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 <stdio.h>
  21
  22 #include "stdlib--.h"  /* For mkstemp that returns safer FDs.  */
  23 #include "system.h"
  24 #include "tmpdir.h"
  25
  26 #include "temp-stream.h"
  27
  28
  29 #if defined __MSDOS__ || defined _WIN32
  30 /* Define this to non-zero on systems for which the regular mechanism
  31    (of unlinking an open file and expecting to be able to write, seek
  32    back to the beginning, then reread it) doesn't work.  E.g., on Windows
  33    and DOS systems.  */
  34 # define DONT_UNLINK_WHILE_OPEN 1
  35 #endif
  36
  37 #if DONT_UNLINK_WHILE_OPEN
  38
  39 /* FIXME-someday: remove all of this DONT_UNLINK_WHILE_OPEN junk.
  40    Using atexit like this is wrong, since it can fail
  41    when called e.g. 32 or more times.
  42    But this isn't a big deal, since the code is used only on WOE/DOS
  43    systems, and few people invoke tac on that many nonseekable files.  */
  44
  45 static char const *file_to_remove;
  46 static FILE *fp_to_close;
  47
  48 static void
  49 unlink_tempfile (void)
  50 {
  51   fclose (fp_to_close);
  52   unlink (file_to_remove);
  53 }
  54
  55 static void
  56 record_or_unlink_tempfile (char const *fn, FILE *fp)
  57 {
  58   if (!file_to_remove)
  59     {
  60       file_to_remove = fn;
  61       fp_to_close = fp;
  62       atexit (unlink_tempfile);
  63     }
  64 }
  65
  66 #else
  67
  68 static void
  69 record_or_unlink_tempfile (char const *fn, MAYBE_UNUSED FILE *fp)
  70 {
  71   unlink (fn);
  72 }
  73
  74 #endif
  75
  76 /* A wrapper around mkstemp that gives us both an open stream pointer,
  77    FP, and the corresponding FILE_NAME.  Always return the same FP/name
  78    pair, rewinding/truncating it upon each reuse.
  79
  80    Note this honors $TMPDIR, unlike the standard defined tmpfile().
  81
  82    Returns TRUE on success.  */
  83 bool
  84 temp_stream (FILE **fp, char **file_name)
  85 {
  86   static char *tempfile = nullptr;
  87   static FILE *tmp_fp;
  88   if (tempfile == nullptr)
  89     {
  90       char *tempbuf = nullptr;
  91       size_t tempbuf_len = 128;
  92
  93       while (true)
  94         {
  95           if (! (tempbuf = realloc (tempbuf, tempbuf_len)))
  96             {
  97               error (0, errno, _("failed to make temporary file name"));
  98               return false;
  99             }
 100
 101           if (path_search (tempbuf, tempbuf_len, nullptr, "cutmp", true) == 0)
 102             break;
 103
 104           if (errno != EINVAL || PATH_MAX / 2 < tempbuf_len)
 105             {
 106               error (0, errno == EINVAL ? ENAMETOOLONG : errno,
 107                      _("failed to make temporary file name"));
 108               return false;
 109             }
 110
 111           tempbuf_len *= 2;
 112         }
 113
 114       tempfile = tempbuf;
 115
 116       /* FIXME: there's a small window between a successful mkstemp call
 117          and the unlink that's performed by record_or_unlink_tempfile.
 118          If we're interrupted in that interval, this code fails to remove
 119          the temporary file.  On systems that define DONT_UNLINK_WHILE_OPEN,
 120          the window is much larger -- it extends to the atexit-called
 121          unlink_tempfile.
 122          FIXME: clean up upon fatal signal.  Don't block them, in case
 123          $TMPDIR is a remote file system.  */
 124
 125       int fd = mkstemp (tempfile);
 126       if (fd < 0)
 127         {
 128           error (0, errno, _("failed to create temporary file %s"),
 129                  quoteaf (tempfile));
 130           goto Reset;
 131         }
 132
 133       tmp_fp = fdopen (fd, (O_BINARY ? "w+b" : "w+"));
 134       if (! tmp_fp)
 135         {
 136           error (0, errno, _("failed to open %s for writing"),
 137                  quoteaf (tempfile));
 138           close (fd);
 139           unlink (tempfile);
 140         Reset:
 141           free (tempfile);
 142           tempfile = nullptr;
 143           return false;
 144         }
 145
 146       record_or_unlink_tempfile (tempfile, tmp_fp);
 147     }
 148   else
 149     {
 150       clearerr (tmp_fp);
 151       if (fseeko (tmp_fp, 0, SEEK_SET) < 0
 152           || ftruncate (fileno (tmp_fp), 0) < 0)
 153         {
 154           error (0, errno, _("failed to rewind stream for %s"),
 155                  quoteaf (tempfile));
 156           return false;
 157         }
 158     }
 159
 160   *fp = tmp_fp;
 161   if (file_name)
 162     *file_name = tempfile;
 163   return true;
 164 }