src/temp-stream.c

   1 /* temp-stream.c -- provide a stream to a per process temp file
   2
   3    Copyright (C) 2023-2024 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation, either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 #include <config.h>
  19
  20 #include <stdbool.h>
  21 #include <stdio.h>
  22
  23 #include "stdlib--.h"  /* For mkstemp that returns safer FDs.  */
  24 #include "system.h"
  25 #include "tmpdir.h"
  26
  27 #include "temp-stream.h"
  28
  29
  30 #if defined __MSDOS__ || defined _WIN32
  31 /* Define this to non-zero on systems for which the regular mechanism
  32    (of unlinking an open file and expecting to be able to write, seek
  33    back to the beginning, then reread it) doesn't work.  E.g., on Windows
  34    and DOS systems.  */
  35 # define DONT_UNLINK_WHILE_OPEN 1
  36 #endif
  37
  38 #if DONT_UNLINK_WHILE_OPEN
  39
  40 /* FIXME-someday: remove all of this DONT_UNLINK_WHILE_OPEN junk.
  41    Using atexit like this is wrong, since it can fail
  42    when called e.g. 32 or more times.
  43    But this isn't a big deal, since the code is used only on WOE/DOS
  44    systems, and few people invoke tac on that many nonseekable files.  */
  45
  46 static char const *file_to_remove;
  47 static FILE *fp_to_close;
  48
  49 static void
  50 unlink_tempfile (void)
  51 {
  52   fclose (fp_to_close);
  53   unlink (file_to_remove);
  54 }
  55
  56 static void
  57 record_or_unlink_tempfile (char const *fn, FILE *fp)
  58 {
  59   if (!file_to_remove)
  60     {
  61       file_to_remove = fn;
  62       fp_to_close = fp;
  63       atexit (unlink_tempfile);
  64     }
  65 }
  66
  67 #else
  68
  69 static void
  70 record_or_unlink_tempfile (char const *fn, MAYBE_UNUSED FILE *fp)
  71 {
  72   unlink (fn);
  73 }
  74
  75 #endif
  76
  77 /* A wrapper around mkstemp that gives us both an open stream pointer,
  78    FP, and the corresponding FILE_NAME.  Always return the same FP/name
  79    pair, rewinding/truncating it upon each reuse.
  80
  81    Note this honors $TMPDIR, unlike the standard defined tmpfile().
  82
  83    Returns TRUE on success.  */
  84 bool
  85 temp_stream (FILE **fp, char **file_name)
  86 {
  87   static char *tempfile = nullptr;
  88   static FILE *tmp_fp;
  89   if (tempfile == nullptr)
  90     {
  91       char *tempbuf = nullptr;
  92       size_t tempbuf_len = 128;
  93
  94       while (true)
  95         {
  96           if (! (tempbuf = realloc (tempbuf, tempbuf_len)))
  97             {
  98               error (0, errno, _("failed to make temporary file name"));
  99               return false;
 100             }
 101
 102           if (path_search (tempbuf, tempbuf_len, nullptr, "cutmp", true) == 0)
 103             break;
 104
 105           if (errno != EINVAL || PATH_MAX / 2 < tempbuf_len)
 106             {
 107               error (0, errno == EINVAL ? ENAMETOOLONG : errno,
 108                      _("failed to make temporary file name"));
 109               return false;
 110             }
 111
 112           tempbuf_len *= 2;
 113         }
 114
 115       tempfile = tempbuf;
 116
 117       /* FIXME: there's a small window between a successful mkstemp call
 118          and the unlink that's performed by record_or_unlink_tempfile.
 119          If we're interrupted in that interval, this code fails to remove
 120          the temporary file.  On systems that define DONT_UNLINK_WHILE_OPEN,
 121          the window is much larger -- it extends to the atexit-called
 122          unlink_tempfile.
 123          FIXME: clean up upon fatal signal.  Don't block them, in case
 124          $TMPDIR is a remote file system.  */
 125
 126       int fd = mkstemp (tempfile);
 127       if (fd < 0)
 128         {
 129           error (0, errno, _("failed to create temporary file %s"),
 130                  quoteaf (tempfile));
 131           goto Reset;
 132         }
 133
 134       tmp_fp = fdopen (fd, (O_BINARY ? "w+b" : "w+"));
 135       if (! tmp_fp)
 136         {
 137           error (0, errno, _("failed to open %s for writing"),
 138                  quoteaf (tempfile));
 139           close (fd);
 140           unlink (tempfile);
 141         Reset:
 142           free (tempfile);
 143           tempfile = nullptr;
 144           return false;
 145         }
 146
 147       record_or_unlink_tempfile (tempfile, tmp_fp);
 148     }
 149   else
 150     {
 151       clearerr (tmp_fp);
 152       if (fseeko (tmp_fp, 0, SEEK_SET) < 0
 153           || ftruncate (fileno (tmp_fp), 0) < 0)
 154         {
 155           error (0, errno, _("failed to rewind stream for %s"),
 156                  quoteaf (tempfile));
 157           return false;
 158         }
 159     }
 160
 161   *fp = tmp_fp;
 162   if (file_name)
 163     *file_name = tempfile;
 164   return true;
 165 }