diff: remove unportable ‘diff -N - f <&-’ feature

* NEWS: Mention this.
* bootstrap.conf (gnulib_modules): Add stdopen.
* doc/diffutils.texi (Comparing Directories):
Do not document behavior if stdin is closed.
* src/diff.c: Include stdopen.h.
(main): Call stdopen early.
(compare_files) [__hpux]: Remove recently-introduced
special case for HP-UX exec with stdin closed.
* tests/new-file: Remove tests of the removed feature.

NEWS

@@ -2,6 +2,15 @@ GNU diffutils NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** Incompatible changes
+
+  diff no longer treats a closed stdin as representing an absent file
+  in usage like 'diff --new-file - foo <&-'.  This feature was rarely
+  if ever used and was not portable to POSIX platforms that reopen
+  stdin on exec, such as SELinux if the process underwent an AT_SECURE
+  transition, or HP-UX even if not setuid.
+  [bug#33965 introduced in 2.8]
+
 
 * Noteworthy changes in release 3.7 (2018-12-31) [stable]
 

bootstrap.conf

@@ -67,6 +67,7 @@ stat
 stat-macros
 stat-time
 stdint
+stdopen
 strcase
 strftime
 strptime

doc/diffutils.texi

@@ -1796,8 +1796,7 @@ second position.)  To do this, use the @option{--new-file}
 (@option{-N}) option.  This option affects command-line arguments as
 well as files found via directory traversal; for example, @samp{diff
 -N a b} treats @file{a} as empty if @file{a} does not exist but
-@file{b} does, and similarly @samp{diff -N - b} treats standard input
-as empty if it is closed but @file{b} exists.
+@file{b} does.
 
 If the older directory contains large files that are not in
 the newer directory, you can make the patch smaller by using the

src/diff.c

@@ -37,6 +37,7 @@
 #include <progname.h>
 #include <sh-quote.h>
 #include <stat-time.h>
+#include <stdopen.h>
 #include <timespec.h>
 #include <version-etc.h>
 #include <xalloc.h>
@@ -297,6 +298,9 @@ main (int argc, char **argv)
   re_set_syntax (RE_SYNTAX_GREP | RE_NO_POSIX_BACKTRACKING);
   excluded = new_exclude ();
   presume_output_tty = false;
+  int stdopen_errno = stdopen ();
+  if (stdopen_errno != 0)
+    error (EXIT_TROUBLE, stdopen_errno, "standard file descriptors");
 
   /* Decode the options.  */
 
@@ -1172,13 +1176,6 @@ compare_files (struct comparison const *parent,
 	      cmp.file[f].desc = STDIN_FILENO;
 	      if (binary && ! isatty (STDIN_FILENO))
 		set_binary_mode (STDIN_FILENO, O_BINARY);
-#ifdef __hpux
-	      /* Recognize file descriptors closed by the parent on HP-UX.  */
-	      int flags = fcntl (STDIN_FILENO, F_GETFL, NULL);
-	      if (flags >= 0 && (flags & FD_CLOEXEC) != 0)
-		cmp.file[f].desc = ERRNO_ENCODE (EBADF);
-	      else
-#endif
 	      if (fstat (STDIN_FILENO, &cmp.file[f].stat) != 0)
 		cmp.file[f].desc = ERRNO_ENCODE (errno);
 	      else

tests/new-file

@@ -10,12 +10,6 @@ echo a > a || fail=1
 echo '0a1
 > a' > exp || fail=1
 
-returns_ 1 diff -N - a <&- > out || fail=1
-compare exp out || fail=1
-
-returns_ 1 diff --unidirectional-new-file - a <&- > out || fail=1
-compare exp out || fail=1
-
 returns_ 1 diff -N b - < a > out || fail=1
 compare exp out || fail=1
 
@@ -25,14 +19,6 @@ compare exp out || fail=1
 echo '1d0
 < a' > exp || fail=1
 
-returns_ 1 diff -N a - <&- > out || fail=1
-compare exp out || fail=1
-
-# With closed standard input, require an exit status of 2
-# and empty stdout.
-returns_ 2 diff --unidirectional-new-file a - <&- > out || fail=1
-compare /dev/null out || fail=1
-
 returns_ 1 diff -N - b < a > out || fail=1
 compare exp out || fail=1