From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sat, 2 Feb 2013 19:18:00 +0000 (-0800)
Subject: Avoid file time stamp bug on MS-Windows.
X-Git-Tag: emacs-24.3.90~173^2~7^2~159
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=8ea41ea99608c7483ef5c91e230a760cf90bfb54;p=emacs.git

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 b90f18eba91..092302d6492 100644
--- 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 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)
 		{