1lockfile API 2============ 3 4The lockfile API serves two purposes: 5 6* Mutual exclusion and atomic file updates. When we want to change a 7 file, we create a lockfile `<filename>.lock`, write the new file 8 contents into it, and then rename the lockfile to its final 9 destination `<filename>`. We create the `<filename>.lock` file with 10 `O_CREAT|O_EXCL` so that we can notice and fail if somebody else has 11 already locked the file, then atomically rename the lockfile to its 12 final destination to commit the changes and unlock the file. 13 14* Automatic cruft removal. If the program exits after we lock a file 15 but before the changes have been committed, we want to make sure 16 that we remove the lockfile. This is done by remembering the 17 lockfiles we have created in a linked list and setting up an 18 `atexit(3)` handler and a signal handler that clean up the 19 lockfiles. This mechanism ensures that outstanding lockfiles are 20 cleaned up if the program exits (including when `die()` is called) 21 or if the program dies on a signal. 22 23Please note that lockfiles only block other writers. Readers do not 24block, but they are guaranteed to see either the old contents of the 25file or the new contents of the file (assuming that the filesystem 26implements `rename(2)` atomically). 27 28 29Calling sequence 30---------------- 31 32The caller: 33 34* Allocates a `struct lock_file` either as a static variable or on the 35 heap, initialized to zeros. Once you use the structure to call the 36 `hold_lock_file_*` family of functions, it belongs to the lockfile 37 subsystem and its storage must remain valid throughout the life of 38 the program (i.e. you cannot use an on-stack variable to hold this 39 structure). 40 41* Attempts to create a lockfile by passing that variable and the path 42 of the final destination (e.g. `$GIT_DIR/index`) to 43 `hold_lock_file_for_update` or `hold_lock_file_for_append`. 44 45* Writes new content for the destination file by writing to the file 46 descriptor returned by those functions (also available via 47 `lock->fd`). 48 49When finished writing, the caller can: 50 51* Close the file descriptor and rename the lockfile to its final 52 destination by calling `commit_lock_file`. 53 54* Close the file descriptor and remove the lockfile by calling 55 `rollback_lock_file`. 56 57* Close the file descriptor without removing or renaming the lockfile 58 by calling `close_lock_file`, and later call `commit_lock_file`, 59 `rollback_lock_file`, or `reopen_lock_file`. 60 61Even after the lockfile is committed or rolled back, the `lock_file` 62object must not be freed or altered by the caller. However, it may be 63reused; just pass it to another call of `hold_lock_file_for_update` or 64`hold_lock_file_for_append`. 65 66If the program exits before you have called one of `commit_lock_file`, 67`rollback_lock_file`, or `close_lock_file`, an `atexit(3)` handler 68will close and remove the lockfile, rolling back any uncommitted 69changes. 70 71If you need to close the file descriptor you obtained from a 72`hold_lock_file_*` function yourself, do so by calling 73`close_lock_file`. You should never call `close(2)` yourself! 74Otherwise the `struct lock_file` structure would still think that the 75file descriptor needs to be closed, and a later call to 76`commit_lock_file` or `rollback_lock_file` or program exit would 77result in duplicate calls to `close(2)`. Worse yet, if you `close(2)` 78and then later open another file descriptor for a completely different 79purpose, then a call to `commit_lock_file` or `rollback_lock_file` 80might close that unrelated file descriptor. 81 82 83Error handling 84-------------- 85 86The `hold_lock_file_*` functions return a file descriptor on success 87or -1 on failure (unless `LOCK_DIE_ON_ERROR` is used; see below). On 88errors, `errno` describes the reason for failure. Errors can be 89reported by passing `errno` to one of the following helper functions: 90 91unable_to_lock_message:: 92 93 Append an appropriate error message to a `strbuf`. 94 95unable_to_lock_error:: 96 97 Emit an appropriate error message using `error()`. 98 99unable_to_lock_die:: 100 101 Emit an appropriate error message and `die()`. 102 103 104Flags 105----- 106 107The following flags can be passed to `hold_lock_file_for_update` or 108`hold_lock_file_for_append`: 109 110LOCK_NODEREF:: 111 112 Usually symbolic links in the destination path are resolved 113 and the lockfile is created by adding ".lock" to the resolved 114 path. If `LOCK_NODEREF` is set, then the lockfile is created 115 by adding ".lock" to the path argument itself. This option is 116 used, for example, when locking a symbolic reference, which 117 for backwards-compatibility reasons can be a symbolic link 118 containing the name of the referred-to-reference. 119 120LOCK_DIE_ON_ERROR:: 121 122 If a lock is already taken for the file, `die()` with an error 123 message. If this option is not specified, trying to lock a 124 file that is already locked returns -1 to the caller. 125 126 127The functions 128------------- 129 130hold_lock_file_for_update:: 131 132 Take a pointer to `struct lock_file`, the path of the file to 133 be locked (e.g. `$GIT_DIR/index`) and a flags argument (see 134 above). Attempt to create a lockfile for the destination and 135 return the file descriptor for writing to the file. 136 137hold_lock_file_for_append:: 138 139 Like `hold_lock_file_for_update`, but before returning copy 140 the existing contents of the file (if any) to the lockfile and 141 position its write pointer at the end of the file. 142 143commit_lock_file:: 144 145 Take a pointer to the `struct lock_file` initialized with an 146 earlier call to `hold_lock_file_for_update` or 147 `hold_lock_file_for_append`, close the file descriptor and 148 rename the lockfile to its final destination. Return 0 upon 149 success or a negative value on failure to `close(2)` or 150 `rename(2)`. 151 152rollback_lock_file:: 153 154 Take a pointer to the `struct lock_file` initialized with an 155 earlier call to `hold_lock_file_for_update` or 156 `hold_lock_file_for_append`, close the file descriptor and 157 remove the lockfile. 158 159close_lock_file:: 160 161 Take a pointer to the `struct lock_file` initialized with an 162 earlier call to `hold_lock_file_for_update` or 163 `hold_lock_file_for_append`, and close the file descriptor. 164 Return 0 upon success or a negative value on failure to 165 close(2). Usually `commit_lock_file` or `rollback_lock_file` 166 should be called after `close_lock_file`. 167 168reopen_lock_file:: 169 170 Re-open a lockfile that has been closed (using 171 `close_lock_file`) but not yet committed or rolled back. This 172 can be used to implement a sequence of operations like the 173 following: 174 175 * Lock file. 176 177 * Write new contents to lockfile, then `close_lock_file` to 178 cause the contents to be written to disk. 179 180 * Pass the name of the lockfile to another program to allow it 181 (and nobody else) to inspect the contents you wrote, while 182 still holding the lock yourself. 183 184 * `reopen_lock_file` to reopen the lockfile. Make further 185 updates to the contents. 186 187 * `commit_lock_file` to make the final version permanent.