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Diffstat (limited to 'Documentation/technical/api-lockfile.txt')
| -rw-r--r-- | Documentation/technical/api-lockfile.txt | 254 |
1 files changed, 200 insertions, 54 deletions
diff --git a/Documentation/technical/api-lockfile.txt b/Documentation/technical/api-lockfile.txt index dd89404..93b5f23 100644 --- a/Documentation/technical/api-lockfile.txt +++ b/Documentation/technical/api-lockfile.txt @@ -3,20 +3,132 @@ 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. +* 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 @@ -24,51 +136,85 @@ 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. |