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, 11 insertions, 3 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;