common: Fix iobuf_peek corner case.

Previously, iobuf_peek on a file smaller than 'buflen' would hang.

* common/iobuf.c (underflow): Generalize by adding a target parameter.
(iobuf_peek): Use this to prevent looping here.
* tests/openpgp/Makefile.am (TESTS): Add new test.
* tests/openpgp/setup.scm (dearmor): Move function...
* tests/openpgp/defs.scm (dearmor): ... here.
* tests/openpgp/issue2419.scm: New file.
* tests/openpgp/samplemsgs/issue2419.asc: Likewise.

GnuPG-bug-id: 2419
Signed-off-by: Justus Winter <justus@g10code.com>

tests/openpgp/issue2419.scm

@@ -0,0 +1,28 @@
+#!/usr/bin/env gpgscm
+
+;; Copyright (C) 2016 g10 Code GmbH
+;;
+;; This file is part of GnuPG.
+;;
+;; GnuPG is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 3 of the License, or
+;; (at your option) any later version.
+;;
+;; GnuPG is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with this program; if not, see <http://www.gnu.org/licenses/>.
+
+(load (with-path "defs.scm"))
+
+(info "Checking iobuf_peek corner case (issue2419)...")
+(lettmp
+ (onebyte)
+ (dearmor (in-srcdir "samplemsgs/issue2419.asc") onebyte)
+ (catch (assert (string-contains? *error* "invalid packet"))
+	(call-popen `(,@GPG --list-packets ,onebyte) "")
+	(error "Expected an error but got none")))