# 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.