cat: allow copying empty files to themselves

Problem reported by Vincent Lefevre in: http://bugs.gnu.org/18449
* src/cat.c (main): Allow copying an empty file to itself.
* tests/misc/cat-self.sh: New test.
* tests/local.mk (all_tests): Add it.

src/cat.c

@@ -527,8 +527,8 @@ main (int argc, char **argv)
   /* I-node number of the output.  */
   ino_t out_ino;
 
-  /* True if the output file should not be the same as any input file.  */
-  bool check_redirection = true;
+  /* True if the output is a regular file.  */
+  bool out_isreg;
 
   /* Nonzero if we have ever read standard input.  */
   bool have_read_stdin = false;
@@ -637,25 +637,9 @@ main (int argc, char **argv)
     error (EXIT_FAILURE, errno, _("standard output"));
 
   outsize = io_blksize (stat_buf);
-  /* Input file can be output file for non-regular files.
-     fstat on pipes returns S_IFSOCK on some systems, S_IFIFO
-     on others, so the checking should not be done for those types,
-     and to allow things like cat < /dev/tty > /dev/tty, checking
-     is not done for device files either.  */
-
-  if (S_ISREG (stat_buf.st_mode))
-    {
-      out_dev = stat_buf.st_dev;
-      out_ino = stat_buf.st_ino;
-    }
-  else
-    {
-      check_redirection = false;
-#ifdef lint  /* Suppress 'used before initialized' warning.  */
-      out_dev = 0;
-      out_ino = 0;
-#endif
-    }
+  out_dev = stat_buf.st_dev;
+  out_ino = stat_buf.st_ino;
+  out_isreg = S_ISREG (stat_buf.st_mode) != 0;
 
   if (! (number || show_ends || squeeze_blank))
     {
@@ -704,14 +688,13 @@ main (int argc, char **argv)
 
       fdadvise (input_desc, 0, 0, FADVISE_SEQUENTIAL);
 
-      /* Compare the device and i-node numbers of this input file with
-         the corresponding values of the (output file associated with)
-         stdout, and skip this input file if they coincide.  Input
-         files cannot be redirected to themselves.  */
+      /* Don't copy a nonempty regular file to itself, as that would
+         merely exhaust the output device.  It's better to catch this
+         error earlier rather than later.  */
 
-      if (check_redirection
+      if (out_isreg
           && stat_buf.st_dev == out_dev && stat_buf.st_ino == out_ino
-          && (input_desc != STDIN_FILENO))
+          && lseek (input_desc, 0, SEEK_CUR) < stat_buf.st_size)
         {
           error (0, 0, _("%s: input file is output file"), infile);
           ok = false;

tests/local.mk

@@ -258,6 +258,7 @@ all_tests =					\
   tests/misc/wc-parallel.sh			\
   tests/misc/cat-proc.sh			\
   tests/misc/cat-buf.sh				\
+  tests/misc/cat-self.sh			\
   tests/misc/base64.pl				\
   tests/misc/basename.pl			\
   tests/misc/close-stdout.sh			\

tests/misc/cat-self.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+# Check that cat operates correctly when the input is the same as the output.
+
+# Copyright 2014 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ cat
+
+echo x >out || framework_failure_
+echo x >out1 || framework_failure_
+cat out >>out && fail=1
+compare out out1 || fail=1
+
+# This example is taken from the POSIX spec for 'cat'.
+echo x >doc || framework_failure_
+echo y >doc.end || framework_failure_
+cat doc doc.end >doc || fail=1
+compare doc doc.end || fail=1
+
+Exit $fail