Add a lockfile function to append to a file

This takes care of copying the original contents into the replacement
file after the lock is held, so that concurrent additions can't miss
each other's changes.

[jc: munged to drop mmap in favor of copy_file.]
Signed-off-by: NDaniel Barkalow <barkalow@iabervon.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -391,6 +391,7 @@ struct lock_file {
 	char filename[PATH_MAX];
 };
 extern int hold_lock_file_for_update(struct lock_file *, const char *path, int);
+extern int hold_lock_file_for_append(struct lock_file *, const char *path, int);
 extern int commit_lock_file(struct lock_file *);
 
 extern int hold_locked_index(struct lock_file *, int);

lockfile.c

@@ -160,6 +160,34 @@ int hold_lock_file_for_update(struct lock_file *lk, const char *path, int die_on
 	return fd;
 }
 
+int hold_lock_file_for_append(struct lock_file *lk, const char *path, int die_on_error)
+{
+	int fd, orig_fd;
+
+	fd = lock_file(lk, path);
+	if (fd < 0) {
+		if (die_on_error)
+			die("unable to create '%s.lock': %s", path, strerror(errno));
+		return fd;
+	}
+
+	orig_fd = open(path, O_RDONLY);
+	if (orig_fd < 0) {
+		if (errno != ENOENT) {
+			if (die_on_error)
+				die("cannot open '%s' for copying", path);
+			close(fd);
+			return error("cannot open '%s' for copying", path);
+		}
+	} else if (copy_fd(orig_fd, fd)) {
+		if (die_on_error)
+			exit(128);
+		close(fd);
+		return -1;
+	}
+	return fd;
+}
+
 int close_lock_file(struct lock_file *lk)
 {
 	int fd = lk->fd;