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.
+* Mutual exclusion and atomic file updates. When we want to change a
+ file, we create a lockfile `<filename>.lock`, write the new file
+ contents into it, and then rename the lockfile to its final
+ destination `<filename>`. We create the `<filename>.lock` file with
+ `O_CREAT|O_EXCL` so that we can notice and fail if somebody else has
+ already locked the file, then atomically rename the lockfile to its
+ final destination to commit the changes and unlock the file.
+
+* Automatic cruft removal. If the program exits after we lock a file
+ but before the changes have been committed, we want to make sure
+ that we remove the lockfile. This is done by remembering the
+ lockfiles we have created in a linked list and setting up an
+ `atexit(3)` handler and a signal handler that clean up the
+ lockfiles. This mechanism ensures that outstanding lockfiles are
+ cleaned up if the program exits (including when `die()` is called)
+ or if the program dies on a signal.
+
+Please note that lockfiles only block other writers. Readers do not
+block, but they are guaranteed to see either the old contents of the
+file or the new contents of the file (assuming that the filesystem
+implements `rename(2)` atomically).
+
+
+Calling sequence
+----------------
+
+The caller:
+
+* Allocates a `struct lock_file` either as a static variable or on the
+ heap, initialized to zeros. Once you use the structure to call the
+ `hold_lock_file_*` family of functions, it belongs to the lockfile
+ subsystem and its storage must remain valid throughout the life of
+ the program (i.e. you cannot use an on-stack variable to hold this
+ structure).
+
+* Attempts to create a lockfile by passing that variable and the path
+ of the final destination (e.g. `$GIT_DIR/index`) to
+ `hold_lock_file_for_update` or `hold_lock_file_for_append`.
+
+* Writes new content for the destination file by either:
+
+ * writing to the file descriptor returned by the `hold_lock_file_*`
+ functions (also available via `lock->fd`).
+
+ * calling `fdopen_lock_file` to get a `FILE` pointer for the open
+ file and writing to the file using stdio.
+
+When finished writing, the caller can:
+
+* Close the file descriptor and rename the lockfile to its final
+ 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`,
+ `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`,
+`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)` or `fclose(3)`
+yourself! Otherwise the `struct lock_file` structure would still think
+that the file descriptor needs to be closed, and a commit or rollback
+would result in duplicate calls to `close(2)`. Worse yet, if you close
+and then later open another file descriptor for a completely different
+purpose, then a commit or rollback might close that unrelated file
+descriptor.
+
+
+Error handling
+--------------
+
+The `hold_lock_file_*` functions return a file descriptor on success
+or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see below). On
+errors, `errno` describes the reason for failure. Errors can be
+reported by passing `errno` to one of the following helper functions:
+
+unable_to_lock_message::
+
+ Append an appropriate error message to a `strbuf`.
+
+unable_to_lock_error::
+
+ Emit an appropriate error message using `error()`.
+
+unable_to_lock_die::
+
+ Emit an appropriate error message and `die()`.
+
+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
+-----
+
+The following flags can be passed to `hold_lock_file_for_update` or
+`hold_lock_file_for_append`:
+
+LOCK_NO_DEREF::
+
+ Usually symbolic links in the destination path are resolved
+ and the lockfile is created by adding ".lock" to the resolved
+ path. If `LOCK_NO_DEREF` is set, then the lockfile is created
+ by adding ".lock" to the path argument itself. This option is
+ used, for example, when locking a symbolic reference, which
+ for backwards-compatibility reasons can be a symbolic link
+ containing the name of the referred-to-reference.
+
+LOCK_DIE_ON_ERROR::
+
+ If a lock is already taken for the file, `die()` with an error
+ message. If this option is not specified, trying to lock a
+ file that is already locked returns -1 to the caller.
The functions
hold_lock_file_for_update::
- Take a pointer to `struct lock_file`, the filename of
- the final destination (e.g. `$GIT_DIR/index`) and a flag
- `die_on_error`. Attempt to create a lockfile for the
- destination and return the file descriptor for writing
- to the file. If `die_on_error` flag is true, it dies if
- a lock is already taken for the file; otherwise it
- returns a negative integer to the caller on failure.
+ Take a pointer to `struct lock_file`, the path of the file to
+ be locked (e.g. `$GIT_DIR/index`) and a flags argument (see
+ above). Attempt to create a lockfile for the destination and
+ return the file descriptor for writing to the file.
+
+hold_lock_file_for_append::
+
+ Like `hold_lock_file_for_update`, but before returning copy
+ the existing contents of the file (if any) to the lockfile and
+ position its write pointer at the end of the file.
+
+fdopen_lock_file::
+
+ Associate a stdio stream with the lockfile. Return NULL
+ (*without* rolling back the lockfile) on error. The stream is
+ closed automatically when `close_lock_file` is called or when
+ the file is committed or rolled back.
+
+get_locked_file_path::
+
+ Return the path of the file that is locked by the specified
+ lock_file object. The caller must free the memory.
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).
+ Take a pointer to the `struct lock_file` initialized with an
+ earlier call to `hold_lock_file_for_update` or
+ `hold_lock_file_for_append`, close the file descriptor, and
+ rename the lockfile to its final destination. Return 0 upon
+ success. On failure, roll back the lock file and return -1,
+ with `errno` set to the value from the failing call to
+ `close(2)` or `rename(2)`. It is a bug to call
+ `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 earlier call to `hold_lock_file_for_update()`,
- close the file descriptor and remove the lockfile.
+ Take a pointer to the `struct lock_file` initialized with an
+ earlier call to `hold_lock_file_for_update` or
+ `hold_lock_file_for_append`, close the file descriptor and
+ remove the lockfile. It is a NOOP to call
+ `rollback_lock_file()` for a `lock_file` object that has
+ already been committed or rolled back.
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.
+
+ Take a pointer to the `struct lock_file` initialized with an
+ earlier call to `hold_lock_file_for_update` or
+ `hold_lock_file_for_append`. Close the file descriptor (and
+ the file pointer if it has been opened using
+ `fdopen_lock_file`). Return 0 upon success. On failure to
+ `close(2)`, return a negative value and roll back the lock
+ file. Usually `commit_lock_file`, `commit_lock_file_to`, or
+ `rollback_lock_file` should eventually be called if
+ `close_lock_file` succeeds.
+
+reopen_lock_file::
+
+ Re-open a lockfile that has been closed (using
+ `close_lock_file`) but not yet committed or rolled back. This
+ can be used to implement a sequence of operations like the
+ following:
+
+ * Lock file.
+
+ * Write new contents to lockfile, then `close_lock_file` to
+ cause the contents to be written to disk.
+
+ * Pass the name of the lockfile to another program to allow it
+ (and nobody else) to inspect the contents you wrote, while
+ still holding the lock yourself.
+
+ * `reopen_lock_file` to reopen the lockfile. Make further
+ updates to the contents.
+
+ * `commit_lock_file` to make the final version permanent.