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

1 files changed, 7 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 7c7c2c30e..c0e5fdbbb 100644
--- a/NEWS
+++ b/NEWS
@@ -8,6 +8,13 @@ GNU coreutils NEWS                                    -*- outline -*-
   processes will not intersperse their output.
   [the bug dates back to the initial implementation]
 
+  head --lines=-N (-n-N) now resets the read pointer of a seekable input file.
+  This means that "head -n-3" no longer consumes all of its input, and lines
+  not output by head may be processed by other programs.  For example, this
+  command now prints the final line, 2, while before it would print nothing:
+    seq 2 > k; (head -n-1 > /dev/null; cat) < k
+  [This bug was present in "the beginning".]
+
   ls --color would mis-color relative-named symlinks in /
   [bug introduced in coreutils-8.17]