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.

3 files changed, 17 insertions, 9 deletions

diff --git a/src/buffer.c b/src/buffer.c
index cea1ddb5ab3..babfba3e65f 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2033,9 +2033,6 @@ DEFUN ("current-buffer", Fcurrent_buffer, Scurrent_buffer, 0, 0, 0,
 void
 set_buffer_internal_1 (register struct buffer *b)
 {
-  register struct buffer *old_buf;
-  register Lisp_Object tail;
-
 #ifdef USE_MMAP_FOR_BUFFERS
   if (b->text->beg == NULL)
     enlarge_buffer_text (b, 0);
@@ -2044,6 +2041,17 @@ set_buffer_internal_1 (register struct buffer *b)
   if (current_buffer == b)
     return;
 
+  set_buffer_internal_2 (b);
+}
+
+/* Like set_buffer_internal_1, but doesn't check whether B is already
+   the current buffer.  Called upon switch of the current thread, see
+   post_acquire_global_lock.  */
+void set_buffer_internal_2 (register struct buffer *b)
+{
+  register struct buffer *old_buf;
+  register Lisp_Object tail;
+
   BUFFER_CHECK_INDIRECTION (b);
 
   old_buf = current_buffer;
diff --git a/src/buffer.h b/src/buffer.h
index 21ad5e3bc0f..854b5b5dd32 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -1082,6 +1082,7 @@ extern void recenter_overlay_lists (struct buffer *, ptrdiff_t);
 extern ptrdiff_t overlay_strings (ptrdiff_t, struct window *, unsigned char **);
 extern void validate_region (Lisp_Object *, Lisp_Object *);
 extern void set_buffer_internal_1 (struct buffer *);
+extern void set_buffer_internal_2 (struct buffer *);
 extern void set_buffer_temp (struct buffer *);
 extern Lisp_Object buffer_local_value (Lisp_Object, Lisp_Object);
 extern void record_buffer (Lisp_Object);
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))
     {