[PATCH v2] gdb/linux-tdep: use thread lwp instead of pid for info proc

[Disconnect3d <dominik.b.czarnota@gmail.com>]

This commit fixes bug #31207 where GDB shows an empty result
of `info proc mappings` when we debug a non-main thread of a Linux
process while the main thread has exitted. In such a case, the
`/proc/<pid>/maps` file renders an empty file and `/proc/<lwp>/maps`
should be used instead.

It is worth noting that the `/proc/<lwp>/` directories are not
rendered by the Linux kernel when listing the procfs directory
but they can be opened and they contain the usual procfs entries,
just for the pid of the thread.

Signed-off-by: Disconnect3d <dominik.b.czarnota@gmail.com>
---
 gdb/linux-tdep.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 141c1199c8a..c141c9c6701 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -842,7 +842,11 @@ 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;
+      /* Use the thread's LWP ID instead of process ID when accessing procfs,
+         since the LWP-specific entries provide more useful information.
+         For example, /proc/<pid>/maps may be empty if the current thread has exited.
+      */
+      pid = inferior_thread ()->ptid.lwp ();
     }
 
   args = skip_spaces (args);
-- 
2.30.2