Fix point motion in cloned buffers

* src/thread.c (post_acquire_global_lock): Call
set_buffer_internal_2 instead of tricking set_buffer_internal_1
into resetting the current buffer even if it didn't change.  This
avoids bug#25165, caused by failing to record the modified values
of point and mark, because current_buffer was set to NULL.  Also,
don't bother re-setting the buffer if there was no thread switch,
as that just wastes cycles.
* src/buffer.c (set_buffer_internal_2): New function, with most of
the body of set_buffer_internal_1, but without the test for B
being identical to the current buffer.
(set_buffer_internal_1): Call set_buffer_internal_2 if B is not
identical to the current buffer.
* src/buffer.h (set_buffer_internal_2): Add prototype.

* test/src/thread-tests.el (thread-sticky-point): New test.

1 files changed, 5 insertions, 6 deletions

diff --git a/src/thread.c b/src/thread.c
index 3e61723f0ab..6e9ca2e256b 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -55,7 +55,6 @@ release_global_lock (void)
 static void
 post_acquire_global_lock (struct thread_state *self)
 {
-  Lisp_Object buffer;
   struct thread_state *prev_thread = current_thread;
 
   /* Do this early on, so that code below could signal errors (e.g.,
@@ -71,12 +70,12 @@ post_acquire_global_lock (struct thread_state *self)
       if (prev_thread != NULL)
         unbind_for_thread_switch (prev_thread);
       rebind_for_thread_switch ();
-    }
 
-  /* We need special handling to re-set the buffer.  */
-  XSETBUFFER (buffer, self->m_current_buffer);
-  self->m_current_buffer = 0;
-  set_buffer_internal (XBUFFER (buffer));
+       /* Set the new thread's current buffer.  This needs to be done
+          even if it is the same buffer as that of the previous thread,
+          because of thread-local bindings.  */
+      set_buffer_internal_2 (current_buffer);
+    }
 
   if (!NILP (current_thread->error_symbol))
     {