBJEXT) ptid.$(OBJEXT) \
+	rsp-low.$(OBJEXT) run-time-clock.$(OBJEXT) \
+	safe-strerror.$(OBJEXT) scoped_mmap.$(OBJEXT) search.$(OBJEXT) \
+	signals.$(OBJEXT) signals-state-save-restore.$(OBJEXT) \
+	tdesc.$(OBJEXT) thread-pool.$(OBJEXT) xml-utils.$(OBJEXT) \
+	$(am__objects_1)
 libgdbsupport_a_OBJECTS = $(am_libgdbsupport_a_OBJECTS)
 AM_V_P = $(am__v_P_@AM_V@)
 am__v_P_ = $(am__v_P_@AM_DEFAULT_V@)
@@ -379,6 +380,7 @@ libgdbsupport_a_SOURCES = \
     gdb_wait.cc \
     gdb_vecs.cc \
     job-control.cc \
+    managed-tty.cc \
     netstuff.cc \
     new-op.cc \
     pathstuff.cc \
@@ -484,6 +486,7 @@ distclean-compile:
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/gdb_vecs.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/gdb_wait.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/job-control.Po@am__quote@
+@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/managed-tty.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/netstuff.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/new-op.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/pathstuff.Po@am__quote@
diff --git a/gdbsupport/managed-tty.cc b/gdbsupport/managed-tty.cc
new file mode 100644
index 00000000000..824cf0985c2
--- /dev/null
+++ b/gdbsupport/managed-tty.cc
@@ -0,0 +1,22 @@
+/* Support for GDB-managed terminals.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "gdbsupport/common-defs.h"
+#include "gdbsupport/managed-tty.h"
+
+bool debug_managed_tty = false;
diff --git a/gdbsupport/managed-tty.h b/gdbsupport/managed-tty.h
new file mode 100644
index 00000000000..d02d6c95659
--- /dev/null
+++ b/gdbsupport/managed-tty.h
@@ -0,0 +1,42 @@
+/* Support for GDB-managed terminals.
+
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef MANAGED_TTY_H
+#define MANAGED_TTY_H
+
+/* Using host-dependent code here is fine, because the support for
+   creating terminals for inferiors is meant to be used with
+   child/native targets.  Note we disable the feature on no-MMU
+   architectures for now because those can't use fork, see
+   fork-inferior.c.  */
+#if (defined (__linux__)				\
+     && !(defined (__UCLIBC__) && defined (HAS_NOMMU)))
+# define GDB_MANAGED_TERMINALS 1
+#else
+# define GDB_MANAGED_TERMINALS 0
+#endif
+
+extern bool debug_managed_tty;
+
+/* Print a "managed-tty" debug statement.  */
+
+#define managed_tty_debug_printf(fmt, ...) \
+  debug_prefixed_printf_cond (debug_managed_tty, "managed-tty",fmt, ##__VA_ARGS__)
+
+#endif /* MANAGED_TTY_H */
-- 
2.26.2