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