hold_locked_index(): align error handling with hold_lockfile_for_update()
[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 *
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).
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
72 * lockfile by calling `close_lock_file()`, and later call
73 * `commit_lock_file()`, `commit_lock_file_to()`,
74 * `rollback_lock_file()`, or `reopen_lock_file()`.
75 *
76 * Even after the lockfile is committed or rolled back, the
77 * `lock_file` object must not be freed or altered by the caller.
78 * However, it may be reused; just pass it to another call of
aae42e43 79 * `hold_lock_file_for_update()`.
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80 *
81 * If the program exits before `commit_lock_file()`,
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82 * `commit_lock_file_to()`, or `rollback_lock_file()` is called, the
83 * tempfile module will close and remove the lockfile, thereby rolling
84 * back any uncommitted changes.
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85 *
86 * If you need to close the file descriptor you obtained from a
87 * `hold_lock_file_for_*()` function yourself, do so by calling
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88 * `close_lock_file()`. See "tempfile.h" for more information.
89 *
90 *
91 * Under the covers, a lockfile is just a tempfile with a few helper
92 * functions. In particular, the state diagram and the cleanup
93 * machinery are all implemented in the tempfile module.
94 *
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95 *
96 * Error handling
97 * --------------
98 *
99 * The `hold_lock_file_for_*()` functions return a file descriptor on
100 * success or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see
101 * "flags" below). On errors, `errno` describes the reason for
102 * failure. Errors can be reported by passing `errno` to
103 * `unable_to_lock_message()` or `unable_to_lock_die()`.
104 *
105 * Similarly, `commit_lock_file`, `commit_lock_file_to`, and
106 * `close_lock_file` return 0 on success. On failure they set `errno`
107 * appropriately, do their best to roll back the lockfile, and return
108 * -1.
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109 */
110
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111#include "tempfile.h"
112
697cc8ef 113struct lock_file {
1a9d15db 114 struct tempfile tempfile;
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115};
116
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
132 * is already locked returns -1 to the caller.
133 */
697cc8ef 134#define LOCK_DIE_ON_ERROR 1
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135
136/*
137 * Usually symbolic links in the destination path are resolved. This
138 * means that (1) the lockfile is created by adding ".lock" to the
139 * resolved path, and (2) upon commit, the resolved path is
140 * overwritten. However, if `LOCK_NO_DEREF` is set, then the lockfile
141 * is created by adding ".lock" to the path argument itself. This
142 * option is used, for example, when detaching a symbolic reference,
143 * which for backwards-compatibility reasons, can be a symbolic link
144 * containing the name of the referred-to-reference.
145 */
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146#define LOCK_NO_DEREF 2
147
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148/*
149 * Attempt to create a lockfile for the file at `path` and return a
150 * file descriptor for writing to it, or -1 on error. If the file is
151 * currently locked, retry with quadratic backoff for at least
152 * timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
153 * timeout_ms is -1, retry indefinitely. The flags argument and error
154 * handling are described above.
155 */
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156extern int hold_lock_file_for_update_timeout(
157 struct lock_file *lk, const char *path,
158 int flags, long timeout_ms);
159
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160/*
161 * Attempt to create a lockfile for the file at `path` and return a
162 * file descriptor for writing to it, or -1 on error. The flags
163 * argument and error handling are described above.
164 */
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165static inline int hold_lock_file_for_update(
166 struct lock_file *lk, const char *path,
167 int flags)
168{
169 return hold_lock_file_for_update_timeout(lk, path, flags, 0);
170}
171
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172/*
173 * Append an appropriate error message to `buf` following the failure
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174 * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
175 * `errno` set by the failing call.
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176 */
177extern void unable_to_lock_message(const char *path, int err,
178 struct strbuf *buf);
044b6a9e 179
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180/*
181 * Emit an appropriate error message and `die()` following the failure
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182 * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
183 * `errno` set by the failing
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184 * call.
185 */
186extern NORETURN void unable_to_lock_die(const char *path, int err);
187
188/*
189 * Associate a stdio stream with the lockfile (which must still be
190 * open). Return `NULL` (*without* rolling back the lockfile) on
191 * error. The stream is closed automatically when `close_lock_file()`
192 * is called or when the file is committed or rolled back.
193 */
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194static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode)
195{
196 return fdopen_tempfile(&lk->tempfile, mode);
197}
2db69de8 198
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199/*
200 * Return the path of the lockfile. The return value is a pointer to a
201 * field within the lock_file object and should not be freed.
202 */
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203static inline const char *get_lock_file_path(struct lock_file *lk)
204{
205 return get_tempfile_path(&lk->tempfile);
206}
b4fb09e4 207
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208static inline int get_lock_file_fd(struct lock_file *lk)
209{
210 return get_tempfile_fd(&lk->tempfile);
211}
212
213static inline FILE *get_lock_file_fp(struct lock_file *lk)
214{
215 return get_tempfile_fp(&lk->tempfile);
216}
c99a4c2d 217
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218/*
219 * Return the path of the file that is locked by the specified
220 * lock_file object. The caller must free the memory.
221 */
222extern char *get_locked_file_path(struct lock_file *lk);
223
224/*
225 * If the lockfile is still open, close it (and the file pointer if it
226 * has been opened using `fdopen_lock_file()`) without renaming the
227 * lockfile over the file being locked. Return 0 upon success. On
228 * failure to `close(2)`, return a negative value and roll back the
229 * lock file. Usually `commit_lock_file()`, `commit_lock_file_to()`,
230 * or `rollback_lock_file()` should eventually be called if
231 * `close_lock_file()` succeeds.
232 */
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233static inline int close_lock_file(struct lock_file *lk)
234{
235 return close_tempfile(&lk->tempfile);
236}
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237
238/*
239 * Re-open a lockfile that has been closed using `close_lock_file()`
240 * but not yet committed or rolled back. This can be used to implement
241 * a sequence of operations like the following:
242 *
243 * * Lock file.
244 *
245 * * Write new contents to lockfile, then `close_lock_file()` to
246 * cause the contents to be written to disk.
247 *
248 * * Pass the name of the lockfile to another program to allow it (and
249 * nobody else) to inspect the contents you wrote, while still
250 * holding the lock yourself.
251 *
252 * * `reopen_lock_file()` to reopen the lockfile. Make further updates
253 * to the contents.
254 *
255 * * `commit_lock_file()` to make the final version permanent.
256 */
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257static inline int reopen_lock_file(struct lock_file *lk)
258{
259 return reopen_tempfile(&lk->tempfile);
260}
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261
262/*
263 * Commit the change represented by `lk`: close the file descriptor
264 * and/or file pointer if they are still open and rename the lockfile
265 * to its final destination. Return 0 upon success. On failure, roll
266 * back the lock file and return -1, with `errno` set to the value
267 * from the failing call to `close(2)` or `rename(2)`. It is a bug to
268 * call `commit_lock_file()` for a `lock_file` object that is not
269 * currently locked.
270 */
271extern int commit_lock_file(struct lock_file *lk);
272
273/*
274 * Like `commit_lock_file()`, but rename the lockfile to the provided
275 * `path`. `path` must be on the same filesystem as the lock file.
276 */
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277static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
278{
279 return rename_tempfile(&lk->tempfile, path);
280}
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281
282/*
283 * Roll back `lk`: close the file descriptor and/or file pointer and
284 * remove the lockfile. It is a NOOP to call `rollback_lock_file()`
285 * for a `lock_file` object that has already been committed or rolled
286 * back.
287 */
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288static inline void rollback_lock_file(struct lock_file *lk)
289{
290 delete_tempfile(&lk->tempfile);
291}
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292
293#endif /* LOCKFILE_H */