head: with --lines=-N (-n-N) reset file pointer on seekable input

* src/head.c (elide_tail_lines_seekable): Reset file pointer
after printing up to an end-relative line-counted offset.
Anoop Sharma reported the problem and suggested the fix.
* tests/misc/head-pos: Add coverage via a very similar, existing test.
Also add coverage for a previously untested block of code.
* tests/misc/head-elide-tail ($READ_BUFSIZE): Update to 8192, to
match the value of BUFSIZ I see today on Fedora 17/x86_64 (unrelated
to this fix).
* NEWS (Bug fixes): Mention it.

Improved-by: Pádraig Brady

2 files changed, 20 insertions, 8 deletions

diff --git a/tests/misc/head-elide-tail b/tests/misc/head-elide-tail
index de4896bc5..85f509d26 100755
--- a/tests/misc/head-elide-tail
+++ b/tests/misc/head-elide-tail
@@ -26,7 +26,7 @@ $ENV{PROG} = 'head';
 @ENV{qw(LANGUAGE LANG LC_ALL)} = ('C') x 3;
 
 # This should match the definition in head.c.
-my $READ_BUFSIZE = 4096;
+my $READ_BUFSIZE = 8192;
 
 my @Tests =
   (
diff --git a/tests/misc/head-pos b/tests/misc/head-pos
index 3d96261f9..fa3284e90 100755
--- a/tests/misc/head-pos
+++ b/tests/misc/head-pos
@@ -21,12 +21,24 @@
 print_ver_ head
 
 (echo a; echo b) > in || framework_failure_
-
-(head -n 1 >/dev/null; cat) < in > out || fail=1
-cat <<EOF > exp
-b
-EOF
-
-compare exp out || fail=1
+echo b > exp || framework_failure_
+
+for i in -1 1; do
+  (head -n $i >/dev/null; cat) < in > out || fail=1
+  compare exp out || fail=1
+done
+
+# Exercise the (start_pos < pos) block in elide_tail_lines_seekable.
+# So far, this is the only test to do that.
+# Do that by creating a file larger than BUFSIZ (I've seen 128K) and
+# elide a suffix of it (by line count) that is also larger than BUFSIZ.
+# 50000 lines times 6 bytes per line gives us enough leeway even on a
+# system with a BUFSIZ of 256K.
+n_lines=50000
+seq 70000 > in2 || framework_failure_
+echo $n_lines > exp-n || framework_failure_
+
+(head -n-$n_lines>/dev/null; wc -l) < in2 > n
+compare exp-n n || fail=1
 
 Exit $fail