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The lockfile API is a handy way to obtain a file that is cleaned
up if you die(). But sometimes you would need this sequence to
work:
1. hold_lock_file_for_update() to get a file descriptor for
writing;
2. write the contents out, without being able to decide if the
results should be committed or rolled back;
3. do something else that makes the decision --- and this
"something else" needs the lockfile not to have an open file
descriptor for writing (e.g. Windows do not want a open file
to be renamed);
4. call commit_lock_file() or rollback_lock_file() as
appropriately.
This adds close_lock_file() you can call between step 2 and 3 in
the above sequence.
Signed-off-by: Junio C Hamano <gitster@pobox.com>
74 lines
2.9 KiB
Text
74 lines
2.9 KiB
Text
lockfile API
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============
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The lockfile API serves two purposes:
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* Mutual exclusion. When we write out a new index file, first
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we create a new file `$GIT_DIR/index.lock`, write the new
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contents into it, and rename it to the final destination
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`$GIT_DIR/index`. We try to create the `$GIT_DIR/index.lock`
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file with O_EXCL so that we can notice and fail when somebody
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else is already trying to update the index file.
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* Automatic cruft removal. After we create the "lock" file, we
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may decide to `die()`, and we would want to make sure that we
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remove the file that has not been committed to its final
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destination. This is done by remembering the lockfiles we
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created in a linked list and cleaning them up from an
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`atexit(3)` handler. Outstanding lockfiles are also removed
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when the program dies on a signal.
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The functions
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-------------
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hold_lock_file_for_update::
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Take a pointer to `struct lock_file`, the filename of
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the final destination (e.g. `$GIT_DIR/index`) and a flag
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`die_on_error`. Attempt to create a lockfile for the
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destination and return the file descriptor for writing
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to the file. If `die_on_error` flag is true, it dies if
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a lock is already taken for the file; otherwise it
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returns a negative integer to the caller on failure.
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commit_lock_file::
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Take a pointer to the `struct lock_file` initialized
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with an earlier call to `hold_lock_file_for_update()`,
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close the file descriptor and rename the lockfile to its
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final destination. Returns 0 upon success, a negative
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value on failure to close(2) or rename(2).
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rollback_lock_file::
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Take a pointer to the `struct lock_file` initialized
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with an earlier call to `hold_lock_file_for_update()`,
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close the file descriptor and remove the lockfile.
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close_lock_file::
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Take a pointer to the `struct lock_file` initialized
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with an earlier call to `hold_lock_file_for_update()`,
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and close the file descriptor. Returns 0 upon success,
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a negative value on failure to close(2).
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Because the structure is used in an `atexit(3)` handler, its
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storage has to stay throughout the life of the program. It
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cannot be an auto variable allocated on the stack.
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Call `commit_lock_file()` or `rollback_lock_file()` when you are
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done writing to the file descriptor. If you do not call either
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and simply `exit(3)` from the program, an `atexit(3)` handler
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will close and remove the lockfile.
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If you need to close the file descriptor you obtained from
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`hold_lock_file_for_update` function yourself, do so by calling
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`close_lock_file()`. You should never call `close(2)` yourself!
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Otherwise the `struct
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lock_file` structure still remembers that the file descriptor
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needs to be closed, and a later call to `commit_lock_file()` or
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`rollback_lock_file()` will result in duplicate calls to
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`close(2)`. Worse yet, if you `close(2)`, open another file
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descriptor for completely different purpose, and then call
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`commit_lock_file()` or `rollback_lock_file()`, they may close
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that unrelated file descriptor.
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