tests/9pfs: guard recent 'Twalk' behaviour fix

Previous 9p patch fixed 'Twalk' request handling, which was previously not
behaving as specified by the 9p2000 protocol spec. This patch adds a new test
case which guards the new 'Twalk' behaviour in question.

More specifically: it sends a 'Twalk' request where the 1st path component
is valid, whereas the 2nd path component transmitted to server does not
exist. The expected behaviour is that 9p server would respond by sending
a 'Rwalk' response with exactly 1 QID (instead of 'Rlerror' response).

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <61bde2f44b87e24b70ec098dfb81765665b2dfcb.1647339025.git.qemu_oss@crudebyte.com>

tests/qtest/virtio-9p-test.c

@@ -669,8 +669,12 @@ static void do_version(QVirtio9P *v9p)
     g_assert_cmpmem(server_version, server_len, version, strlen(version));
 }
 
-/* utility function: walk to requested dir and return fid for that dir */
+/*
-static uint32_t do_walk(QVirtio9P *v9p, const char *path)
+ * utility function: walk to requested dir and return fid for that dir and
+ * the QIDs of server response
+ */
+static uint32_t do_walk_rqids(QVirtio9P *v9p, const char *path, uint16_t *nwqid,
+                              v9fs_qid **wqid)
 {
     char **wnames;
     P9Req *req;
@@ -680,12 +684,18 @@ static uint32_t do_walk(QVirtio9P *v9p, const char *path)
 
     req = v9fs_twalk(v9p, 0, fid, nwnames, wnames, 0);
     v9fs_req_wait_for_reply(req, NULL);
-    v9fs_rwalk(req, NULL, NULL);
+    v9fs_rwalk(req, nwqid, wqid);
 
     split_free(&wnames);
     return fid;
 }
 
+/* utility function: walk to requested dir and return fid for that dir */
+static uint32_t do_walk(QVirtio9P *v9p, const char *path)
+{
+    return do_walk_rqids(v9p, path, NULL, NULL);
+}
+
 /* utility function: walk to requested dir and expect passed error response */
 static void do_walk_expect_error(QVirtio9P *v9p, const char *path, uint32_t err)
 {
@@ -1079,9 +1089,33 @@ static void fs_walk_nonexistent(void *obj, void *data, QGuestAllocator *t_alloc)
     alloc = t_alloc;
 
     do_attach(v9p);
+    /*
+     * The 9p2000 protocol spec says: "If the first element cannot be walked
+     * for any reason, Rerror is returned."
+     */
     do_walk_expect_error(v9p, "non-existent", ENOENT);
 }
 
+static void fs_walk_2nd_nonexistent(void *obj, void *data,
+                                    QGuestAllocator *t_alloc)
+{
+    QVirtio9P *v9p = obj;
+    alloc = t_alloc;
+    uint16_t nwqid;
+    g_autofree v9fs_qid *wqid = NULL;
+    g_autofree char *path = g_strdup_printf(
+        QTEST_V9FS_SYNTH_WALK_FILE "/non-existent", 0
+    );
+
+    do_attach(v9p);
+    do_walk_rqids(v9p, path, &nwqid, &wqid);
+    /*
+     * The 9p2000 protocol spec says: "nwqid is therefore either nwname or the
+     * index of the first elementwise walk that failed."
+     */
+    assert(nwqid == 1);
+}
+
 static void fs_walk_none(void *obj, void *data, QGuestAllocator *t_alloc)
 {
     QVirtio9P *v9p = obj;
@@ -1548,6 +1582,8 @@ static void register_virtio_9p_test(void)
                  fs_walk_dotdot,  &opts);
     qos_add_test("synth/walk/non_existent", "virtio-9p", fs_walk_nonexistent,
                   &opts);
+    qos_add_test("synth/walk/2nd_non_existent", "virtio-9p",
+                 fs_walk_2nd_nonexistent, &opts);
     qos_add_test("synth/lopen/basic", "virtio-9p", fs_lopen,  &opts);
     qos_add_test("synth/write/basic", "virtio-9p", fs_write,  &opts);
     qos_add_test("synth/flush/success", "virtio-9p", fs_flush_success,