Avoid file time stamp bug on MS-Windows.

* fileio.c (Fwrite_region): Don't use the heuristic on empty files,
as FAT32 doesn't update time stamps when truncating them.
Also, check that a file time stamp is not a multiple of 100 ns;
this should catch all instances of the problem on MS-Windows,
as its native file system resolution is 100 ns or worse, and
checking for a non-multiple of 100 ns should impose only a small
overhead on systems with ns resolution.

Fixes: debbugs:13149

1 files changed, 15 insertions, 4 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 99c5f7a5837..e76f2bc2420 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -5020,11 +5020,22 @@ This calls `write-region-annotate-functions' at the start, and
           if (fstat (desc1, &st1) == 0
               && st.st_dev == st1.st_dev && st.st_ino == st1.st_ino)
             {
+              /* Use the heuristic if it appears to be valid.  With neither
+                 O_EXCL nor O_TRUNC, if Emacs happened to write nothing to the
+                 file, the time stamp won't change.  Also, some non-POSIX
+                 systems don't update an empty file's time stamp when
+                 truncating it.  Finally, file systems with 100 ns or worse
+                 resolution sometimes seem to have bugs: on a system with ns
+                 resolution, checking ns % 100 incorrectly avoids the heuristic
+                 1% of the time, but the problem should be temporary as we will
+                 try again on the next time stamp.  */
+              bool use_heuristic
+                = ((open_flags & (O_EXCL | O_TRUNC)) != 0
+                   && st.st_size != 0
+                   && EMACS_NSECS (modtime) % 100 != 0);
+
               EMACS_TIME modtime1 = get_stat_mtime (&st1);
-              /* 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
+              if (use_heuristic
                   && EMACS_TIME_EQ (modtime, modtime1)
                   && st.st_size == st1.st_size)
                 {