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authorJunio C Hamano <gitster@pobox.com>2015-08-25 21:57:09 (GMT)
committerJunio C Hamano <gitster@pobox.com>2015-08-25 21:57:09 (GMT)
commitdb86e61cbbc4c59a0886366bbf392498e64b53c8 (patch)
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parent9e9033166b3a6b34aad917891b9210fa194fdfd9 (diff)
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Merge branch 'mh/tempfile'
The "lockfile" API has been rebuilt on top of a new "tempfile" API. * mh/tempfile: credential-cache--daemon: use tempfile module credential-cache--daemon: delete socket from main() gc: use tempfile module to handle gc.pid file lock_repo_for_gc(): compute the path to "gc.pid" only once diff: use tempfile module setup_temporary_shallow(): use tempfile module write_shared_index(): use tempfile module register_tempfile(): new function to handle an existing temporary file tempfile: add several functions for creating temporary files prepare_tempfile_object(): new function, extracted from create_tempfile() tempfile: a new module for handling temporary files commit_lock_file(): use get_locked_file_path() lockfile: add accessor get_lock_file_path() lockfile: add accessors get_lock_file_fd() and get_lock_file_fp() create_bundle(): duplicate file descriptor to avoid closing it twice lockfile: move documentation to lockfile.h and lockfile.c
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-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.