[PATCH] Limit the switch_to_thread() calls in startup_inferior()

[Kamil Rytarowski <n54@gmx.com>]

Do not jump over the threads during the startup unless we encounter
TARGET_WAITKIND_STOPPED with SIGTRAP or TARGET_WAITKIND_EXECD.

Otherwise whenever a startup-with-shell processes signals on the
startup stage, it might indicate to switch to a non-existing
thread or a special-thread number (target lwp=0 on NetBSD means
that a signal was directed to all threads within a process).

This caused a crash with tcsh on NetBSD, where the tcsh shell
runs startup detection of the hostname. This action involves
spwaning a new process through fork.

GDB crashes this way:
$ SHELL=tcsh /usr/bin/gdb echo
(gdb) r
Starting program: /bin/echo
/usr/src/external/gpl3/gdb/lib/libgdb/../../dist/gdb/thread.c:1309: internal-error: void switch_to_thread(thread_info*): Assertion `thr != NULL' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.

And the core-dump is as follows:

(top-gdb) bt
During symbol reading: incomplete CFI data; unspecified registers (e.g., rax) at 0x7f7ff5b698a4
    file=0x135314f "../../gdb/thread.c", line=1309, fmt=0x1353069 "%s: Assertion `%s' failed.", ap=0x7f7fffffdc08) at ../../gdb/utils.c:414
    at ../../gdb/inf-ptrace.c:117
args=0x0, from_tty=1, run_how=RUN_NORMAL) at ../../gdb/infcmd.c:493
--Type <RET> for more, q to quit, c to continue without paging--
---
 gdb/nat/fork-inferior.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gdb/nat/fork-inferior.c b/gdb/nat/fork-inferior.c
index 7ba0126871d..b6c20a8fac1 100644
--- a/gdb/nat/fork-inferior.c
+++ b/gdb/nat/fork-inferior.c
@@ -503,7 +503,6 @@ startup_inferior (process_stratum_target *proc_target, pid_t pid, int ntraps,
 	  case TARGET_WAITKIND_SYSCALL_ENTRY:
 	  case TARGET_WAITKIND_SYSCALL_RETURN:
 	    /* Ignore gracefully during startup of the inferior.  */
-	    switch_to_thread (proc_target, event_ptid);
 	    break;

 	  case TARGET_WAITKIND_SIGNALLED:
@@ -536,7 +535,9 @@ startup_inferior (process_stratum_target *proc_target, pid_t pid, int ntraps,

 	  case TARGET_WAITKIND_STOPPED:
 	    resume_signal = ws.value.sig;
-	    switch_to_thread (proc_target, event_ptid);
+	    /* Ignore gracefully the !TRAP signals intercepted from the shell.  */
+	    if (resume_signal == GDB_SIGNAL_TRAP)
+	      switch_to_thread (proc_target, event_ptid);
 	    break;
 	}

--
2.28.0