technical/api-lockfile.txt

lockfile API
============

The lockfile API serves two purposes:

* Mutual exclusion. When we write out a new index file, first
 we create a new file `$GIT_DIR/index.lock`, write the new
 contents into it, and rename it to the final destination
 `$GIT_DIR/index`. We try to create the `$GIT_DIR/index.lock`
 file with O_EXCL so that we can notice and fail when somebody
 else is already trying to update the index file.

* Automatic cruft removal. After we create the "lock" file, we
 may decide to `die()`, and we would want to make sure that we
 remove the file that has not been committed to its final
 destination. This is done by remembering the lockfiles we
 created in a linked list and cleaning them up from an
 `atexit(3)` handler. Outstanding lockfiles are also removed
 when the program dies on a signal.


The functions
-------------

hold_lock_file_for_update::

Take a pointer to `struct lock_file`, the filename of
the final destination (e.g. `$GIT_DIR/index`) and a flag
`die_on_error`. Attempt to create a lockfile for the
destination and return the file descriptor for writing
to the file. If `die_on_error` flag is true, it dies if
a lock is already taken for the file; otherwise it
returns a negative integer to the caller on failure.

commit_lock_file::

Take a pointer to the `struct lock_file` initialized
with an earlier call to `hold_lock_file_for_update()`,
close the file descriptor and rename the lockfile to its
final destination. Returns 0 upon success, a negative
value on failure to close(2) or rename(2).

rollback_lock_file::

Take a pointer to the `struct lock_file` initialized
with an earlier call to `hold_lock_file_for_update()`,
close the file descriptor and remove the lockfile.

close_lock_file::
Take a pointer to the `struct lock_file` initialized
with an earlier call to `hold_lock_file_for_update()`,
and close the file descriptor. Returns 0 upon success,
a negative value on failure to close(2).

Because the structure is used in an `atexit(3)` handler, its
storage has to stay throughout the life of the program. It
cannot be an auto variable allocated on the stack.

Call `commit_lock_file()` or `rollback_lock_file()` when you are
done writing to the file descriptor. If you do not call either
and simply `exit(3)` from the program, an `atexit(3)` handler
will close and remove the lockfile.

If you need to close the file descriptor you obtained from
`hold_lock_file_for_update` function yourself, do so by calling
`close_lock_file()`. You should never call `close(2)` yourself!
Otherwise the `struct
lock_file` structure still remembers that the file descriptor
needs to be closed, and a later call to `commit_lock_file()` or
`rollback_lock_file()` will result in duplicate calls to
`close(2)`. Worse yet, if you `close(2)`, open another file
descriptor for completely different purpose, and then call
`commit_lock_file()` or `rollback_lock_file()`, they may close
that unrelated file descriptor.