Subject: Re: bug#79367: 31.0.50; magit-commit sometimes doesn't work if diff-hl-update-async is t

From: Eli Zaretskii <[email-masked]>
Date: Wed, 03 Sep 2025 19:42:47 +0300

> From: Spencer Baugh <[email-masked]>
> Cc: [email-masked],  [email-masked],  [email-masked]
> Date: Wed, 03 Sep 2025 12:03:42 -0400
> 
> Eli Zaretskii <[email-masked]> writes:
> 
> >> >> But I think we need to call set_proc_thread when the server process
> >> >> is not locked as well.  Otherwise the `thread' member of the
> >> >> fd_callback_info may still be a dangling pointer.
> >> >
> >> > Oh, you mean the below?  I agree, it is safer.
> >> 
> >> We should not do that - if the thread member of fd_callback_info is a
> >> dangling pointer, that indicates there's a bug elsewhere.
> >
> > So how about adding an assertion before we clear it?
> 
> Sure.  fd_callback_info.thread specifically should always be NULL at the
> time we're using the fd_callback_info slot for some new fd, so we should
> assert that.

So, this:

diff --git a/src/process.c b/src/process.c
index d6efac5..8f5be5e 100644
--- a/src/process.c
+++ b/src/process.c
@@ -5078,6 +5078,10 @@ server_accept_connection (Lisp_Object server, int channel)
   fcntl (s, F_SETFL, O_NONBLOCK);
 
   p = XPROCESS (proc);
+  /* make_process calls pset_thread, but if the server process is not
+     locked to any thread, we need to undo what make_process did.  */
+  if (NILP (ps->thread))
+    pset_thread (p, Qnil);
 
   /* Build new contact information for this setup.  */
   contact = Fcopy_sequence (ps->childp);
@@ -5117,6 +5121,19 @@ server_accept_connection (Lisp_Object server, int channel)
     add_process_read_fd (s);
   if (s > max_desc)
     max_desc = s;
+  /* If the server process is locked to this thread, lock the client
+     process to the same thread, otherwise clear the thread of its I/O
+     descriptors.  */
+  if (NILP (ps->thread))
+    {
+      eassert (!fd_callback_info[p->infd].thread);
+      set_proc_thread (p, NULL);
+    }
+  else
+    {
+      eassert (XTHREAD (ps->thread) == current_thread);
+      set_proc_thread (p, XTHREAD (ps->thread));
+    }
 
   /* Setup coding system for new process based on server process.
      This seems to be the proper thing to do, as the coding system