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

diff --git a/src/ChangeLog b/src/ChangeLog
index b90f18eba910757736258510876ef2b10be3a7a7..092302d6492c31b40b164a099c101acd72177a70 100644 (file)
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,14 @@
+2013-02-02  Paul Eggert  <eggert@cs.ucla.edu>
+
+       Avoid file time stamp bug on MS-Windows (Bug#13149).
+       * 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.
+
 2013-02-02  Eli Zaretskii  <eliz@gnu.org>
 
        Avoid encoding file names on MS-Windows when they need to be run

diff --git a/src/fileio.c b/src/fileio.c
index 99c5f7a5837230c0599052781ec22ccdecf3779e..e76f2bc2420068e8183c2003402b48fad8c3a545 100644 (file)
--- 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)
                {