[PATCH] Make `linux_info_proc` prefer using the LWP over the PID

[Matheus Branco Borella <dark.ryu.550@gmail.com>]

Fixes: https://sourceware.org/bugzilla/show_bug.cgi?id=31207

Normally, `linux_info_proc` would use the PID to determine which subfolder in
`/proc` to read information from. While this is usually fine, it breaks down
after the main thread exits, at which point the information in `/proc/$pid`
becomes become unreliable, if it is available at all. While it is the case
that most programs terminate after their main thread exits, some may continue
running from detached threads, in which case `info proc` will start misbehaving.

This patch addresses this by making it so that the LWP - the Lightweight Process
ID, that, in the case of GNU/Linux is the number of the process backing up the
thread[1] - is prefered over the PID. By doing this, `linux_info_proc` will
always access valid procfs information, even after the main thread exits.

[1]: https://man7.org/linux/man-pages/man2/clone.2.html
---
 gdb/linux-tdep.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 82e8bc3db3c..2c91e298d45 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -840,7 +840,14 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
       if (current_inferior ()->fake_pid_p)
 	error (_("Can't determine the current process's PID: you must name one."));
 
-      pid = current_inferior ()->pid;
+      /* Seeing as, when the main thread exits, the information in /proc/$pid
+       * becomes unreliable, we should prefer using the current TID, whenever
+       * possible. */
+      pid = inferior_ptid.lwp ();
+
+      /* And fall back to the actual PID only when the TID is not available. */
+      if (pid == 0)
+	pid = current_inferior ()->pid;
     }
 
   args = skip_spaces (args);
-- 
2.40.1