mu: Test splicing child containers when there is only one thread

This test reproduces a regression introduced by commit 97101f1f82
("mu: Prune empty container when an only child gets promoted to the
root set").

When the results of a mu-find query contain only a one thread:

$ mu find --threads --fields 'd s' ''
Sat 09 Aug 2014 07:00:00 PM CEST [mu4e] Test Message
  `-> Sat 09 Aug 2014 08:30:00 PM CEST Re: [mu4e] Test Message

... and we narrow down the query in such a way that the root message
gets excluded, then a crash occurs:

$ mu find --threads --fields 'd s' '' date:2014-08-09/20:00..2014-08-09/21:00
**
ERROR:mu-container.c:117:mu_container_append_siblings: assertion failed: (c)
Aborted (core dumped)

Reported-by: Josiah Schwab <jschwab@gmail.com>

.gitignore

@@ -54,6 +54,7 @@ msg2pdf
 test-mu-cmd
 test-mu-cmd-cfind
 test-mu-contacts
+test-mu-container
 test-mu-date
 test-mu-flags
 test-mu-maildir

lib/tests/Makefile.am

@@ -70,6 +70,10 @@ TEST_PROGS += test-mu-flags
 test_mu_flags_SOURCES= test-mu-flags.c dummy.cc
 test_mu_flags_LDADD=  libtestmucommon.la
 
+TEST_PROGS += test-mu-container
+test_mu_container_SOURCES= test-mu-container.c dummy.cc
+test_mu_container_LDADD= libtestmucommon.la
+
 # we need to use dummy.cc to enforce c++ linking...
 BUILT_SOURCES=					\
 	dummy.cc

lib/tests/test-mu-container.c

@@ -0,0 +1,83 @@
+/* -*-mode: c; tab-width: 8; indent-tabs-mode: t; c-basic-offset: 8 -*-*/
+
+/*
+** Copyright (C) 2014  Jakub Sitnicki <jsitnicki@gmail.com>
+**
+** This program 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, or (at your option) any
+** later version.
+**
+** This program 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, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif /*HAVE_CONFIG_H*/
+
+#include <glib.h>
+
+#include "test-mu-common.h"
+#include "mu-container.h"
+
+static gboolean
+container_has_children (const MuContainer *c)
+{
+	return c && c->child;
+}
+
+static gboolean
+container_is_sibling_of (const MuContainer *c, const MuContainer *sibling)
+{
+	const MuContainer *cur;
+
+	for (cur = c; cur; cur = cur->next) {
+		if (cur == sibling)
+			return TRUE;
+	}
+
+	return container_is_sibling_of (sibling, c);
+}
+
+static void
+test_mu_container_splice_children_when_parent_has_no_siblings (void)
+{
+	MuContainer *child, *parent, *root_set;
+
+	child = mu_container_new (NULL, 0, "child");
+	parent = mu_container_new (NULL, 0, "parent");
+	parent = mu_container_append_children (parent, child);
+
+	root_set = parent;
+	root_set = mu_container_splice_children (root_set, parent);
+
+	g_assert (root_set != NULL);
+	g_assert (!container_has_children(parent));
+	g_assert (container_is_sibling_of (root_set, child));
+
+	mu_container_destroy(parent);
+	mu_container_destroy(child);
+}
+
+int
+main (int argc, char *argv[])
+{
+	g_test_init (&argc, &argv, NULL);
+
+	g_test_add_func ("/mu-container/mu-container-splice-children-when-parent-has-no-siblings",
+			 test_mu_container_splice_children_when_parent_has_no_siblings);
+
+	g_log_set_handler (NULL,
+			   G_LOG_LEVEL_MASK | G_LOG_FLAG_FATAL| G_LOG_FLAG_RECURSION,
+			   (GLogFunc)black_hole, NULL);
+
+	return g_test_run ();
+}