Merge branch 'jk/partial-clone-sparse-blob'
[git/git.git] / lockfile.h
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1#ifndef LOCKFILE_H
2#define LOCKFILE_H
3
4/*
5 * File write-locks as used by Git.
6 *
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7 * The lockfile API serves two purposes:
8 *
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.
17 *
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.
26 *
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).
31 *
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32 * Most of the heavy lifting is done by the tempfile module (see
33 * "tempfile.h").
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34 *
35 * Calling sequence
36 * ----------------
37 *
38 * The caller:
39 *
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40 * * Allocates a `struct lock_file` with whatever storage duration you
41 * desire. The struct does not have to be initialized before being
42 * used, but it is good practice to do so using by setting it to
43 * all-zeros (or using the LOCK_INIT macro). This puts the object in a
44 * consistent state that allows you to call rollback_lock_file() even
45 * if the lock was never taken (in which case it is a noop).
2db69de8 46 *
aae42e43 47 * * Attempts to create a lockfile by calling `hold_lock_file_for_update()`.
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48 *
49 * * Writes new content for the destination file by either:
50 *
51 * * writing to the file descriptor returned by the
52 * `hold_lock_file_for_*()` functions (also available via
53 * `lock->fd`).
54 *
55 * * calling `fdopen_lock_file()` to get a `FILE` pointer for the
56 * open file and writing to the file using stdio.
57 *
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58 * Note that the file descriptor returned by hold_lock_file_for_update()
59 * is marked O_CLOEXEC, so the new contents must be written by the
60 * current process, not a spawned one.
61 *
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62 * When finished writing, the caller can:
63 *
64 * * Close the file descriptor and rename the lockfile to its final
65 * destination by calling `commit_lock_file()` or
66 * `commit_lock_file_to()`.
67 *
68 * * Close the file descriptor and remove the lockfile by calling
69 * `rollback_lock_file()`.
70 *
71 * * Close the file descriptor without removing or renaming the
83a3069a 72 * lockfile by calling `close_lock_file_gently()`, and later call
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73 * `commit_lock_file()`, `commit_lock_file_to()`,
74 * `rollback_lock_file()`, or `reopen_lock_file()`.
75 *
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76 * After the lockfile is committed or rolled back, the `lock_file`
77 * object can be discarded or reused.
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78 *
79 * If the program exits before `commit_lock_file()`,
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80 * `commit_lock_file_to()`, or `rollback_lock_file()` is called, the
81 * tempfile module will close and remove the lockfile, thereby rolling
82 * back any uncommitted changes.
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83 *
84 * If you need to close the file descriptor you obtained from a
85 * `hold_lock_file_for_*()` function yourself, do so by calling
83a3069a 86 * `close_lock_file_gently()`. See "tempfile.h" for more information.
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87 *
88 *
89 * Under the covers, a lockfile is just a tempfile with a few helper
90 * functions. In particular, the state diagram and the cleanup
91 * machinery are all implemented in the tempfile module.
92 *
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93 *
94 * Error handling
95 * --------------
96 *
97 * The `hold_lock_file_for_*()` functions return a file descriptor on
98 * success or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see
99 * "flags" below). On errors, `errno` describes the reason for
100 * failure. Errors can be reported by passing `errno` to
101 * `unable_to_lock_message()` or `unable_to_lock_die()`.
102 *
103 * Similarly, `commit_lock_file`, `commit_lock_file_to`, and
104 * `close_lock_file` return 0 on success. On failure they set `errno`
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105 * appropriately and return -1. The `commit` variants (but not `close`)
106 * do their best to delete the temporary file before returning.
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107 */
108
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109#include "tempfile.h"
110
697cc8ef 111struct lock_file {
076aa2cb 112 struct tempfile *tempfile;
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113};
114
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115#define LOCK_INIT { NULL }
116
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117/* String appended to a filename to derive the lockfile name: */
118#define LOCK_SUFFIX ".lock"
119#define LOCK_SUFFIX_LEN 5
120
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121
122/*
123 * Flags
124 * -----
125 *
aae42e43 126 * The following flags can be passed to `hold_lock_file_for_update()`.
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127 */
128
129/*
130 * If a lock is already taken for the file, `die()` with an error
131 * message. If this flag is not specified, trying to lock a file that
3f061bf5 132 * is already locked silently returns -1 to the caller, or ...
2db69de8 133 */
697cc8ef 134#define LOCK_DIE_ON_ERROR 1
2db69de8 135
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136/*
137 * ... this flag can be passed instead to return -1 and give the usual
138 * error message upon an error.
139 */
b22d7484 140#define LOCK_REPORT_ON_ERROR 4
3f061bf5 141
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142/*
143 * Usually symbolic links in the destination path are resolved. This
144 * means that (1) the lockfile is created by adding ".lock" to the
145 * resolved path, and (2) upon commit, the resolved path is
146 * overwritten. However, if `LOCK_NO_DEREF` is set, then the lockfile
147 * is created by adding ".lock" to the path argument itself. This
148 * option is used, for example, when detaching a symbolic reference,
149 * which for backwards-compatibility reasons, can be a symbolic link
150 * containing the name of the referred-to-reference.
151 */
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152#define LOCK_NO_DEREF 2
153
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154/*
155 * Attempt to create a lockfile for the file at `path` and return a
156 * file descriptor for writing to it, or -1 on error. If the file is
157 * currently locked, retry with quadratic backoff for at least
158 * timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
159 * timeout_ms is -1, retry indefinitely. The flags argument and error
160 * handling are described above.
161 */
55454427 162int hold_lock_file_for_update_timeout(
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163 struct lock_file *lk, const char *path,
164 int flags, long timeout_ms);
165
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166/*
167 * Attempt to create a lockfile for the file at `path` and return a
168 * file descriptor for writing to it, or -1 on error. The flags
169 * argument and error handling are described above.
170 */
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171static inline int hold_lock_file_for_update(
172 struct lock_file *lk, const char *path,
173 int flags)
174{
175 return hold_lock_file_for_update_timeout(lk, path, flags, 0);
176}
177
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178/*
179 * Return a nonzero value iff `lk` is currently locked.
180 */
181static inline int is_lock_file_locked(struct lock_file *lk)
182{
076aa2cb 183 return is_tempfile_active(lk->tempfile);
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184}
185
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186/*
187 * Append an appropriate error message to `buf` following the failure
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188 * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
189 * `errno` set by the failing call.
2db69de8 190 */
55454427 191void unable_to_lock_message(const char *path, int err,
ad6dad09 192 struct strbuf *buf);
044b6a9e 193
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194/*
195 * Emit an appropriate error message and `die()` following the failure
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196 * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
197 * `errno` set by the failing
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198 * call.
199 */
55454427 200NORETURN void unable_to_lock_die(const char *path, int err);
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201
202/*
203 * Associate a stdio stream with the lockfile (which must still be
204 * open). Return `NULL` (*without* rolling back the lockfile) on
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205 * error. The stream is closed automatically when
206 * `close_lock_file_gently()` is called or when the file is committed or
207 * rolled back.
2db69de8 208 */
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209static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode)
210{
076aa2cb 211 return fdopen_tempfile(lk->tempfile, mode);
1a9d15db 212}
2db69de8 213
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214/*
215 * Return the path of the lockfile. The return value is a pointer to a
216 * field within the lock_file object and should not be freed.
217 */
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218static inline const char *get_lock_file_path(struct lock_file *lk)
219{
076aa2cb 220 return get_tempfile_path(lk->tempfile);
1a9d15db 221}
b4fb09e4 222
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223static inline int get_lock_file_fd(struct lock_file *lk)
224{
076aa2cb 225 return get_tempfile_fd(lk->tempfile);
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226}
227
228static inline FILE *get_lock_file_fp(struct lock_file *lk)
229{
076aa2cb 230 return get_tempfile_fp(lk->tempfile);
1a9d15db 231}
c99a4c2d 232
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233/*
234 * Return the path of the file that is locked by the specified
235 * lock_file object. The caller must free the memory.
236 */
55454427 237char *get_locked_file_path(struct lock_file *lk);
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238
239/*
240 * If the lockfile is still open, close it (and the file pointer if it
241 * has been opened using `fdopen_lock_file()`) without renaming the
242 * lockfile over the file being locked. Return 0 upon success. On
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243 * failure to `close(2)`, return a negative value (the lockfile is not
244 * rolled back). Usually `commit_lock_file()`, `commit_lock_file_to()`,
83a3069a 245 * or `rollback_lock_file()` should eventually be called.
2db69de8 246 */
83a3069a 247static inline int close_lock_file_gently(struct lock_file *lk)
1a9d15db 248{
076aa2cb 249 return close_tempfile_gently(lk->tempfile);
1a9d15db 250}
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251
252/*
83a3069a 253 * Re-open a lockfile that has been closed using `close_lock_file_gently()`
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254 * but not yet committed or rolled back. This can be used to implement
255 * a sequence of operations like the following:
256 *
257 * * Lock file.
258 *
83a3069a 259 * * Write new contents to lockfile, then `close_lock_file_gently()` to
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260 * cause the contents to be written to disk.
261 *
262 * * Pass the name of the lockfile to another program to allow it (and
263 * nobody else) to inspect the contents you wrote, while still
264 * holding the lock yourself.
265 *
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266 * * `reopen_lock_file()` to reopen the lockfile, truncating the existing
267 * contents. Write out the new contents.
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268 *
269 * * `commit_lock_file()` to make the final version permanent.
270 */
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271static inline int reopen_lock_file(struct lock_file *lk)
272{
076aa2cb 273 return reopen_tempfile(lk->tempfile);
1a9d15db 274}
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275
276/*
277 * Commit the change represented by `lk`: close the file descriptor
278 * and/or file pointer if they are still open and rename the lockfile
279 * to its final destination. Return 0 upon success. On failure, roll
280 * back the lock file and return -1, with `errno` set to the value
281 * from the failing call to `close(2)` or `rename(2)`. It is a bug to
282 * call `commit_lock_file()` for a `lock_file` object that is not
283 * currently locked.
284 */
55454427 285int commit_lock_file(struct lock_file *lk);
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286
287/*
288 * Like `commit_lock_file()`, but rename the lockfile to the provided
289 * `path`. `path` must be on the same filesystem as the lock file.
290 */
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291static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
292{
293 return rename_tempfile(&lk->tempfile, path);
294}
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295
296/*
297 * Roll back `lk`: close the file descriptor and/or file pointer and
298 * remove the lockfile. It is a NOOP to call `rollback_lock_file()`
299 * for a `lock_file` object that has already been committed or rolled
300 * back.
301 */
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302static inline void rollback_lock_file(struct lock_file *lk)
303{
304 delete_tempfile(&lk->tempfile);
305}
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306
307#endif /* LOCKFILE_H */