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lockfile.c: document the various states of lock_file objects

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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@alum.mit.edu>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Michael Haggerty 2014-10-01 12:28:14 +02:00 committed by Junio C Hamano
parent 04e57d4d32
commit 0a06f14837
1 changed files with 42 additions and 0 deletions
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lockfile.c
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@ -4,6 +4,48 @@
#include "cache.h"
#include "sigchain.h"
/*
* File write-locks as used by Git.
*
* For an overview of how to use the lockfile API, please see
*
* Documentation/technical/api-lockfile.txt
*
* This module keeps track of all locked files in lock_file_list for
* use at cleanup. This list and the lock_file objects that comprise
* it must be kept in self-consistent states at all time, because the
* program can be interrupted any time by a signal, in which case the
* signal handler will walk through the list attempting to clean up
* any open lock files.
*
* A lockfile 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 lockfile created by
* 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.
*/
static struct lock_file *lock_file_list;
static void remove_lock_file(void)