diff options
Diffstat (limited to 'lockfile.h')
| -rw-r--r-- | lockfile.h | 84 |
1 files changed, 51 insertions, 33 deletions
diff --git a/lockfile.h b/lockfile.h index 3d301937b0..35403ccc0d 100644 --- a/lockfile.h +++ b/lockfile.h @@ -37,12 +37,12 @@ * * 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_for_*()` 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). + * * Allocates a `struct lock_file` with whatever storage duration you + * desire. The struct does not have to be initialized before being + * used, but it is good practice to do so using by setting it to + * all-zeros (or using the LOCK_INIT macro). This puts the object in a + * consistent state that allows you to call rollback_lock_file() even + * if the lock was never taken (in which case it is a noop). * * * Attempts to create a lockfile by calling `hold_lock_file_for_update()`. * @@ -55,6 +55,10 @@ * * calling `fdopen_lock_file()` to get a `FILE` pointer for the * open file and writing to the file using stdio. * + * Note that the file descriptor returned by hold_lock_file_for_update() + * is marked O_CLOEXEC, so the new contents must be written by the + * current process, not a spawned one. + * * When finished writing, the caller can: * * * Close the file descriptor and rename the lockfile to its final @@ -65,14 +69,12 @@ * `rollback_lock_file()`. * * * Close the file descriptor without removing or renaming the - * lockfile by calling `close_lock_file()`, and later call + * lockfile by calling `close_lock_file_gently()`, 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()`. + * After the lockfile is committed or rolled back, the `lock_file` + * object can be discarded or reused. * * If the program exits before `commit_lock_file()`, * `commit_lock_file_to()`, or `rollback_lock_file()` is called, the @@ -81,7 +83,7 @@ * * If you need to close the file descriptor you obtained from a * `hold_lock_file_for_*()` function yourself, do so by calling - * `close_lock_file()`. See "tempfile.h" for more information. + * `close_lock_file_gently()`. See "tempfile.h" for more information. * * * Under the covers, a lockfile is just a tempfile with a few helper @@ -100,16 +102,18 @@ * * 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. + * appropriately and return -1. The `commit` variants (but not `close`) + * do their best to delete the temporary file before returning. */ #include "tempfile.h" struct lock_file { - struct tempfile tempfile; + struct tempfile *tempfile; }; +#define LOCK_INIT { NULL } + /* String appended to a filename to derive the lockfile name: */ #define LOCK_SUFFIX ".lock" #define LOCK_SUFFIX_LEN 5 @@ -125,11 +129,17 @@ struct lock_file { /* * If a lock is already taken for the file, `die()` with an error * message. If this flag is not specified, trying to lock a file that - * is already locked returns -1 to the caller. + * is already locked silently returns -1 to the caller, or ... */ #define LOCK_DIE_ON_ERROR 1 /* + * ... this flag can be passed instead to return -1 and give the usual + * error message upon an error. + */ +#define LOCK_REPORT_ON_ERROR 4 + +/* * Usually symbolic links in the destination path are resolved. This * means that (1) the lockfile is created by adding ".lock" to the * resolved path, and (2) upon commit, the resolved path is @@ -166,6 +176,14 @@ static inline int hold_lock_file_for_update( } /* + * Return a nonzero value iff `lk` is currently locked. + */ +static inline int is_lock_file_locked(struct lock_file *lk) +{ + return is_tempfile_active(lk->tempfile); +} + +/* * Append an appropriate error message to `buf` following the failure * of `hold_lock_file_for_update()` to lock `path`. `err` should be the * `errno` set by the failing call. @@ -184,12 +202,13 @@ extern NORETURN void unable_to_lock_die(const char *path, int err); /* * Associate a stdio stream with the lockfile (which must still be * open). 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. + * error. The stream is closed automatically when + * `close_lock_file_gently()` is called or when the file is committed or + * rolled back. */ static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode) { - return fdopen_tempfile(&lk->tempfile, mode); + return fdopen_tempfile(lk->tempfile, mode); } /* @@ -198,17 +217,17 @@ static inline FILE *fdopen_lock_file(struct lock_file *lk, const char *mode) */ static inline const char *get_lock_file_path(struct lock_file *lk) { - return get_tempfile_path(&lk->tempfile); + return get_tempfile_path(lk->tempfile); } static inline int get_lock_file_fd(struct lock_file *lk) { - return get_tempfile_fd(&lk->tempfile); + return get_tempfile_fd(lk->tempfile); } static inline FILE *get_lock_file_fp(struct lock_file *lk) { - return get_tempfile_fp(&lk->tempfile); + return get_tempfile_fp(lk->tempfile); } /* @@ -221,38 +240,37 @@ extern char *get_locked_file_path(struct lock_file *lk); * If the lockfile is still open, close it (and the file pointer if it * has been opened using `fdopen_lock_file()`) without renaming the * lockfile over the file being locked. 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. + * failure to `close(2)`, return a negative value (the lockfile is not + * rolled back). Usually `commit_lock_file()`, `commit_lock_file_to()`, + * or `rollback_lock_file()` should eventually be called. */ -static inline int close_lock_file(struct lock_file *lk) +static inline int close_lock_file_gently(struct lock_file *lk) { - return close_tempfile(&lk->tempfile); + return close_tempfile_gently(lk->tempfile); } /* - * Re-open a lockfile that has been closed using `close_lock_file()` + * Re-open a lockfile that has been closed using `close_lock_file_gently()` * 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 + * * Write new contents to lockfile, then `close_lock_file_gently()` 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. + * * `reopen_lock_file()` to reopen the lockfile, truncating the existing + * contents. Write out the new contents. * * * `commit_lock_file()` to make the final version permanent. */ static inline int reopen_lock_file(struct lock_file *lk) { - return reopen_tempfile(&lk->tempfile); + return reopen_tempfile(lk->tempfile); } /* |