tempfile.h

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  50  * object is no longer valid and should not be reused.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