1lockfile API 2============ 3 4The lockfile API serves two purposes: 5 6* Mutual exclusion. When we write out a new index file, first 7 we create a new file `$GIT_DIR/index.lock`, write the new 8 contents into it, and rename it to the final destination 9 `$GIT_DIR/index`. We try to create the `$GIT_DIR/index.lock` 10 file with O_EXCL so that we can notice and fail when somebody 11 else is already trying to update the index file. 12 13* Automatic cruft removal. After we create the "lock" file, we 14 may decide to `die()`, and we would want to make sure that we 15 remove the file that has not been committed to its final 16 destination. This is done by remembering the lockfiles we 17 created in a linked list and cleaning them up from an 18 `atexit(3)` handler. Outstanding lockfiles are also removed 19 when the program dies on a signal. 20 21 22The functions 23------------- 24 25hold_lock_file_for_update:: 26 27 Take a pointer to `struct lock_file`, the filename of 28 the final destination (e.g. `$GIT_DIR/index`) and a flag 29 `die_on_error`. Attempt to create a lockfile for the 30 destination and return the file descriptor for writing 31 to the file. If `die_on_error` flag is true, it dies if 32 a lock is already taken for the file; otherwise it 33 returns a negative integer to the caller on failure. 34 35commit_lock_file:: 36 37 Take a pointer to the `struct lock_file` initialized 38 with an earlier call to `hold_lock_file_for_update()`, 39 close the file descriptor and rename the lockfile to its 40 final destination. 41 42rollback_lock_file:: 43 44 Take a pointer to the `struct lock_file` initialized 45 with an earlier call to `hold_lock_file_for_update()`, 46 close the file descriptor and remove the lockfile. 47 48Because the structure is used in an `atexit(3)` handler, its 49storage has to stay throughout the life of the program. It 50cannot be an auto variable allocated on the stack. 51 52Call `commit_lock_file()` or `rollback_lock_file()` when you are 53done writing to the file descriptor. If you do not call either 54and simply `exit(3)` from the program, an `atexit(3)` handler 55will close and remove the lockfile. 56 57You should not close the file descriptor you obtained from 58`hold_lock_file_for_update` function yourself. The `struct 59lock_file` structure still remembers that the file descriptor 60needs to be closed, and a later call to `commit_lock_file()` or 61`rollback_lock_file()` will result in duplicate calls to 62`close(2)`. Worse yet, if you `close(2)`, open another file 63descriptor for completely different purpose, and then call 64`commit_lock_file()` or `rollback_lock_file()`, they may close 65that unrelated file descriptor.