Merge pull request #23365 from yashykt/deadlockws

Fix possible deadlock in RemoveExternalConnectivityWatcher

src/core/ext/filters/client_channel/client_channel.cc

@@ -184,13 +184,19 @@ class ChannelData {
 
   void RemoveExternalConnectivityWatcher(grpc_closure* on_complete,
                                          bool cancel) {
+    ExternalConnectivityWatcher* watcher = nullptr;
+    {
       MutexLock lock(&external_watchers_mu_);
       auto it = external_watchers_.find(on_complete);
       if (it != external_watchers_.end()) {
-      if (cancel) it->second->Cancel();
+        watcher = it->second;
         external_watchers_.erase(it);
       }
     }
+    // watcher->Cancel() will hop into the WorkSerializer, so we have to unlock
+    // the mutex before calling it.
+    if (watcher != nullptr && cancel) watcher->Cancel();
+  }
 
   int NumExternalConnectivityWatchers() const {
     MutexLock lock(&external_watchers_mu_);