commit_lock_file_to(): refactor a helper out of commit_lock_file()

commit_locked_index(), when writing to an alternate index file,
duplicates (poorly) the code in commit_lock_file(). And anyway, it
shouldn't have to know so much about the internal workings of lockfile
objects. So extract a new function commit_lock_file_to() that does the
work common to the two functions, and call it from both
commit_lock_file() and commit_locked_index().

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-lockfile.txt

@@ -49,35 +49,34 @@ The caller:
 When finished writing, the caller can:
 
 * Close the file descriptor and rename the lockfile to its final
-  destination by calling `commit_lock_file`.
+  destination by calling `commit_lock_file` or `commit_lock_file_to`.
 
 * Close the file descriptor and remove the lockfile by calling
   `rollback_lock_file`.
 
 * Close the file descriptor without removing or renaming the lockfile
   by calling `close_lock_file`, and later call `commit_lock_file`,
-  `rollback_lock_file`, or `reopen_lock_file`.
+  `commit_lock_file_to`, `rollback_lock_file`, or `reopen_lock_file`.
 
 Even after the lockfile is committed or rolled back, the `lock_file`
 object must not be freed or altered by the caller. However, it may be
 reused; just pass it to another call of `hold_lock_file_for_update` or
 `hold_lock_file_for_append`.
 
 If the program exits before you have called one of `commit_lock_file`,
-`rollback_lock_file`, or `close_lock_file`, an `atexit(3)` handler
-will close and remove the lockfile, rolling back any uncommitted
-changes.
+`commit_lock_file_to`, `rollback_lock_file`, or `close_lock_file`, an
+`atexit(3)` handler will close and remove the lockfile, rolling back
+any uncommitted changes.
 
 If you need to close the file descriptor you obtained from a
 `hold_lock_file_*` function yourself, do so by calling
 `close_lock_file`. You should never call `close(2)` yourself!
 Otherwise the `struct lock_file` structure would still think that the
-file descriptor needs to be closed, and a later call to
-`commit_lock_file` or `rollback_lock_file` or program exit would
+file descriptor needs to be closed, and a commit or rollback would
 result in duplicate calls to `close(2)`. Worse yet, if you `close(2)`
 and then later open another file descriptor for a completely different
-purpose, then a call to `commit_lock_file` or `rollback_lock_file`
-might close that unrelated file descriptor.
+purpose, then a commit or rollback might close that unrelated file
+descriptor.
 
 
 Error handling
@@ -100,9 +99,9 @@ unable_to_lock_die::
 
 	Emit an appropriate error message and `die()`.
 
-Similarly, `commit_lock_file` and `close_lock_file` return 0 on
-success. On failure they set `errno` appropriately, do their best to
-roll back the lockfile, and return -1.
+Similarly, `commit_lock_file`, `commit_lock_file_to`, and
+`close_lock_file` return 0 on success. On failure they set `errno`
+appropriately, do their best to roll back the lockfile, and return -1.
 
 
 Flags
@@ -156,6 +155,12 @@ commit_lock_file::
 	`commit_lock_file` for a `lock_file` object that is not
 	currently locked.
 
+commit_lock_file_to::
+
+	Like `commit_lock_file()`, except that it takes an explicit
+	`path` argument to which the lockfile should be renamed. The
+	`path` must be on the same filesystem as the lock file.
+
 rollback_lock_file::
 
 	Take a pointer to the `struct lock_file` initialized with an
@@ -172,8 +177,9 @@ close_lock_file::
 	`hold_lock_file_for_append`, and close the file descriptor.
 	Return 0 upon success. On failure to `close(2)`, return a
 	negative value and roll back the lock file. Usually
-	`commit_lock_file` or `rollback_lock_file` should eventually
-	be called if `close_lock_file` succeeds.
+	`commit_lock_file`, `commit_lock_file_to`, or
+	`rollback_lock_file` should eventually be called if
+	`close_lock_file` succeeds.
 
 reopen_lock_file::
 

cache.h

@@ -590,6 +590,7 @@ extern void unable_to_lock_message(const char *path, int err,
 extern NORETURN void unable_to_lock_die(const char *path, int err);
 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_to(struct lock_file *, const char *path);
 extern int commit_lock_file(struct lock_file *);
 extern int reopen_lock_file(struct lock_file *);
 extern void update_index_if_able(struct index_state *, struct lock_file *);

lockfile.c

@@ -43,9 +43,9 @@
  *   Same as the previous state, except that the lockfile is closed
  *   and fd is -1.
  *
- * - Unlocked (after commit_lock_file(), rollback_lock_file(), a
- *   failed attempt to lock, or a failed close_lock_file()).  In this
- *   state:
+ * - Unlocked (after commit_lock_file(), commit_lock_file_to(),
+ *   rollback_lock_file(), a failed attempt to lock, or a failed
+ *   close_lock_file()).  In this state:
  *   - active is unset
  *   - filename is empty (usually, though there are transitory
  *     states in which this condition doesn't hold). Client code should
@@ -284,23 +284,15 @@ int reopen_lock_file(struct lock_file *lk)
 	return lk->fd;
 }
 
-int commit_lock_file(struct lock_file *lk)
+int commit_lock_file_to(struct lock_file *lk, const char *path)
 {
-	static struct strbuf result_file = STRBUF_INIT;
-	int err;
-
 	if (!lk->active)
-		die("BUG: attempt to commit unlocked object");
+		die("BUG: attempt to commit unlocked object to \"%s\"", path);
 
 	if (close_lock_file(lk))
 		return -1;
 
-	/* remove ".lock": */
-	strbuf_add(&result_file, lk->filename.buf,
-		   lk->filename.len - LOCK_SUFFIX_LEN);
-	err = rename(lk->filename.buf, result_file.buf);
-	strbuf_reset(&result_file);
-	if (err) {
+	if (rename(lk->filename.buf, path)) {
 		int save_errno = errno;
 		rollback_lock_file(lk);
 		errno = save_errno;
@@ -312,6 +304,26 @@ int commit_lock_file(struct lock_file *lk)
 	return 0;
 }
 
+int commit_lock_file(struct lock_file *lk)
+{
+	static struct strbuf result_file = STRBUF_INIT;
+	int err;
+
+	if (!lk->active)
+		die("BUG: attempt to commit unlocked object");
+
+	if (lk->filename.len <= LOCK_SUFFIX_LEN ||
+	    strcmp(lk->filename.buf + lk->filename.len - LOCK_SUFFIX_LEN, LOCK_SUFFIX))
+		die("BUG: lockfile filename corrupt");
+
+	/* remove ".lock": */
+	strbuf_add(&result_file, lk->filename.buf,
+		   lk->filename.len - LOCK_SUFFIX_LEN);
+	err = commit_lock_file_to(lk, result_file.buf);
+	strbuf_reset(&result_file);
+	return err;
+}
+
 int hold_locked_index(struct lock_file *lk, int die_on_error)
 {
 	return hold_lock_file_for_update(lk, get_index_file(),

read-cache.c

@@ -2041,17 +2041,10 @@ void set_alternate_index_output(const char *name)
 
 static int commit_locked_index(struct lock_file *lk)
 {
-	if (alternate_index_output) {
-		if (close_lock_file(lk))
-			return -1;
-		if (rename(lk->filename.buf, alternate_index_output))
-			return -1;
-		lk->active = 0;
-		strbuf_reset(&lk->filename);
-		return 0;
-	} else {
+	if (alternate_index_output)
+		return commit_lock_file_to(lk, alternate_index_output);
+	else
 		return commit_lock_file(lk);
-	}
 }
 
 static int do_write_locked_index(struct index_state *istate, struct lock_file *lock,