balancer: Enforce embedding the SubConn interface in implementations (#7758)

balancer/balancer.go

@@ -155,6 +155,9 @@ type SubConn interface {
 	// indicate the shutdown operation.  This may be delivered before
 	// in-progress RPCs are complete and the actual connection is closed.
 	Shutdown()
+	// enforceEmbedding is an unexported method to force implementers embed
+	// this interface, allowing gRPC to add methods without breaking users.
+	enforceEmbedding()
 }
 
 // NewSubConnOptions contains options to create new SubConn.

balancer/base/balancer_test.go

@@ -41,6 +41,7 @@ func (c *testClientConn) NewSubConn(addrs []resolver.Address, opts balancer.NewS
 func (c *testClientConn) UpdateState(balancer.State) {}
 
 type testSubConn struct {
+	balancer.SubConn
 	updateState func(balancer.SubConnState)
 }
 

balancer_wrapper.go

@@ -254,6 +254,7 @@ func (ccb *ccBalancerWrapper) Target() string {
 // acBalancerWrapper is a wrapper on top of ac for balancers.
 // It implements balancer.SubConn interface.
 type acBalancerWrapper struct {
+	balancer.SubConn
 	ac            *addrConn          // read-only
 	ccb           *ccBalancerWrapper // read-only
 	stateListener func(balancer.SubConnState)

internal/testutils/balancer.go

@@ -32,6 +32,7 @@ import (
 
 // TestSubConn implements the SubConn interface, to be used in tests.
 type TestSubConn struct {
+	balancer.SubConn
 	tcc           *BalancerClientConn // the CC that owns this SubConn
 	id            string
 	ConnectCh     chan struct{}