lockfile API
============
-Talk about <lockfile.c>, things like:
+The lockfile API serves two purposes:
-* lockfile lifetime -- atexit(3) looks at them, do not put them on the
- stack;
-* hold_lock_file_for_update()
-* commit_lock_file()
-* rollback_rock_file()
+* 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 `$GIT_DIR/index.lock`
+ file with O_EXCL so that we can notice and fail when somebody
+ else is already trying to update the index file.
-(JC, Dscho, Shawn)
+* Automatic cruft removal. After we create the "lock" file, we
+ may decide to `die()`, and we would want to make sure that we
+ remove the file that has not been committed to its final
+ destination. This is done by remembering the lockfiles we
+ created in a linked list and cleaning them up from an
+ `atexit(3)` handler. Outstanding lockfiles are also removed
+ when the program dies on a signal.
+
+
+The functions
+-------------
+
+hold_lock_file_for_update::
+
+ Take a pointer to `struct lock_file`, the filename of
+ the final destination (e.g. `$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.