From fc6fce9a16bc52aca11cd8f1ad2632943fe94201 Mon Sep 17 00:00:00 2001
From: Paul Eggert <address@hidden>
Date: Thu, 17 Nov 2016 21:05:42 -0800
Subject: [PATCH 07/10] grep: treat -f /dev/null like -m0

* NEWS: Document this.
* src/grep.c (main): With -f /dev/null, don't bother to read the
input.  This is what FreeBSD grep does.
* tests/Makefile.am (TESTS): Add skip-read.
* tests/skip-read: New file.
---
 NEWS              |  5 +++++
 src/grep.c        |  5 ++++-
 tests/Makefile.am |  1 +
 tests/skip-read   | 15 +++++++++++++++
 4 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100755 tests/skip-read

diff --git a/NEWS b/NEWS
index a165377..06a186a 100644
--- a/NEWS
+++ b/NEWS
@@ -16,6 +16,11 @@ GNU grep NEWS                                    -*- outline -*-
 
   grep's use of getprogname no longer causes a build failure on HP-UX.
 
+** Improvements
+
+  grep no longer reads the input in a few more cases when it is easy
+  to see that matching cannot succeed, e.g., 'grep -f /dev/null'.
+
 
 * Noteworthy changes in release 2.26 (2016-10-02) [stable]
 
diff --git a/src/grep.c b/src/grep.c
index fe56e9c..317a0d5 100644
--- a/src/grep.c
+++ b/src/grep.c
@@ -2866,7 +2866,10 @@ main (int argc, char **argv)
   if (O_BINARY && !isatty (STDOUT_FILENO))
     set_binary_mode (STDOUT_FILENO, O_BINARY);
 
-  if (max_count == 0)
+  /* If it is easy to see that matching cannot succeed (e.g., 'grep -f
+     /dev/null'), fail without reading the input.  */
+  if (max_count == 0
+      || (keycc == 0 && out_invert && !match_lines && !match_words))
     return EXIT_FAILURE;
 
   /* Prefer sysconf for page size, as getpagesize typically returns int.  */
diff --git a/tests/Makefile.am b/tests/Makefile.am
index f4c82f4..b6f0df3 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -143,6 +143,7 @@ TESTS =						\
   reversed-range-endpoints			\
   sjis-mb					\
   skip-device					\
+  skip-read					\
   spencer1					\
   spencer1-locale				\
   status					\
diff --git a/tests/skip-read b/tests/skip-read
new file mode 100755
index 0000000..627d362
--- /dev/null
+++ b/tests/skip-read
@@ -0,0 +1,15 @@
+#!/bin/sh
+# Check that grep skips reading in some cases.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+for opts in '-m0 y' '-f /dev/null' '-v ""'; do
+  for matcher in '' -E -F; do
+    eval returns_ 1 grep $opts $matcher no-such-file > out || fail=1
+    compare /dev/null out || fail=1
+  done
+done
+
+Exit $fail
-- 
2.7.4