use wrapped TransportCredentials instead of handling handshake in Dialer, so that grpc library will use correct :scheme

grpcurl.go

@@ -605,33 +605,44 @@ func BlockingDial(ctx context.Context, network, address string, creds credential
 		}
 	}
 
+	// custom credentials and dialer will notify on error via the
+	// writeResult function
+	if creds != nil {
+		creds = &errSignalingCreds{
+			TransportCredentials: creds,
+			writeResult:          writeResult,
+		}
+	}
 	dialer := func(ctx context.Context, address string) (net.Conn, error) {
+		// NB: We *could* handle the TLS handshake ourselves, in the custom
+		// dialer (instead of customizing both the dialer and the credentials).
+		// But that requires using WithInsecure dial option (so that the gRPC
+		// library doesn't *also* try to do a handshake). And that would mean
+		// that the library would send the wrong ":scheme" metaheader to
+		// servers: it would send "http" instead of "https" because it is
+		// unaware that TLS is actually in use.
 		conn, err := (&net.Dialer{}).DialContext(ctx, network, address)
 		if err != nil {
 			writeResult(err)
-			return nil, err
 		}
-		if creds != nil {
-			conn, _, err = creds.ClientHandshake(ctx, address, conn)
-			if err != nil {
-				writeResult(err)
-				return nil, err
-			}
-		}
-		return conn, nil
+		return conn, err
 	}
 
 	// Even with grpc.FailOnNonTempDialError, this call will usually timeout in
 	// the face of TLS handshake errors. So we can't rely on grpc.WithBlock() to
 	// know when we're done. So we run it in a goroutine and then use result
-	// channel to either get the channel or fail-fast.
+	// channel to either get the connection or fail-fast.
 	go func() {
 		opts = append(opts,
 			grpc.WithBlock(),
 			grpc.FailOnNonTempDialError(true),
 			grpc.WithContextDialer(dialer),
-			grpc.WithInsecure(), // we are handling TLS, so tell grpc not to
 		)
+		if creds == nil {
+			opts = append(opts, grpc.WithInsecure())
+		} else {
+			opts = append(opts, grpc.WithTransportCredentials(creds))
+		}
 		conn, err := grpc.DialContext(ctx, address, opts...)
 		var res interface{}
 		if err != nil {
@@ -652,3 +663,18 @@ func BlockingDial(ctx context.Context, network, address string, creds credential
 		return nil, ctx.Err()
 	}
 }
+
+// errSignalingCreds is a wrapper around a TransportCredentials value, but
+// it will use the writeResult function to notify on error.
+type errSignalingCreds struct {
+	credentials.TransportCredentials
+	writeResult func(res interface{})
+}
+
+func (c *errSignalingCreds) ClientHandshake(ctx context.Context, addr string, rawConn net.Conn) (net.Conn, credentials.AuthInfo, error) {
+	conn, auth, err := c.TransportCredentials.ClientHandshake(ctx, addr, rawConn)
+	if err != nil {
+		c.writeResult(err)
+	}
+	return conn, auth, err
+}