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authorMichael Haggerty <mhagger@alum.mit.edu>2014-10-01 10:28:14 (GMT)
committerJunio C Hamano <gitster@pobox.com>2014-10-01 20:45:11 (GMT)
commit0a06f148373285822baf5f3e83696732a556a2d1 (patch)
treed47697a7722e108215e97e65d229bcd3c39b5ca9
parent04e57d4d32541bc5dba553a31f09aa2ee456bdad (diff)
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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>
-rw-r--r--lockfile.c42
1 files changed, 42 insertions, 0 deletions
diff --git a/lockfile.c b/lockfile.c
index 81143e5..2680dc9 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -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)