tempfile.h

   1 #ifndef TEMPFILE_H
   2 #define TEMPFILE_H
   3
   4 /*
   5  * Handle temporary files.
   6  *
   7  * The tempfile API allows temporary files to be created, deleted, and
   8  * atomically renamed. Temporary files that are still active when the
   9  * program ends are cleaned up automatically. Lockfiles (see
  10  * "lockfile.h") are built on top of this API.
  11  *
  12  *
  13  * Calling sequence
  14  * ----------------
  15  *
  16  * The caller:
  17  *
  18  * * Allocates a `struct tempfile` either as a static variable or on
  19  *   the heap, initialized to zeros. Once you use the structure to
  20  *   call `create_tempfile()`, it belongs to the tempfile subsystem
  21  *   and its storage must remain valid throughout the life of the
  22  *   program (i.e. you cannot use an on-stack variable to hold this
  23  *   structure).
  24  *
  25  * * Attempts to create a temporary file by calling
  26  *   `create_tempfile()`.
  27  *
  28  * * Writes new content to the file by either:
  29  *
  30  *   * writing to the file descriptor returned by `create_tempfile()`
  31  *     (also available via `tempfile->fd`).
  32  *
  33  *   * calling `fdopen_tempfile()` to get a `FILE` pointer for the
  34  *     open file and writing to the file using stdio.
  35  *
  36  *   Note that the file descriptor returned by create_tempfile()
  37  *   is marked O_CLOEXEC, so the new contents must be written by
  38  *   the current process, not any spawned one.
  39  *
  40  * When finished writing, the caller can:
  41  *
  42  * * Close the file descriptor and remove the temporary file by
  43  *   calling `delete_tempfile()`.
  44  *
  45  * * Close the temporary file and rename it atomically to a specified
  46  *   filename by calling `rename_tempfile()`. This relinquishes
  47  *   control of the file.
  48  *
  49  * * Close the file descriptor without removing or renaming the
  50  *   temporary file by calling `close_tempfile()`, and later call
  51  *   `delete_tempfile()` or `rename_tempfile()`.
  52  *
  53  * Even after the temporary file is renamed or deleted, the `tempfile`
  54  * object must not be freed or altered by the caller. However, it may
  55  * be reused; just pass it to another call of `create_tempfile()`.
  56  *
  57  * If the program exits before `rename_tempfile()` or
  58  * `delete_tempfile()` is called, an `atexit(3)` handler will close
  59  * and remove the temporary file.
  60  *
  61  * If you need to close the file descriptor yourself, do so by calling
  62  * `close_tempfile()`. You should never call `close(2)` or `fclose(3)`
  63  * yourself, otherwise the `struct tempfile` structure would still
  64  * think that the file descriptor needs to be closed, and a later
  65  * cleanup would result in duplicate calls to `close(2)`. Worse yet,
  66  * if you close and then later open another file descriptor for a
  67  * completely different purpose, then the unrelated file descriptor
  68  * might get closed.
  69  *
  70  *
  71  * Error handling
  72  * --------------
  73  *
  74  * `create_tempfile()` returns a file descriptor on success or -1 on
  75  * failure. On errors, `errno` describes the reason for failure.
  76  *
  77  * `delete_tempfile()`, `rename_tempfile()`, and `close_tempfile()`
  78  * return 0 on success. On failure they set `errno` appropriately, do
  79  * their best to delete the temporary file, and return -1.
  80  */
  81
  82 struct tempfile {
  83         struct tempfile *volatile next;
  84         volatile sig_atomic_t active;
  85         volatile int fd;
  86         FILE *volatile fp;
  87         volatile pid_t owner;
  88         char on_list;
  89         struct strbuf filename;
  90 };
  91
  92 /*
  93  * Attempt to create a temporary file at the specified `path`. Return
  94  * a file descriptor for writing to it, or -1 on error. It is an error
  95  * if a file already exists at that path.
  96  */
  97 extern int create_tempfile(struct tempfile *tempfile, const char *path);
  98
  99 /*
 100  * Register an existing file as a tempfile, meaning that it will be
 101  * deleted when the program exits. The tempfile is considered closed,
 102  * but it can be worked with like any other closed tempfile (for
 103  * example, it can be opened using reopen_tempfile()).
 104  */
 105 extern void register_tempfile(struct tempfile *tempfile, const char *path);
 106
 107
 108 /*
 109  * mks_tempfile functions
 110  *
 111  * The following functions attempt to create and open temporary files
 112  * with names derived automatically from a template, in the manner of
 113  * mkstemps(), and arrange for them to be deleted if the program ends
 114  * before they are deleted explicitly. There is a whole family of such
 115  * functions, named according to the following pattern:
 116  *
 117  *     x?mks_tempfile_t?s?m?()
 118  *
 119  * The optional letters have the following meanings:
 120  *
 121  *   x - die if the temporary file cannot be created.
 122  *
 123  *   t - create the temporary file under $TMPDIR (as opposed to
 124  *       relative to the current directory). When these variants are
 125  *       used, template should be the pattern for the filename alone,
 126  *       without a path.
 127  *
 128  *   s - template includes a suffix that is suffixlen characters long.
 129  *
 130  *   m - the temporary file should be created with the specified mode
 131  *       (otherwise, the mode is set to 0600).
 132  *
 133  * None of these functions modify template. If the caller wants to
 134  * know the (absolute) path of the file that was created, it can be
 135  * read from tempfile->filename.
 136  *
 137  * On success, the functions return a file descriptor that is open for
 138  * writing the temporary file. On errors, they return -1 and set errno
 139  * appropriately (except for the "x" variants, which die() on errors).
 140  */
 141
 142 /* See "mks_tempfile functions" above. */
 143 extern int mks_tempfile_sm(struct tempfile *tempfile,
 144                            const char *template, int suffixlen, int mode);
 145
 146 /* See "mks_tempfile functions" above. */
 147 static inline int mks_tempfile_s(struct tempfile *tempfile,
 148                                  const char *template, int suffixlen)
 149 {
 150         return mks_tempfile_sm(tempfile, template, suffixlen, 0600);
 151 }
 152
 153 /* See "mks_tempfile functions" above. */
 154 static inline int mks_tempfile_m(struct tempfile *tempfile,
 155                                  const char *template, int mode)
 156 {
 157         return mks_tempfile_sm(tempfile, template, 0, mode);
 158 }
 159
 160 /* See "mks_tempfile functions" above. */
 161 static inline int mks_tempfile(struct tempfile *tempfile,
 162                                const char *template)
 163 {
 164         return mks_tempfile_sm(tempfile, template, 0, 0600);
 165 }
 166
 167 /* See "mks_tempfile functions" above. */
 168 extern int mks_tempfile_tsm(struct tempfile *tempfile,
 169                             const char *template, int suffixlen, int mode);
 170
 171 /* See "mks_tempfile functions" above. */
 172 static inline int mks_tempfile_ts(struct tempfile *tempfile,
 173                                   const char *template, int suffixlen)
 174 {
 175         return mks_tempfile_tsm(tempfile, template, suffixlen, 0600);
 176 }
 177
 178 /* See "mks_tempfile functions" above. */
 179 static inline int mks_tempfile_tm(struct tempfile *tempfile,
 180                                   const char *template, int mode)
 181 {
 182         return mks_tempfile_tsm(tempfile, template, 0, mode);
 183 }
 184
 185 /* See "mks_tempfile functions" above. */
 186 static inline int mks_tempfile_t(struct tempfile *tempfile,
 187                                  const char *template)
 188 {
 189         return mks_tempfile_tsm(tempfile, template, 0, 0600);
 190 }
 191
 192 /* See "mks_tempfile functions" above. */
 193 extern int xmks_tempfile_m(struct tempfile *tempfile,
 194                            const char *template, int mode);
 195
 196 /* See "mks_tempfile functions" above. */
 197 static inline int xmks_tempfile(struct tempfile *tempfile,
 198                                 const char *template)
 199 {
 200         return xmks_tempfile_m(tempfile, template, 0600);
 201 }
 202
 203 /*
 204  * Associate a stdio stream with the temporary file (which must still
 205  * be open). Return `NULL` (*without* deleting the file) on error. The
 206  * stream is closed automatically when `close_tempfile()` is called or
 207  * when the file is deleted or renamed.
 208  */
 209 extern FILE *fdopen_tempfile(struct tempfile *tempfile, const char *mode);
 210
 211 static inline int is_tempfile_active(struct tempfile *tempfile)
 212 {
 213         return tempfile->active;
 214 }
 215
 216 /*
 217  * Return the path of the lockfile. The return value is a pointer to a
 218  * field within the lock_file object and should not be freed.
 219  */
 220 extern const char *get_tempfile_path(struct tempfile *tempfile);
 221
 222 extern int get_tempfile_fd(struct tempfile *tempfile);
 223 extern FILE *get_tempfile_fp(struct tempfile *tempfile);
 224
 225 /*
 226  * If the temporary file is still open, close it (and the file pointer
 227  * too, if it has been opened using `fdopen_tempfile()`) without
 228  * deleting the file. Return 0 upon success. On failure to `close(2)`,
 229  * return a negative value and delete the file. Usually
 230  * `delete_tempfile()` or `rename_tempfile()` should eventually be
 231  * called if `close_tempfile()` succeeds.
 232  */
 233 extern int close_tempfile(struct tempfile *tempfile);
 234
 235 /*
 236  * Re-open a temporary file that has been closed using
 237  * `close_tempfile()` but not yet deleted or renamed. This can be used
 238  * to implement a sequence of operations like the following:
 239  *
 240  * * Create temporary file.
 241  *
 242  * * Write new contents to file, then `close_tempfile()` to cause the
 243  *   contents to be written to disk.
 244  *
 245  * * Pass the name of the temporary file to another program to allow
 246  *   it (and nobody else) to inspect or even modify the file's
 247  *   contents.
 248  *
 249  * * `reopen_tempfile()` to reopen the temporary file. Make further
 250  *   updates to the contents.
 251  *
 252  * * `rename_tempfile()` to move the file to its permanent location.
 253  */
 254 extern int reopen_tempfile(struct tempfile *tempfile);
 255
 256 /*
 257  * Close the file descriptor and/or file pointer and remove the
 258  * temporary file associated with `tempfile`. It is a NOOP to call
 259  * `delete_tempfile()` for a `tempfile` object that has already been
 260  * deleted or renamed.
 261  */
 262 extern void delete_tempfile(struct tempfile *tempfile);
 263
 264 /*
 265  * Close the file descriptor and/or file pointer if they are still
 266  * open, and atomically rename the temporary file to `path`. `path`
 267  * must be on the same filesystem as the lock file. Return 0 on
 268  * success. On failure, delete the temporary file and return -1, with
 269  * `errno` set to the value from the failing call to `close(2)` or
 270  * `rename(2)`. It is a bug to call `rename_tempfile()` for a
 271  * `tempfile` object that is not currently active.
 272  */
 273 extern int rename_tempfile(struct tempfile *tempfile, const char *path);
 274
 275 #endif /* TEMPFILE_H */