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lockfile.c: document the various states of lock_file objects
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>
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lockfile.c
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lockfile.c
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#include "cache.h"
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#include "sigchain.h"
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/*
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* File write-locks as used by Git.
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*
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* For an overview of how to use the lockfile API, please see
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*
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* Documentation/technical/api-lockfile.txt
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*
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* This module keeps track of all locked files in lock_file_list for
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* use at cleanup. This list and the lock_file objects that comprise
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* it must be kept in self-consistent states at all time, because the
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* program can be interrupted any time by a signal, in which case the
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* signal handler will walk through the list attempting to clean up
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* any open lock files.
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*
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* A lockfile is owned by the process that created it. The lock_file
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* object has an "owner" field that records its owner. This field is
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* used to prevent a forked process from closing a lockfile created by
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* its parent.
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*
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* A lock_file object can be in several states:
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*
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* - Uninitialized. In this state the object's on_list field must be
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* zero but the rest of its contents need not be initialized. As
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* soon as the object is used in any way, it is irrevocably
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* registered in the lock_file_list, and on_list is set.
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*
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* - Locked, lockfile open (after hold_lock_file_for_update(),
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* hold_lock_file_for_append(), or reopen_lock_file()). In this
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* state, the lockfile exists, filename holds the filename of the
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* lockfile, fd holds a file descriptor open for writing to the
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* lockfile, and owner holds the PID of the process that locked the
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* file.
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*
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* - Locked, lockfile closed (after close_lock_file()). Same as the
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* previous state, except that the lockfile is closed and fd is -1.
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*
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* - Unlocked (after commit_lock_file(), rollback_lock_file(), or a
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* failed attempt to lock). In this state, filename[0] == '\0' and
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* fd is -1. The object is left registered in the lock_file_list,
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* and on_list is set.
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*/
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static struct lock_file *lock_file_list;
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static void remove_lock_file(void)
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