fix newPendingTransactions subscription deadlock issue (backport evmos#933)

Author: crypto-facs

CHANGELOG.md

@@ -40,6 +40,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 ### Bug Fixes
 
 * (rpc) [crypto-org-chain#42](https://github.com/crypto-org-chain/ethermint/pull/42) Fix `eth_getLogs` when specify blockHash without address/topics, and limit the response size.
+* (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. (backport: #933)
 
 ### Bug Fixes
 

rpc/websockets.go

@@ -706,8 +706,16 @@ 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()