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044b6a9efe
Currently, there is only one attempt to lock a file. If it fails, the whole operation fails. But it might sometimes be advantageous to try acquiring a file lock a few times before giving up. So add a new function, hold_lock_file_for_update_timeout(), that allows a timeout to be specified. Make hold_lock_file_for_update() a thin wrapper around the new function. If timeout_ms is positive, then retry for at least that many milliseconds to acquire the lock. On each failed attempt, use select() to wait for a backoff time that increases quadratically (capped at 1 second) and has a random component to prevent two processes from getting synchronized. If timeout_ms is negative, retry indefinitely. In a moment we will switch to using the new function when locking packed-refs. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>
100 lines
3.5 KiB
C
100 lines
3.5 KiB
C
#ifndef LOCKFILE_H
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#define LOCKFILE_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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* The possible states of a lock_file object are as follows:
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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:
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* - the lockfile exists
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* - active is set
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* - filename holds the filename of the lockfile
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* - fd holds a file descriptor open for writing to the lockfile
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* - fp holds a pointer to an open FILE object if and only if
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* fdopen_lock_file() has been called on the object
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* - owner holds the PID of the process that locked the file
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*
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* - Locked, lockfile closed (after successful close_lock_file()).
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* Same as the previous state, except that the lockfile is closed
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* and fd is -1.
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*
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* - Unlocked (after commit_lock_file(), commit_lock_file_to(),
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* rollback_lock_file(), a failed attempt to lock, or a failed
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* close_lock_file()). In this state:
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* - active is unset
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* - filename is empty (usually, though there are transitory
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* states in which this condition doesn't hold). Client code should
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* *not* rely on the filename being empty in this state.
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* - fd is -1
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* - the object is left registered in the lock_file_list, and
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* on_list is set.
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*/
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struct lock_file {
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struct lock_file *volatile next;
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volatile sig_atomic_t active;
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volatile int fd;
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FILE *volatile fp;
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volatile pid_t owner;
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char on_list;
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struct strbuf filename;
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};
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/* String appended to a filename to derive the lockfile name: */
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#define LOCK_SUFFIX ".lock"
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#define LOCK_SUFFIX_LEN 5
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#define LOCK_DIE_ON_ERROR 1
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#define LOCK_NO_DEREF 2
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extern void unable_to_lock_message(const char *path, int err,
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struct strbuf *buf);
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extern NORETURN void unable_to_lock_die(const char *path, int err);
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extern int hold_lock_file_for_update_timeout(
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struct lock_file *lk, const char *path,
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int flags, long timeout_ms);
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static inline int hold_lock_file_for_update(
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struct lock_file *lk, const char *path,
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int flags)
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{
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return hold_lock_file_for_update_timeout(lk, path, flags, 0);
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}
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extern int hold_lock_file_for_append(struct lock_file *lk, const char *path,
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int flags);
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extern FILE *fdopen_lock_file(struct lock_file *, const char *mode);
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extern char *get_locked_file_path(struct lock_file *);
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extern int commit_lock_file_to(struct lock_file *, const char *path);
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extern int commit_lock_file(struct lock_file *);
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extern int reopen_lock_file(struct lock_file *);
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extern int close_lock_file(struct lock_file *);
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extern void rollback_lock_file(struct lock_file *);
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#endif /* LOCKFILE_H */
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