abci/server: recover from app panics in socket server (#3809)

fixes #3800

CHANGELOG_PENDING.md

@@ -17,5 +17,6 @@
 ### FEATURES:
 
 ### IMPROVEMENTS:
+- [abci] \#3809 Recover from application panics in `server/socket_server.go` to allow socket cleanup (@ruseinov)
 
 ### BUG FIXES:

abci/server/socket_server.go

@@ -146,6 +146,16 @@ func (s *SocketServer) waitForClose(closeConn chan error, connID int) {
 func (s *SocketServer) handleRequests(closeConn chan error, conn net.Conn, responses chan<- *types.Response) {
 	var count int
 	var bufReader = bufio.NewReader(conn)
+
+	defer func() {
+		// make sure to recover from any app-related panics to allow proper socket cleanup
+		r := recover()
+		if r != nil {
+			closeConn <- fmt.Errorf("recovered from panic: %v", r)
+			s.appMtx.Unlock()
+		}
+	}()
+
 	for {
 
 		var req = &types.Request{}
@@ -154,7 +164,7 @@ func (s *SocketServer) handleRequests(closeConn chan error, conn net.Conn, respo
 			if err == io.EOF {
 				closeConn <- err
 			} else {
-				closeConn <- fmt.Errorf("Error reading message: %v", err.Error())
+				closeConn <- fmt.Errorf("error reading message: %v", err)
 			}
 			return
 		}