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Diffstat (limited to 'Documentation/technical/api-lockfile.txt')
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diff --git a/Documentation/technical/api-lockfile.txt b/Documentation/technical/api-lockfile.txt new file mode 100644 index 0000000..dd89404 --- /dev/null +++ b/Documentation/technical/api-lockfile.txt @@ -0,0 +1,74 @@ +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. |