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lockfile: move documentation to lockfile.h and lockfile.c 

Rearrange/rewrite it somewhat to fit its new environment.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
maint
Michael Haggerty 10 years ago committed by Junio C Hamano
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  1. 220
      Documentation/technical/api-lockfile.txt
  2. 53
      lockfile.c
  3. 290
      lockfile.h

220
Documentation/technical/api-lockfile.txt
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@ -1,220 +0,0 @@ @@ -1,220 +0,0 @@
lockfile API
============

The lockfile API serves two purposes:

* Mutual exclusion and atomic file updates. When we want to change a
file, we create a lockfile `<filename>.lock`, write the new file
contents into it, and then rename the lockfile to its final
destination `<filename>`. We create the `<filename>.lock` file with
`O_CREAT|O_EXCL` so that we can notice and fail if somebody else has
already locked the file, then atomically rename the lockfile to its
final destination to commit the changes and unlock the file.

* Automatic cruft removal. If the program exits after we lock a file
but before the changes have been committed, we want to make sure
that we remove the lockfile. This is done by remembering the
lockfiles we have created in a linked list and setting up an
`atexit(3)` handler and a signal handler that clean up the
lockfiles. This mechanism ensures that outstanding lockfiles are
cleaned up if the program exits (including when `die()` is called)
or if the program dies on a signal.

Please note that lockfiles only block other writers. Readers do not
block, but they are guaranteed to see either the old contents of the
file or the new contents of the file (assuming that the filesystem
implements `rename(2)` atomically).


Calling sequence
----------------

The caller:

* Allocates a `struct lock_file` either as a static variable or on the
heap, initialized to zeros. Once you use the structure to call the
`hold_lock_file_*` family of functions, it belongs to the lockfile
subsystem and its storage must remain valid throughout the life of
the program (i.e. you cannot use an on-stack variable to hold this
structure).

* Attempts to create a lockfile by passing that variable and the path
of the final destination (e.g. `$GIT_DIR/index`) to
`hold_lock_file_for_update` or `hold_lock_file_for_append`.

* Writes new content for the destination file by either:

* writing to the file descriptor returned by the `hold_lock_file_*`
functions (also available via `lock->fd`).

* calling `fdopen_lock_file` to get a `FILE` pointer for the open
file and writing to the file using stdio.

When finished writing, the caller can:

* Close the file descriptor and rename the lockfile to its final
destination by calling `commit_lock_file` or `commit_lock_file_to`.

* Close the file descriptor and remove the lockfile by calling
`rollback_lock_file`.

* Close the file descriptor without removing or renaming the lockfile
by calling `close_lock_file`, and later call `commit_lock_file`,
`commit_lock_file_to`, `rollback_lock_file`, or `reopen_lock_file`.

Even after the lockfile is committed or rolled back, the `lock_file`
object must not be freed or altered by the caller. However, it may be
reused; just pass it to another call of `hold_lock_file_for_update` or
`hold_lock_file_for_append`.

If the program exits before you have called one of `commit_lock_file`,
`commit_lock_file_to`, `rollback_lock_file`, or `close_lock_file`, an
`atexit(3)` handler will close and remove the lockfile, rolling back
any uncommitted changes.

If you need to close the file descriptor you obtained from a
`hold_lock_file_*` function yourself, do so by calling
`close_lock_file`. You should never call `close(2)` or `fclose(3)`
yourself! Otherwise the `struct lock_file` structure would still think
that the file descriptor needs to be closed, and a commit or rollback
would result in duplicate calls to `close(2)`. Worse yet, if you close
and then later open another file descriptor for a completely different
purpose, then a commit or rollback might close that unrelated file
descriptor.


Error handling
--------------

The `hold_lock_file_*` functions return a file descriptor on success
or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see below). On
errors, `errno` describes the reason for failure. Errors can be
reported by passing `errno` to one of the following helper functions:

unable_to_lock_message::

Append an appropriate error message to a `strbuf`.

unable_to_lock_error::

Emit an appropriate error message using `error()`.

unable_to_lock_die::

Emit an appropriate error message and `die()`.

Similarly, `commit_lock_file`, `commit_lock_file_to`, and
`close_lock_file` return 0 on success. On failure they set `errno`
appropriately, do their best to roll back the lockfile, and return -1.


Flags
-----

The following flags can be passed to `hold_lock_file_for_update` or
`hold_lock_file_for_append`:

LOCK_NO_DEREF::

Usually symbolic links in the destination path are resolved
and the lockfile is created by adding ".lock" to the resolved
path. If `LOCK_NO_DEREF` is set, then the lockfile is created
by adding ".lock" to the path argument itself. This option is
used, for example, when locking a symbolic reference, which
for backwards-compatibility reasons can be a symbolic link
containing the name of the referred-to-reference.

LOCK_DIE_ON_ERROR::

If a lock is already taken for the file, `die()` with an error
message. If this option is not specified, trying to lock a
file that is already locked returns -1 to the caller.


The functions
-------------

hold_lock_file_for_update::

Take a pointer to `struct lock_file`, the path of the file to
be locked (e.g. `$GIT_DIR/index`) and a flags argument (see
above). Attempt to create a lockfile for the destination and
return the file descriptor for writing to the file.

hold_lock_file_for_append::

Like `hold_lock_file_for_update`, but before returning copy
the existing contents of the file (if any) to the lockfile and
position its write pointer at the end of the file.

fdopen_lock_file::

Associate a stdio stream with the lockfile. Return NULL
(*without* rolling back the lockfile) on error. The stream is
closed automatically when `close_lock_file` is called or when
the file is committed or rolled back.

get_locked_file_path::

Return the path of the file that is locked by the specified
lock_file object. The caller must free the memory.

commit_lock_file::

Take a pointer to the `struct lock_file` initialized with an
earlier call to `hold_lock_file_for_update` or
`hold_lock_file_for_append`, close the file descriptor, and
rename the lockfile to its final destination. Return 0 upon
success. On failure, roll back the lock file and return -1,
with `errno` set to the value from the failing call to
`close(2)` or `rename(2)`. It is a bug to call
`commit_lock_file` for a `lock_file` object that is not
currently locked.

commit_lock_file_to::

Like `commit_lock_file()`, except that it takes an explicit
`path` argument to which the lockfile should be renamed. The
`path` must be on the same filesystem as the lock file.

rollback_lock_file::

Take a pointer to the `struct lock_file` initialized with an
earlier call to `hold_lock_file_for_update` or
`hold_lock_file_for_append`, close the file descriptor and
remove the lockfile. It is a NOOP to call
`rollback_lock_file()` for a `lock_file` object that has
already been committed or rolled back.

close_lock_file::

Take a pointer to the `struct lock_file` initialized with an
earlier call to `hold_lock_file_for_update` or
`hold_lock_file_for_append`. Close the file descriptor (and
the file pointer if it has been opened using
`fdopen_lock_file`). Return 0 upon success. On failure to
`close(2)`, return a negative value and roll back the lock
file. Usually `commit_lock_file`, `commit_lock_file_to`, or
`rollback_lock_file` should eventually be called if
`close_lock_file` succeeds.

reopen_lock_file::

Re-open a lockfile that has been closed (using
`close_lock_file`) but not yet committed or rolled back. This
can be used to implement a sequence of operations like the
following:

* Lock file.

* Write new contents to lockfile, then `close_lock_file` to
cause the contents to be written to disk.

* Pass the name of the lockfile to another program to allow it
(and nobody else) to inspect the contents you wrote, while
still holding the lock yourself.

* `reopen_lock_file` to reopen the lockfile. Make further
updates to the contents.

* `commit_lock_file` to make the final version permanent.

53
lockfile.c
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@ -1,6 +1,59 @@ @@ -1,6 +1,59 @@
/*
* Copyright (c) 2005, Junio C Hamano
*/

/*
* State diagram and cleanup
* -------------------------
*
* 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.
*
* The possible states of a `lock_file` object are as follows:
*
* - 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 `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
* - `active` is set
* - `filename` holds the filename of the lockfile
* - `fd` holds a file descriptor open for writing to the lockfile
* - `fp` holds a pointer to an open `FILE` object if and only if
* `fdopen_lock_file()` has been called on the object
* - `owner` holds the PID of the process that locked the file
*
* - Locked, lockfile closed (after successful `close_lock_file()`).
* Same as the previous state, except that the lockfile is closed
* and `fd` is -1.
*
* - Unlocked (after `commit_lock_file()`, `commit_lock_file_to()`,
* `rollback_lock_file()`, a failed attempt to lock, or a failed
* `close_lock_file()`). In this state:
*
* - `active` is unset
* - `filename` is empty (usually, though there are transitory
* states in which this condition doesn't hold). Client code should
* *not* rely on the filename being empty in this state.
* - `fd` is -1
* - the object is left registered in the `lock_file_list`, and
* `on_list` is set.
*
* A lockfile is owned by the process that created it. The `lock_file`
* has an `owner` field that records the owner's PID. This field is
* used to prevent a forked process from closing a lockfile created by
* its parent.
*/

#include "cache.h"
#include "lockfile.h"
#include "sigchain.h"

290
lockfile.h
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@ -4,54 +4,103 @@ @@ -4,54 +4,103 @@
/*
* 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.
*
* The possible states of a lock_file object are as follows:
*
* - 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
* - active is set
* - filename holds the filename of the lockfile
* - fd holds a file descriptor open for writing to the lockfile
* - fp holds a pointer to an open FILE object if and only if
* fdopen_lock_file() has been called on the object
* - owner holds the PID of the process that locked the file
*
* - Locked, lockfile closed (after successful close_lock_file()).
* Same as the previous state, except that the lockfile is closed
* and fd is -1.
*
* - Unlocked (after commit_lock_file(), commit_lock_file_to(),
* rollback_lock_file(), a failed attempt to lock, or a failed
* close_lock_file()). In this state:
* - active is unset
* - filename is empty (usually, though there are transitory
* states in which this condition doesn't hold). Client code should
* *not* rely on the filename being empty in this state.
* - fd is -1
* - the object is left registered in the lock_file_list, and
* on_list is set.
* The lockfile API serves two purposes:
*
* * Mutual exclusion and atomic file updates. When we want to change
* a file, we create a lockfile `<filename>.lock`, write the new
* file contents into it, and then rename the lockfile to its final
* destination `<filename>`. We create the `<filename>.lock` file
* with `O_CREAT|O_EXCL` so that we can notice and fail if somebody
* else has already locked the file, then atomically rename the
* lockfile to its final destination to commit the changes and
* unlock the file.
*
* * Automatic cruft removal. If the program exits after we lock a
* file but before the changes have been committed, we want to make
* sure that we remove the lockfile. This is done by remembering the
* lockfiles we have created in a linked list and setting up an
* `atexit(3)` handler and a signal handler that clean up the
* lockfiles. This mechanism ensures that outstanding lockfiles are
* cleaned up if the program exits (including when `die()` is
* called) or if the program is terminated by a signal.
*
* Please note that lockfiles only block other writers. Readers do not
* block, but they are guaranteed to see either the old contents of
* the file or the new contents of the file (assuming that the
* filesystem implements `rename(2)` atomically).
*
*
* Calling sequence
* ----------------
*
* The caller:
*
* * Allocates a `struct lock_file` either as a static variable or on
* the heap, initialized to zeros. Once you use the structure to
* call the `hold_lock_file_for_*()` family of functions, it belongs
* to the lockfile subsystem and its storage must remain valid
* throughout the life of the program (i.e. you cannot use an
* on-stack variable to hold this structure).
*
* * Attempts to create a lockfile by calling
* `hold_lock_file_for_update()` or `hold_lock_file_for_append()`.
*
* * Writes new content for the destination file by either:
*
* * writing to the file descriptor returned by the
* `hold_lock_file_for_*()` functions (also available via
* `lock->fd`).
*
* * calling `fdopen_lock_file()` to get a `FILE` pointer for the
* open file and writing to the file using stdio.
*
* When finished writing, the caller can:
*
* * Close the file descriptor and rename the lockfile to its final
* destination by calling `commit_lock_file()` or
* `commit_lock_file_to()`.
*
* * Close the file descriptor and remove the lockfile by calling
* `rollback_lock_file()`.
*
* * Close the file descriptor without removing or renaming the
* lockfile by calling `close_lock_file()`, and later call
* `commit_lock_file()`, `commit_lock_file_to()`,
* `rollback_lock_file()`, or `reopen_lock_file()`.
*
* Even after the lockfile is committed or rolled back, the
* `lock_file` object must not be freed or altered by the caller.
* However, it may be reused; just pass it to another call of
* `hold_lock_file_for_update()` or `hold_lock_file_for_append()`.
*
* If the program exits before `commit_lock_file()`,
* `commit_lock_file_to()`, or `rollback_lock_file()` is called, an
* `atexit(3)` handler will close and remove the lockfile, thereby
* rolling back any uncommitted changes.
*
* If you need to close the file descriptor you obtained from a
* `hold_lock_file_for_*()` function yourself, do so by calling
* `close_lock_file()`. You should never call `close(2)` or
* `fclose(3)` yourself, otherwise the `struct lock_file` structure
* would still think that the file descriptor needs to be closed, and
* a commit or rollback would result in duplicate calls to `close(2)`.
* Worse yet, if you close and then later open another file descriptor
* for a completely different purpose, then a commit or rollback might
* close that unrelated file descriptor.
*
* Error handling
* --------------
*
* The `hold_lock_file_for_*()` functions return a file descriptor on
* success or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see
* "flags" below). On errors, `errno` describes the reason for
* failure. Errors can be reported by passing `errno` to
* `unable_to_lock_message()` or `unable_to_lock_die()`.
*
* Similarly, `commit_lock_file`, `commit_lock_file_to`, and
* `close_lock_file` return 0 on success. On failure they set `errno`
* appropriately, do their best to roll back the lockfile, and return
* -1.
*/

struct lock_file {
@ -68,16 +117,51 @@ struct lock_file { @@ -68,16 +117,51 @@ struct lock_file {
#define LOCK_SUFFIX ".lock"
#define LOCK_SUFFIX_LEN 5


/*
* Flags
* -----
*
* The following flags can be passed to `hold_lock_file_for_update()`
* or `hold_lock_file_for_append()`.
*/

/*
* If a lock is already taken for the file, `die()` with an error
* message. If this flag is not specified, trying to lock a file that
* is already locked returns -1 to the caller.
*/
#define LOCK_DIE_ON_ERROR 1

/*
* Usually symbolic links in the destination path are resolved. This
* means that (1) the lockfile is created by adding ".lock" to the
* resolved path, and (2) upon commit, the resolved path is
* overwritten. However, if `LOCK_NO_DEREF` is set, then the lockfile
* is created by adding ".lock" to the path argument itself. This
* option is used, for example, when detaching a symbolic reference,
* which for backwards-compatibility reasons, can be a symbolic link
* containing the name of the referred-to-reference.
*/
#define LOCK_NO_DEREF 2

extern void unable_to_lock_message(const char *path, int err,
struct strbuf *buf);
extern NORETURN void unable_to_lock_die(const char *path, int err);
/*
* Attempt to create a lockfile for the file at `path` and return a
* file descriptor for writing to it, or -1 on error. If the file is
* currently locked, retry with quadratic backoff for at least
* timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
* timeout_ms is -1, retry indefinitely. The flags argument and error
* handling are described above.
*/
extern int hold_lock_file_for_update_timeout(
struct lock_file *lk, const char *path,
int flags, long timeout_ms);

/*
* Attempt to create a lockfile for the file at `path` and return a
* file descriptor for writing to it, or -1 on error. The flags
* argument and error handling are described above.
*/
static inline int hold_lock_file_for_update(
struct lock_file *lk, const char *path,
int flags)
@ -85,15 +169,101 @@ static inline int hold_lock_file_for_update( @@ -85,15 +169,101 @@ static inline int hold_lock_file_for_update(
return hold_lock_file_for_update_timeout(lk, path, flags, 0);
}

extern int hold_lock_file_for_append(struct lock_file *lk, const char *path,
int flags);
/*
* Like `hold_lock_file_for_update()`, but before returning copy the
* existing contents of the file (if any) to the lockfile and position
* its write pointer at the end of the file. The flags argument and
* error handling are described above.
*/
extern int hold_lock_file_for_append(struct lock_file *lk,
const char *path, int flags);

/*
* Append an appropriate error message to `buf` following the failure
* of `hold_lock_file_for_update()` or `hold_lock_file_for_append()`
* to lock `path`. `err` should be the `errno` set by the failing
* call.
*/
extern void unable_to_lock_message(const char *path, int err,
struct strbuf *buf);

extern FILE *fdopen_lock_file(struct lock_file *, const char *mode);
extern char *get_locked_file_path(struct lock_file *);
extern int commit_lock_file_to(struct lock_file *, const char *path);
extern int commit_lock_file(struct lock_file *);
extern int reopen_lock_file(struct lock_file *);
extern int close_lock_file(struct lock_file *);
extern void rollback_lock_file(struct lock_file *);
/*
* Emit an appropriate error message and `die()` following the failure
* of `hold_lock_file_for_update()` or `hold_lock_file_for_append()`
* to lock `path`. `err` should be the `errno` set by the failing
* call.
*/
extern NORETURN void unable_to_lock_die(const char *path, int err);

/*
* Associate a stdio stream with the lockfile (which must still be
* open). Return `NULL` (*without* rolling back the lockfile) on
* error. The stream is closed automatically when `close_lock_file()`
* is called or when the file is committed or rolled back.
*/
extern FILE *fdopen_lock_file(struct lock_file *lk, const char *mode);

/*
* Return the path of the file that is locked by the specified
* lock_file object. The caller must free the memory.
*/
extern char *get_locked_file_path(struct lock_file *lk);

/*
* If the lockfile is still open, close it (and the file pointer if it
* has been opened using `fdopen_lock_file()`) without renaming the
* lockfile over the file being locked. Return 0 upon success. On
* failure to `close(2)`, return a negative value and roll back the
* lock file. Usually `commit_lock_file()`, `commit_lock_file_to()`,
* or `rollback_lock_file()` should eventually be called if
* `close_lock_file()` succeeds.
*/
extern int close_lock_file(struct lock_file *lk);

/*
* Re-open a lockfile that has been closed using `close_lock_file()`
* but not yet committed or rolled back. This can be used to implement
* a sequence of operations like the following:
*
* * Lock file.
*
* * Write new contents to lockfile, then `close_lock_file()` to
* cause the contents to be written to disk.
*
* * Pass the name of the lockfile to another program to allow it (and
* nobody else) to inspect the contents you wrote, while still
* holding the lock yourself.
*
* * `reopen_lock_file()` to reopen the lockfile. Make further updates
* to the contents.
*
* * `commit_lock_file()` to make the final version permanent.
*/
extern int reopen_lock_file(struct lock_file *lk);

/*
* Commit the change represented by `lk`: close the file descriptor
* and/or file pointer if they are still open and rename the lockfile
* to its final destination. Return 0 upon success. On failure, roll
* back the lock file and return -1, with `errno` set to the value
* from the failing call to `close(2)` or `rename(2)`. It is a bug to
* call `commit_lock_file()` for a `lock_file` object that is not
* currently locked.
*/
extern int commit_lock_file(struct lock_file *lk);

/*
* Like `commit_lock_file()`, but rename the lockfile to the provided
* `path`. `path` must be on the same filesystem as the lock file.
*/
extern int commit_lock_file_to(struct lock_file *lk, const char *path);

/*
* Roll back `lk`: close the file descriptor and/or file pointer and
* remove the lockfile. It is a NOOP to call `rollback_lock_file()`
* for a `lock_file` object that has already been committed or rolled
* back.
*/
extern void rollback_lock_file(struct lock_file *lk);

#endif /* LOCKFILE_H */

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