git-multimail: update to release 1.2.0
[git.git] / lockfile.h
blob3d301937b0a7e84ccfb03798be1be4047f0654cd
1 #ifndef LOCKFILE_H
2 #define LOCKFILE_H
4 /*
5 * File write-locks as used by Git.
7 * The lockfile API serves two purposes:
9 * * Mutual exclusion and atomic file updates. When we want to change
10 * a file, we create a lockfile `<filename>.lock`, write the new
11 * file contents into it, and then rename the lockfile to its final
12 * destination `<filename>`. We create the `<filename>.lock` file
13 * with `O_CREAT|O_EXCL` so that we can notice and fail if somebody
14 * else has already locked the file, then atomically rename the
15 * lockfile to its final destination to commit the changes and
16 * unlock the file.
18 * * Automatic cruft removal. If the program exits after we lock a
19 * file but before the changes have been committed, we want to make
20 * sure that we remove the lockfile. This is done by remembering the
21 * lockfiles we have created in a linked list and setting up an
22 * `atexit(3)` handler and a signal handler that clean up the
23 * lockfiles. This mechanism ensures that outstanding lockfiles are
24 * cleaned up if the program exits (including when `die()` is
25 * called) or if the program is terminated by a signal.
27 * Please note that lockfiles only block other writers. Readers do not
28 * block, but they are guaranteed to see either the old contents of
29 * the file or the new contents of the file (assuming that the
30 * filesystem implements `rename(2)` atomically).
32 * Most of the heavy lifting is done by the tempfile module (see
33 * "tempfile.h").
35 * Calling sequence
36 * ----------------
38 * The caller:
40 * * Allocates a `struct lock_file` either as a static variable or on
41 * the heap, initialized to zeros. Once you use the structure to
42 * call the `hold_lock_file_for_*()` family of functions, it belongs
43 * to the lockfile subsystem and its storage must remain valid
44 * throughout the life of the program (i.e. you cannot use an
45 * on-stack variable to hold this structure).
47 * * Attempts to create a lockfile by calling `hold_lock_file_for_update()`.
49 * * Writes new content for the destination file by either:
51 * * writing to the file descriptor returned by the
52 * `hold_lock_file_for_*()` functions (also available via
53 * `lock->fd`).
55 * * calling `fdopen_lock_file()` to get a `FILE` pointer for the
56 * open file and writing to the file using stdio.
58 * When finished writing, the caller can:
60 * * Close the file descriptor and rename the lockfile to its final
61 * destination by calling `commit_lock_file()` or
62 * `commit_lock_file_to()`.
64 * * Close the file descriptor and remove the lockfile by calling
65 * `rollback_lock_file()`.
67 * * Close the file descriptor without removing or renaming the
68 * lockfile by calling `close_lock_file()`, and later call
69 * `commit_lock_file()`, `commit_lock_file_to()`,
70 * `rollback_lock_file()`, or `reopen_lock_file()`.
72 * Even after the lockfile is committed or rolled back, the
73 * `lock_file` object must not be freed or altered by the caller.
74 * However, it may be reused; just pass it to another call of
75 * `hold_lock_file_for_update()`.
77 * If the program exits before `commit_lock_file()`,
78 * `commit_lock_file_to()`, or `rollback_lock_file()` is called, the
79 * tempfile module will close and remove the lockfile, thereby rolling
80 * back any uncommitted changes.
82 * If you need to close the file descriptor you obtained from a
83 * `hold_lock_file_for_*()` function yourself, do so by calling
84 * `close_lock_file()`. See "tempfile.h" for more information.
87 * Under the covers, a lockfile is just a tempfile with a few helper
88 * functions. In particular, the state diagram and the cleanup
89 * machinery are all implemented in the tempfile module.
92 * Error handling
93 * --------------
95 * The `hold_lock_file_for_*()` functions return a file descriptor on
96 * success or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see
97 * "flags" below). On errors, `errno` describes the reason for
98 * failure. Errors can be reported by passing `errno` to
99 * `unable_to_lock_message()` or `unable_to_lock_die()`.
101 * Similarly, `commit_lock_file`, `commit_lock_file_to`, and
102 * `close_lock_file` return 0 on success. On failure they set `errno`
103 * appropriately, do their best to roll back the lockfile, and return
104 * -1.
107 #include "tempfile.h"
109 struct lock_file {
110 struct tempfile tempfile;
113 /* String appended to a filename to derive the lockfile name: */
114 #define LOCK_SUFFIX ".lock"
115 #define LOCK_SUFFIX_LEN 5
119 * Flags
120 * -----
122 * The following flags can be passed to `hold_lock_file_for_update()`.
126 * If a lock is already taken for the file, `die()` with an error
127 * message. If this flag is not specified, trying to lock a file that
128 * is already locked returns -1 to the caller.
130 #define LOCK_DIE_ON_ERROR 1
133 * Usually symbolic links in the destination path are resolved. This
134 * means that (1) the lockfile is created by adding ".lock" to the
135 * resolved path, and (2) upon commit, the resolved path is
136 * overwritten. However, if `LOCK_NO_DEREF` is set, then the lockfile
137 * is created by adding ".lock" to the path argument itself. This
138 * option is used, for example, when detaching a symbolic reference,
139 * which for backwards-compatibility reasons, can be a symbolic link
140 * containing the name of the referred-to-reference.
142 #define LOCK_NO_DEREF 2
145 * Attempt to create a lockfile for the file at `path` and return a
146 * file descriptor for writing to it, or -1 on error. If the file is
147 * currently locked, retry with quadratic backoff for at least
148 * timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
149 * timeout_ms is -1, retry indefinitely. The flags argument and error
150 * handling are described above.
152 extern int hold_lock_file_for_update_timeout(
153 struct lock_file *lk, const char *path,
154 int flags, long timeout_ms);
157 * Attempt to create a lockfile for the file at `path` and return a
158 * file descriptor for writing to it, or -1 on error. The flags
159 * argument and error handling are described above.
161 static inline int hold_lock_file_for_update(
162 struct lock_file *lk, const char *path,
163 int flags)
165 return hold_lock_file_for_update_timeout(lk, path, flags, 0);
169 * Append an appropriate error message to `buf` following the failure
170 * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
171 * `errno` set by the failing call.
173 extern void unable_to_lock_message(const char *path, int err,
174 struct strbuf *buf);
177 * Emit an appropriate error message and `die()` following the failure
178 * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
179 * `errno` set by the failing
180 * call.
182 extern NORETURN void unable_to_lock_die(const char *path, int err);
185 * Associate a stdio stream with the lockfile (which must still be
186 * open). Return `NULL` (*without* rolling back the lockfile) on
187 * error. The stream is closed automatically when `close_lock_file()`
188 * is called or when the file is committed or rolled back.
190 static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode)
192 return fdopen_tempfile(&lk->tempfile, mode);
196 * Return the path of the lockfile. The return value is a pointer to a
197 * field within the lock_file object and should not be freed.
199 static inline const char *get_lock_file_path(struct lock_file *lk)
201 return get_tempfile_path(&lk->tempfile);
204 static inline int get_lock_file_fd(struct lock_file *lk)
206 return get_tempfile_fd(&lk->tempfile);
209 static inline FILE *get_lock_file_fp(struct lock_file *lk)
211 return get_tempfile_fp(&lk->tempfile);
215 * Return the path of the file that is locked by the specified
216 * lock_file object. The caller must free the memory.
218 extern char *get_locked_file_path(struct lock_file *lk);
221 * If the lockfile is still open, close it (and the file pointer if it
222 * has been opened using `fdopen_lock_file()`) without renaming the
223 * lockfile over the file being locked. Return 0 upon success. On
224 * failure to `close(2)`, return a negative value and roll back the
225 * lock file. Usually `commit_lock_file()`, `commit_lock_file_to()`,
226 * or `rollback_lock_file()` should eventually be called if
227 * `close_lock_file()` succeeds.
229 static inline int close_lock_file(struct lock_file *lk)
231 return close_tempfile(&lk->tempfile);
235 * Re-open a lockfile that has been closed using `close_lock_file()`
236 * but not yet committed or rolled back. This can be used to implement
237 * a sequence of operations like the following:
239 * * Lock file.
241 * * Write new contents to lockfile, then `close_lock_file()` to
242 * cause the contents to be written to disk.
244 * * Pass the name of the lockfile to another program to allow it (and
245 * nobody else) to inspect the contents you wrote, while still
246 * holding the lock yourself.
248 * * `reopen_lock_file()` to reopen the lockfile. Make further updates
249 * to the contents.
251 * * `commit_lock_file()` to make the final version permanent.
253 static inline int reopen_lock_file(struct lock_file *lk)
255 return reopen_tempfile(&lk->tempfile);
259 * Commit the change represented by `lk`: close the file descriptor
260 * and/or file pointer if they are still open and rename the lockfile
261 * to its final destination. Return 0 upon success. On failure, roll
262 * back the lock file and return -1, with `errno` set to the value
263 * from the failing call to `close(2)` or `rename(2)`. It is a bug to
264 * call `commit_lock_file()` for a `lock_file` object that is not
265 * currently locked.
267 extern int commit_lock_file(struct lock_file *lk);
270 * Like `commit_lock_file()`, but rename the lockfile to the provided
271 * `path`. `path` must be on the same filesystem as the lock file.
273 static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
275 return rename_tempfile(&lk->tempfile, path);
279 * Roll back `lk`: close the file descriptor and/or file pointer and
280 * remove the lockfile. It is a NOOP to call `rollback_lock_file()`
281 * for a `lock_file` object that has already been committed or rolled
282 * back.
284 static inline void rollback_lock_file(struct lock_file *lk)
286 delete_tempfile(&lk->tempfile);
289 #endif /* LOCKFILE_H */