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, 8 insertions, 0 deletions

diff --git a/src/head.c b/src/head.c
index d7e83b716..0d5e1b2c6 100644
--- a/src/head.c
+++ b/src/head.c
@@ -667,6 +667,14 @@ elide_tail_lines_seekable (const char *pretty_filename, int fd,
                  Don't bother testing for failure for such a small amount.
                  Any failure will be detected upon close.  */
               fwrite (buffer, 1, n + 1, stdout);
+
+              /* Set file pointer to the byte after what we've output.  */
+              if (lseek (fd, pos + n + 1, SEEK_SET) < 0)
+                {
+                  error (0, errno, "%s: failed to reset file pointer",
+                         quote (pretty_filename));
+                  return false;
+                }
               return true;
             }
         }