Fix timestamp bug when write-region appends nothing.

* fileio.c (Fwrite_region): When neither O_EXCL nor O_TRUNC is used,
the file's time stamp doesn't change if Emacs happens to write nothing
to the file, and on a buggy file system this could cause Emacs to
incorrectly infer that the file system doesn't have the bug.
Avoid this problem by inhibiting the inference in this case.

Fixes: debbugs:13149

2 files changed, 14 insertions, 1 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 9682da24235..b4c3195973c 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,12 @@
+2013-02-01  Paul Eggert  <eggert@cs.ucla.edu>
+
+        Fix timestamp bug when write-region appends nothing (Bug#13149).
+        * fileio.c (Fwrite_region): When neither O_EXCL nor O_TRUNC is used,
+        the file's time stamp doesn't change if Emacs happens to write nothing
+        to the file, and on a buggy file system this could cause Emacs to
+        incorrectly infer that the file system doesn't have the bug.
+        Avoid this problem by inhibiting the inference in this case.
+
 2013-02-01  Dmitry Antipov  <dmantipov@yandex.ru>
 
         * window.h (struct window): Convert base_line_number, base_line_pos
diff --git a/src/fileio.c b/src/fileio.c
index a8218fcd797..fb6ecfedeb4 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -5036,7 +5036,11 @@ This calls `write-region-annotate-functions' at the start, and
               && st.st_dev == st1.st_dev && st.st_ino == st1.st_ino)
             {
               EMACS_TIME modtime1 = get_stat_mtime (&st1);
-              if (EMACS_TIME_EQ (modtime, modtime1)
+              /* If neither O_EXCL nor O_TRUNC is used, and Emacs happened to
+                 write nothing to the file, the file's time stamp won't change
+                 so it should not be used in this heuristic.  */
+              if ((open_flags & (O_EXCL | O_TRUNC)) != 0
+                  && EMACS_TIME_EQ (modtime, modtime1)
                   && st.st_size == st1.st_size)
                 {
                   timestamp_file_system = st.st_dev;