fix newPendingTransactions subscription deadlock issue (#933)

CHANGELOG.md

@@ -78,6 +78,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (evm) [tharsis#871](https://github.com/tharsis/ethermint/pull/871) Set correct nonce in `EthCall` and `EstimateGas` grpc query.
 * (rpc) [tharsis#878](https://github.com/tharsis/ethermint/pull/878) Workaround to make GetBlock RPC api report correct block gas used.
 * (rpc) [tharsis#900](https://github.com/tharsis/ethermint/pull/900) newPendingTransactions filter return ethereum tx hash.
+* (rpc) [tharsis#933](https://github.com/tharsis/ethermint/pull/933) Fix newPendingTransactions subscription deadlock when a Websocket client exits without unsubscribing and the node errors.
 
 ## [v0.9.0] - 2021-12-01
 

rpc/websockets.go

@@ -712,8 +712,16 @@ func (api *pubSubAPI) subscribePendingTransactions(wsConn *wsConn) (rpc.ID, erro
 							api.logger.Debug("error writing header, will drop peer", "error", err.Error())
 
 							try(func() {
+								// Release the initial read lock in .RUnlock() before
+								// invoking .Lock() to avoid the deadlock in
+								// https://github.com/tharsis/ethermint/issues/821#issuecomment-1033959984
+								// and as documented at https://pkg.go.dev/sync#RWMutex
+								api.filtersMu.RUnlock()
 								api.filtersMu.Lock()
-								defer api.filtersMu.Unlock()
+								defer func() {
+									api.filtersMu.Unlock()
+									api.filtersMu.RLock()
+								}()
 
 								if err != websocket.ErrCloseSent {
 									_ = wsSub.wsConn.Close()