Re: [PATCH] Per-inferior target_terminal state, fix PR gdb/13211, more

Simon Marchi
2018-03-30T14:54:09Z

On 2017-12-04 02:44 PM, Pedro Alves wrote:
> (This applies on top of:
>
> [PATCH] linux-nat: Eliminate custom target_terminal_{inferior,ours}, stop using set_sigint_trap
> https://sourceware.org/ml/gdb-patches/2017-11/msg00368.html
> )
>
> In my multi-target branch I ran into problems with GDB's terminal
> handling that exist in master as well, with multi-inferior debugging.
>
> This patch adds a testcase for said problems
> (gdb.multi/multi-term-settings.exp), fixes the problems, fixes PR
> gdb/13211 as well (and adds a testcase for that too,
> gdb.base/interrupt-daemon.exp).
>
> The basis of the problem I ran into is the following. Consider a
> scenario where you have:
>
> - inferior 1 - started with "attach", process is running on some
> other terminal.
>
> - inferior 2 - started with "run", process is sharing gdb's terminal.
>
> In this scenario, when you stop/resume both inferiors, you want GDB to
> save/restore the terminal settings of inferior 2, the one that is
> sharing GDB's terminal. I.e., you want inferior 2 to "own" the
> terminal (in target_terminal::is_ours/target_terminal::is_inferior
> sense).
>
> Unfortunately, that's not what you get currently. Because GDB doesn't
> know whether an attached inferior is actually sharing GDB's terminal,
> it tries to save/restore its settings anyway, ignoring errors. In
> this case, this is pointless, because inferior 1 is running on a
> different terminal, but GDB doesn't know better.
>
> And then, because it is only possible to have the terminal settings of
> a single inferior be in effect at a time, or make one inferior/pgrp be
> the terminal's foreground pgrp (aka, only one inferior can "own" the
> terminal, ignoring fork children here), if GDB happens to try to
> restore the terminal settings of inferior 1 first, then GDB never
> restores the terminal settings of inferior 2.
>
> This patch fixes that and a few things more along the way:
>
> - Moves enum target_terminal::terminal_state out of the
> target_terminal class (it's currently private) and makes it a
> scoped enum so that it can be easily used elsewhere.
>
> - Replaces the inflow.c:terminal_is_ours boolean with a
> target_terminal_state variable. This allows distinguishing is_ours
> and is_ours_for_output states. This allows finally making
> child_terminal_ours_1 do something with its "output_only"
> parameter.
>
> - Makes each inferior have its own copy of the
> is_ours/is_ours_for_output/is_inferior state.
>
> - Adds a way for GDB to tell whether the inferior is sharing GDB's
> terminal. Works best on Linux and Solaris; the fallback works just
> as well as currently.
>
> - With that, we can remove the inf->attach_flag tests from
> child_terminal_inferior/child_terminal_ours.
>
> - Currently target_ops.to_ours is responsible for both saving the
> current inferior's terminal state, and restoring gdb's state.
> Because each inferior has its own terminal state (possibly handled
> by different targets in a multi-target world, even), we need to
> split the inferior-saving part from the gdb-restoring part. The
> patch adds a new target_ops.to_save_inferior target method for
> that.
>
> - Adds a new target_terminal::save_inferior() function, so that
> sequences like:
>
> scoped_restore_terminal_state save_state;
> target_terminal::ours_for_output ();
>
> ... restore back inferiors that were
> target_terminal_state::is_inferior before back to is_inferior, and
> leaves inferiors that were is_ours alone.
>
> - Along the way, this adds a default implementation of
> target_pass_ctrlc to inflow.c (for inf-child.c), that handles
> passing the Ctrl-C to a process running on GDB's terminal or to
> some other process otherwise.
>
> - Similarly, adds a new target default implementation of
> target_interrupt, for the "interrupt" command. The current
> implementation of this hook in inf-ptrace.c kills the whole process
> group, but that's incorrect/undesirable because we may not be
> attached to all processes in the process group. And also, it's
> incorrect because inferior_process_group() doesn't really return
> the inferior's real process group id if the inferior is not a
> process group leader... This is the cause of PR gdb/13211 [1],
> which this patch fixes. While at it, that target method's "ptid"
> parameter is eliminated, because it's not really used.
>
> - A new test is included that exercises and fixes PR gdb/13211, and
> also fixes a GDB issue reported on stackoverflow that I ran into
> while working on this [2]. The problem is similar to PR gdb/13211,
> except that it also triggers with Ctrl-C. When debugging a daemon
> (i.e., a process that disconnects from the controlling terminal and
> is not a process group leader, then Ctrl-C doesn't work, you just
> can't interrupt the inferior at all, resulting in a hung debug
> session. The problem is that since the inferior is no longer
> associated with gdb's session / controlling terminal, then trying
> to put the inferior in the foreground fails. And so Ctrl-C never
> reaches the inferior directly. pass_signal is only used when the
> inferior is attached, but that is not the case here. This is fixed
> by the new child_pass_ctrlc. Without the fix, the new
> interrupt-daemon.exp testcase fails with timeout waiting for a
> SIGINT that never arrives.
>
> [1] PR gdb/13211 - Async / Process group and interrupt not working
> https://sourceware.org/bugzilla/show_bug.cgi?id=13211
>
> [2] GDB not reacting Ctrl-C when after fork() and setsid()
> https://stackoverflow.com/questions/46101292/gdb-not-reacting-ctrl-c-when-after-fork-and-setsid
>
> Note this patch does _not_ fix:
>
> - PR gdb/14559 - The 'interrupt' command does not work if sigwait is in use
> https://sourceware.org/bugzilla/show_bug.cgi?id=14559
>
> - PR gdb/9425 - When using "sigwait" GDB doesn't trap SIGINT. Ctrl+C terminates program when should break gdb.
> https://sourceware.org/bugzilla/show_bug.cgi?id=9425
>
> The only way to fix that that I know of (without changing the kernel)
> is to make GDB put inferiors in a separate session (create a
> pseudo-tty master/slave pair, make the inferior run with the slave as
> its terminal, and have gdb pump output/input on the master end). I
> have a follow up series that builds on top of this one that does that,
> but that's too invasive for 8.1, I think. While this one fixes a
> couple bugs already, and I think having it in 8.1 would simplify
> backports for (a future) 8.1.1.

Hi Pedro,

I found a regression caused by this commit, I filed a bug here:

https://sourceware.org/bugzilla/show_bug.cgi?id=23020

I'll take a look, but I'm always confused by the terminal target/ours switching
flow, so I don't have high hopes of finding a good fix.

Simon