client: add sidechannel support

Implements gitlab-com/gl-infra/scalability#1303.

This change adds publicly exported code that allows Gitaly clients to accept sidechannel connections coming back from a Gitaly server they have connected to. This is done via a new dial function DialSidechannel.

We need this code to be public so that Workhorse can import it. Without this code, Workhorse won't be able to use sidechannel RPC's.

client/dial.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"gitlab.com/gitlab-org/gitaly/v14/internal/gitaly/client"
+	"gitlab.com/gitlab-org/gitaly/v14/internal/sidechannel"
 	"google.golang.org/grpc"
 	healthpb "google.golang.org/grpc/health/grpc_health_v1"
 )
@@ -30,6 +31,15 @@ func Dial(rawAddress string, connOpts []grpc.DialOption) (*grpc.ClientConn, erro
 	return DialContext(context.Background(), rawAddress, connOpts)
 }
 
+// DialSidechannel configures the dialer to establish a Gitaly
+// backchannel connection instead of a regular gRPC connection. It also
+// injects sr as a sidechannel registry, so that Gitaly can establish
+// sidechannels back to the client.
+func DialSidechannel(ctx context.Context, rawAddress string, sr *SidechannelRegistry, connOpts []grpc.DialOption) (*grpc.ClientConn, error) {
+	clientHandshaker := sidechannel.NewClientHandshaker(sr.logger, sr.registry)
+	return client.Dial(ctx, rawAddress, connOpts, clientHandshaker)
+}
+
 // FailOnNonTempDialError helps to identify if remote listener is ready to accept new connections.
 func FailOnNonTempDialError() []grpc.DialOption {
 	return []grpc.DialOption{