We have nice set of placeholders, but nobody stepped in to fill the gap in the API documentation, so I am doing it myself. Signed-off-by: Junio C Hamano <gitster@xxxxxxxxx> --- Documentation/technical/api-lockfile.txt | 67 ++++++++++++++++++++++++++--- 1 files changed, 60 insertions(+), 7 deletions(-) diff --git a/Documentation/technical/api-lockfile.txt b/Documentation/technical/api-lockfile.txt index 73ac102..5b1553e 100644 --- a/Documentation/technical/api-lockfile.txt +++ b/Documentation/technical/api-lockfile.txt @@ -1,12 +1,65 @@ 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. + +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. + +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. + +You should not close the file descriptor you obtained from +`hold_lock_file_for_update` function yourself. 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. -- 1.5.4.rc3.14.g44397 - To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
