Add config option to enable 'fsync()' of object files

As explained in the documentation[*] this is totally useless on
filesystems that do ordered/journalled data writes, but it can be a
useful safety feature on filesystems like HFS+ that only journal the
metadata, not the actual file contents.

It defaults to off, although we could presumably in theory some day
auto-enable it on a per-filesystem basis.

[*] Yes, I updated the docs for the thing.  Hell really _has_ frozen
    over, and the four horsemen are probably just beyond the horizon.
    EVERYBODY PANIC!

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -372,6 +372,14 @@ core.whitespace::
   does not trigger if the character before such a carriage-return
   is not a whitespace (not enabled by default).
 
+core.fsyncobjectfiles::
+	This boolean will enable 'fsync()' when writing object files.
++
+This is a total waste of time and effort on a filesystem that orders
+data writes properly, but can be useful for filesystems that do not use
+journalling (traditional UNIX filesystems) or that only journal metadata
+and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback").
+
 alias.*::
 	Command aliases for the linkgit:git[1] command wrapper - e.g.
 	after defining "alias.last = cat-file commit HEAD", the invocation

cache.h

@@ -435,6 +435,7 @@ extern size_t packed_git_window_size;
 extern size_t packed_git_limit;
 extern size_t delta_base_cache_limit;
 extern int auto_crlf;
+extern int fsync_object_files;
 
 enum safe_crlf {
 	SAFE_CRLF_FALSE = 0,

config.c

@@ -460,6 +460,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.fsyncobjectfiles")) {
+		fsync_object_files = git_config_bool(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }

environment.c

@@ -29,6 +29,7 @@ const char *apply_default_whitespace;
 int zlib_compression_level = Z_BEST_SPEED;
 int core_compression_level;
 int core_compression_seen;
+int fsync_object_files;
 size_t packed_git_window_size = DEFAULT_PACKED_GIT_WINDOW_SIZE;
 size_t packed_git_limit = DEFAULT_PACKED_GIT_LIMIT;
 size_t delta_base_cache_limit = 16 * 1024 * 1024;

sha1_file.c

@@ -2083,7 +2083,8 @@ int hash_sha1_file(const void *buf, unsigned long len, const char *type,
 /* Finalize a file on disk, and close it. */
 static void close_sha1_file(int fd)
 {
-	/* For safe-mode, we could fsync_or_die(fd, "sha1 file") here */
+	if (fsync_object_files)
+		fsync_or_die(fd, "sha1 file");
 	fchmod(fd, 0444);
 	if (close(fd) != 0)
 		die("unable to write sha1 file");