librbd/migration/HttpClient: avoid hitting an assert in advance_state()

If the shutdown gets delayed until the state transition from
STATE_RESET_CONNECTING completes and the reconnect is successful
(i.e. next_state is STATE_READY), we eventually hit "unexpected
state transition" assert in advance_state().  The reason is that
advance_state() would update m_state and call disconnect() under
STATE_READY instead of STATE_SHUTTING_DOWN.  After the disconnect
maybe_finalize_shutdown() would enter advance_state() again with
STATE_SHUTDOWN as next_state, but the transition to that from
STATE_READY is invalid.

Plug this by not transitioning to next_state if current_state is
STATE_SHUTTING_DOWN.

Signed-off-by: Ilya Dryomov <[email protected]>

src/librbd/migration/HttpClient.cc

@@ -63,14 +63,13 @@ class HttpClient<I>::HttpSession : public HttpSessionInterface {
     m_on_shutdown = on_finish;
 
     auto current_state = m_state;
+    m_state = STATE_SHUTTING_DOWN;
+
     if (current_state == STATE_UNINITIALIZED) {
       // never initialized or resolve/connect failed
       on_finish->complete(0);
       return;
-    }
-
-    m_state = STATE_SHUTTING_DOWN;
-    if (current_state != STATE_READY) {
+    } else if (current_state != STATE_READY) {
       // delay shutdown until current state transition completes
       return;
     }
@@ -501,7 +500,10 @@ class HttpClient<I>::HttpSession : public HttpSessionInterface {
                    << "next_state=" << next_state << ", "
                    << "r=" << r << dendl;
 
-    m_state = next_state;
+    if (current_state != STATE_SHUTTING_DOWN) {
+      m_state = next_state;
+    }
+
     if (current_state == STATE_CONNECTING) {
       if (next_state == STATE_UNINITIALIZED) {
         shutdown_socket();