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-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.