embed: fix blocking Close before gRPC server start

If 'StartEtcd' returns before starting gRPC server
(e.g. mismatch snapshot, misconfiguration),
receiving from grpcServerC blocks forever. This patch
just closes the channel to not block on grpcServerC,
and proceeds to next stop operations in Close.

This was masking the issues in etcd-io#7834

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

embed/etcd.go

@@ -83,6 +83,10 @@ func StartEtcd(inCfg *Config) (e *Etcd, err error) {
 		return
 	}
 	if e.sctxs, err = startClientListeners(cfg); err != nil {
+		// errored before starting gRPC server for serveCtx.grpcServerC
+		for _, sctx := range e.sctxs {
+			close(sctx.grpcServerC)
+		}
 		return
 	}
 	for _, sctx := range e.sctxs {
@@ -127,6 +131,10 @@ func StartEtcd(inCfg *Config) (e *Etcd, err error) {
 	}
 
 	if e.Server, err = etcdserver.NewServer(srvcfg); err != nil {
+		// errored before starting gRPC server for serveCtx.grpcServerC
+		for _, sctx := range e.sctxs {
+			close(sctx.grpcServerC)
+		}
 		return
 	}