Re: [PATCH v1 2/2] gdb: rely on the first non-exited thread TPID when reading Linux procfs files

[Simon Marchi <simark@simark.ca>]

On 7/28/26 10:33 AM, Matthieu Longo wrote:
> diff --git a/gdb/inferior.h b/gdb/inferior.h
> index 5c7a52319e7..91dc4e636fd 100644
> --- a/gdb/inferior.h
> +++ b/gdb/inferior.h
> @@ -513,6 +513,17 @@ class inferior : public refcounted_object,
>    /* Find (non-exited) thread PTID of this inferior.  */
>    thread_info *find_thread (ptid_t ptid);
>  
> +  /* Return the first non-exited thread of this inferior.
> +
> +     This is only a best-effort choice of a thread that is expected to still
> +     exist in the target.  A thread may have exited after GDB last updated its
> +     thread list, or GDB/gdbserver may have observed the exit but not yet
> +     propagated it through all layers.  Therefore the returned thread is not
> +     guaranteed to still be alive when it is later accessed.  This avoids the
> +     common case where the current thread has exited, but callers must still be
> +     prepared for the selected thread to no longer exist.  */
> +  ptid_t first_non_exited_thread () const;

I think it would be more consistent with the other "find thread" methods
to return the `thread_info *`

> diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
> index 8c53ffd5e89..9bdcc55a0e1 100644
> --- a/gdb/linux-tdep.c
> +++ b/gdb/linux-tdep.c
> @@ -457,6 +457,44 @@ linux_has_shared_address_space (struct gdbarch *gdbarch)
>    return linux_is_uclinux ();
>  }
>  
> +/* Return a PTID that identifies the current process and can be used to
> +   access procfs safely.
> +
> +   The returned PTID is that of the thread-group leader whenever it is
> +   still alive.  If the leader has already exited, the PTID of the first
> +   non-exited thread in the current inferior is returned instead.
> +   This ensures that the returned PTID always refers to a live thread
> +   whose procfs entries are present and populated.  */
> +static ptid_t
> +get_process_reference_ptid (bool verbose = false)
> +{
> +  /* Get the current thread.  */
> +  thread_info *thr = inferior_thread ();
> +
> +  /* Construct the PTID of the thread-group leader.  On Linux,
> +     the leader's LWP ID is equal to the process ID.  */
> +  ptid_t leader_ptid (thr->ptid.pid (), thr->ptid.pid ());
> +
> +  /* Use the thread-group leader if it is still alive.  Otherwise, use
> +     the first thread that has not exited.  */
> +  thread_info *leader_thr
> +    = current_inferior ()->find_thread (leader_ptid);
> +  ptid_t ptid = (leader_thr == nullptr
> +		 ? current_inferior ()->first_non_exited_thread ()
> +		 : leader_ptid);
> +
> +  if (!verbose)
> +    return ptid;
> +
> +  if (leader_thr != nullptr)
> +    gdb_printf (_("process %d\n"), leader_ptid.pid ());
> +  else
> +    gdb_printf (_("process %d [Note: information where gathered from LWP %ld " \
> +		  "as the thread-group leader (LWP=%ld) already exited.]\n"),
> +		ptid.pid (), ptid.lwp (), leader_ptid.lwp ());

I don't think the printfs should be part of this getter function.  Move
it to a separate function called by whoever needs it.  Or just inline
it, it looks like there is just a single caller that needs it
(linux_info_proc).

And really (IIUC), this function only really needs to return a tid (the
id of one LWP), not a ptid.

Simon