Windows: Skip fstat/lstat optimization in write_entry()

Commit e4c7292 (write_entry(): use fstat() instead of lstat() when file
is open, 2009-02-09) introduced an optimization of write_entry().
Unfortunately, we cannot take advantage of this optimization on Windows
because there is no guarantee that the time stamps are updated before the
file is closed:

  "The only guarantee about a file timestamp is that the file time is
   correctly reflected when the handle that makes the change is closed."

(http://msdn.microsoft.com/en-us/library/ms724290(VS.85).aspx)

The failure of this optimization on Windows can be observed most easily by
running a 'git checkout' that has to update several large files. In this
case, 'git checkout' will report modified files, but infact only the
timestamps were incorrectly recorded in the index, as can be verified by a
subsequent 'git diff', which shows no change.

Dmitry Potapov reports the same fix needs on Cygwin; this commit contains
his updates for that.

Signed-off-by: Johannes Sixt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: j6t

Makefile

@@ -167,6 +167,10 @@ all::
 # Define NO_EXTERNAL_GREP if you don't want "git grep" to ever call
 # your external grep (e.g., if your system lacks grep, if its grep is
 # broken, or spawning external process is slower than built-in grep git has).
+#
+# Define UNRELIABLE_FSTAT if your system's fstat does not return the same
+# information on a not yet closed file that lstat would return for the same
+# file after it was closed.
 
 GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -803,6 +807,7 @@ ifeq ($(uname_S),HP-UX)
 endif
 ifneq (,$(findstring CYGWIN,$(uname_S)))
 	COMPAT_OBJS += compat/cygwin.o
+	UNRELIABLE_FSTAT = UnfortunatelyYes
 endif
 ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_PREAD = YesPlease
@@ -829,6 +834,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_ST_BLOCKS_IN_STRUCT_STAT = YesPlease
 	NO_NSEC = YesPlease
 	USE_WIN32_MMAP = YesPlease
+	UNRELIABLE_FSTAT = UnfortunatelyYes
 	COMPAT_CFLAGS += -D__USE_MINGW_ACCESS -DNOGDI -Icompat -Icompat/regex -Icompat/fnmatch
 	COMPAT_CFLAGS += -DSNPRINTF_SIZE_CORR=1
 	COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"
@@ -1107,6 +1113,9 @@ endif
 ifdef NO_EXTERNAL_GREP
 	BASIC_CFLAGS += -DNO_EXTERNAL_GREP
 endif
+ifdef UNRELIABLE_FSTAT
+	BASIC_CFLAGS += -DUNRELIABLE_FSTAT
+endif
 
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK=NoThanks

entry.c

@@ -147,7 +147,8 @@ static int write_entry(struct cache_entry *ce, char *path, const struct checkout
 
 		wrote = write_in_full(fd, new, size);
 		/* use fstat() only when path == ce->name */
-		if (state->refresh_cache && !to_tempfile && !state->base_dir_len) {
+		if (fstat_is_reliable() &&
+		    state->refresh_cache && !to_tempfile && !state->base_dir_len) {
 			fstat(fd, &st);
 			fstat_done = 1;
 		}

git-compat-util.h

@@ -408,4 +408,10 @@ void git_qsort(void *base, size_t nmemb, size_t size,
 #endif
 #endif
 
+#ifdef UNRELIABLE_FSTAT
+#define fstat_is_reliable() 0
+#else
+#define fstat_is_reliable() 1
+#endif
+
 #endif