[PATCH] shuf: exit without reading if would never output

[Pádraig Brady]

* src/shuf.c (main): If -n0 specified then no data would ever be output,
so exit without reading input.
* tests/misc/shuf.sh: Augment the related test with this case.
---
 src/shuf.c         |    6 ++++--
 tests/misc/shuf.sh |    6 ++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/shuf.c b/src/shuf.c
index bbf3a86..0fabb0b 100644
--- a/src/shuf.c
+++ b/src/shuf.c
@@ -463,7 +463,8 @@ main (int argc, char **argv)
           break;
 
         case 1:
-          if (! (STREQ (operand[0], "-") || freopen (operand[0], "r", stdin)))
+          if (! (STREQ (operand[0], "-") || ! head_lines
+                 || freopen (operand[0], "r", stdin)))
             error (EXIT_FAILURE, errno, "%s", operand[0]);
           break;
 
@@ -474,7 +475,8 @@ main (int argc, char **argv)
 
       fadvise (stdin, FADVISE_SEQUENTIAL);
 
-      if (head_lines != SIZE_MAX && input_size () > RESERVOIR_MIN_INPUT)
+      if (head_lines != SIZE_MAX && (! head_lines
+                                     || input_size () > RESERVOIR_MIN_INPUT))
         {
           use_reservoir_sampling = true;
           n_lines = SIZE_MAX;   /* unknown number of input lines, for now.  */
diff --git a/tests/misc/shuf.sh b/tests/misc/shuf.sh
index 9c9526d..171b25a 100755
--- a/tests/misc/shuf.sh
+++ b/tests/misc/shuf.sh
@@ -57,4 +57,10 @@ cmp out exp || { fail=1; echo "missing NUL terminator?" 
1>&2; }
 timeout 10 shuf -i1-$SIZE_MAX -n2 >/dev/null ||
   { fail=1; echo "couldn't get a small subset" >&2; }
 
+# Ensure shuf -n0 doesn't read any input or open specified files
+touch unreadable || framework_failure_
+chmod 0 unreadable || framework_failure_
+shuf -n0 unreadable || fail=1
+shuf -n1 unreadable && fail=1
+
 Exit $fail
-- 
1.7.7.6