Document the valid states of lock_file objects, how they get into each state, and how the state is encoded in the object's fields. Signed-off-by: Michael Haggerty <mhagger@xxxxxxxxxxxx> Reviewed-by: Ronnie Sahlberg <sahlberg@xxxxxxxxxx> --- lockfile.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) diff --git a/lockfile.c b/lockfile.c index 00c972c..b6fe848 100644 --- a/lockfile.c +++ b/lockfile.c @@ -4,6 +4,63 @@ #include "cache.h" #include "sigchain.h" +/* + * File write-locks as used by Git. + * + * When a file at $FILENAME needs to be written, it is done as + * follows: + * + * 1. Obtain a lock on the file by creating a lockfile at path + * $FILENAME.lock. The file is opened for read/write using O_CREAT + * and O_EXCL mode to ensure that it doesn't already exist. Such a + * lock file is respected by writers *but not by readers*. + * + * Usually, if $FILENAME is a symlink, then it is resolved, and the + * file ultimately pointed to is the one that is locked and later + * replaced. However, if LOCK_NODEREF is used, then $FILENAME + * itself is locked and later replaced, even if it is a symlink. + * + * 2. Write the new file contents to the lockfile. + * + * 3. Move the lockfile to its final destination using rename(2). + * + * Instead of (3), the change can be rolled back by deleting lockfile. + * + * This module keeps track of all locked files in lock_file_list. + * When the first file is locked, it registers an atexit(3) handler; + * when the program exits, the handler rolls back any files that have + * been locked but were never committed or rolled back. + * + * A lock_file is owned by the process that created it. The lock_file + * object has an "owner" field that records its owner. This field is + * used to prevent a forked process from closing a lock_file of its + * parent. + * + * A lock_file object can be in several states: + * + * - Uninitialized. In this state the object's on_list field must be + * zero but the rest of its contents need not be initialized. As + * soon as the object is used in any way, it is irrevocably + * registered in the lock_file_list, and on_list is set. + * + * - Locked, lockfile open (after hold_lock_file_for_update(), + * hold_lock_file_for_append(), or reopen_lock_file()). In this + * state, the lockfile exists, filename holds the filename of the + * lockfile, fd holds a file descriptor open for writing to the + * lockfile, and owner holds the PID of the process that locked the + * file. + * + * - Locked, lockfile closed (after close_lock_file()). Same as the + * previous state, except that the lockfile is closed and fd is -1. + * + * - Unlocked (after commit_lock_file(), rollback_lock_file(), or a + * failed attempt to lock). In this state, filename[0] == '\0' and + * fd is -1. The object is left registered in the lock_file_list, + * and on_list is set. + * + * See Documentation/api-lockfile.txt for more information. + */ + static struct lock_file *lock_file_list; static void remove_lock_file(void) -- 2.1.0 -- 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