lockfile.h

   1 #ifndef LOCKFILE_H
   2 #define LOCKFILE_H
   3
   4 /*
   5  * File write-locks as used by Git.
   6  *
   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  *
  32  * Most of the heavy lifting is done by the tempfile module (see
  33  * "tempfile.h").
  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  *
  47  * * Attempts to create a lockfile by calling `hold_lock_file_for_update()`.
  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  *
  58  * When finished writing, the caller can:
  59  *
  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()`.
  63  *
  64  * * Close the file descriptor and remove the lockfile by calling
  65  *   `rollback_lock_file()`.
  66  *
  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()`.
  71  *
  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()`.
  76  *
  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.
  81  *
  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.
  85  *
  86  *
  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.
  90  *
  91  *
  92  * Error handling
  93  * --------------
  94  *
  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()`.
 100  *
 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.
 105  */
 106
 107 #include "tempfile.h"
 108
 109 struct lock_file {
 110         struct tempfile tempfile;
 111 };
 112
 113 /* String appended to a filename to derive the lockfile name: */
 114 #define LOCK_SUFFIX ".lock"
 115 #define LOCK_SUFFIX_LEN 5
 116
 117
 118 /*
 119  * Flags
 120  * -----
 121  *
 122  * The following flags can be passed to `hold_lock_file_for_update()`.
 123  */
 124
 125 /*
 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.
 129  */
 130 #define LOCK_DIE_ON_ERROR 1
 131
 132 /*
 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.
 141  */
 142 #define LOCK_NO_DEREF 2
 143
 144 /*
 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.
 151  */
 152 extern int hold_lock_file_for_update_timeout(
 153                 struct lock_file *lk, const char *path,
 154                 int flags, long timeout_ms);
 155
 156 /*
 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.
 160  */
 161 static inline int hold_lock_file_for_update(
 162                 struct lock_file *lk, const char *path,
 163                 int flags)
 164 {
 165         return hold_lock_file_for_update_timeout(lk, path, flags, 0);
 166 }
 167
 168 /*
 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.
 172  */
 173 extern void unable_to_lock_message(const char *path, int err,
 174                                    struct strbuf *buf);
 175
 176 /*
 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.
 181  */
 182 extern NORETURN void unable_to_lock_die(const char *path, int err);
 183
 184 /*
 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.
 189  */
 190 static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode)
 191 {
 192         return fdopen_tempfile(&lk->tempfile, mode);
 193 }
 194
 195 /*
 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.
 198  */
 199 static inline const char *get_lock_file_path(struct lock_file *lk)
 200 {
 201         return get_tempfile_path(&lk->tempfile);
 202 }
 203
 204 static inline int get_lock_file_fd(struct lock_file *lk)
 205 {
 206         return get_tempfile_fd(&lk->tempfile);
 207 }
 208
 209 static inline FILE *get_lock_file_fp(struct lock_file *lk)
 210 {
 211         return get_tempfile_fp(&lk->tempfile);
 212 }
 213
 214 /*
 215  * Return the path of the file that is locked by the specified
 216  * lock_file object. The caller must free the memory.
 217  */
 218 extern char *get_locked_file_path(struct lock_file *lk);
 219
 220 /*
 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.
 228  */
 229 static inline int close_lock_file(struct lock_file *lk)
 230 {
 231         return close_tempfile(&lk->tempfile);
 232 }
 233
 234 /*
 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:
 238  *
 239  * * Lock file.
 240  *
 241  * * Write new contents to lockfile, then `close_lock_file()` to
 242  *   cause the contents to be written to disk.
 243  *
 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.
 247  *
 248  * * `reopen_lock_file()` to reopen the lockfile. Make further updates
 249  *   to the contents.
 250  *
 251  * * `commit_lock_file()` to make the final version permanent.
 252  */
 253 static inline int reopen_lock_file(struct lock_file *lk)
 254 {
 255         return reopen_tempfile(&lk->tempfile);
 256 }
 257
 258 /*
 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.
 266  */
 267 extern int commit_lock_file(struct lock_file *lk);
 268
 269 /*
 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.
 272  */
 273 static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
 274 {
 275         return rename_tempfile(&lk->tempfile, path);
 276 }
 277
 278 /*
 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.
 283  */
 284 static inline void rollback_lock_file(struct lock_file *lk)
 285 {
 286         delete_tempfile(&lk->tempfile);
 287 }
 288
 289 #endif /* LOCKFILE_H */