Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/google.golang.org/grpc/health/producer.go

@@ -0,0 +1,106 @@
+/*
+ *
+ * Copyright 2024 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package health
+
+import (
+	"context"
+	"sync"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/internal"
+	"google.golang.org/grpc/status"
+)
+
+func init() {
+	producerBuilderSingleton = &producerBuilder{}
+	internal.RegisterClientHealthCheckListener = registerClientSideHealthCheckListener
+}
+
+type producerBuilder struct{}
+
+var producerBuilderSingleton *producerBuilder
+
+// Build constructs and returns a producer and its cleanup function.
+func (*producerBuilder) Build(cci any) (balancer.Producer, func()) {
+	p := &healthServiceProducer{
+		cc:     cci.(grpc.ClientConnInterface),
+		cancel: func() {},
+	}
+	return p, func() {
+		p.mu.Lock()
+		defer p.mu.Unlock()
+		p.cancel()
+	}
+}
+
+type healthServiceProducer struct {
+	// The following fields are initialized at build time and read-only after
+	// that and therefore do not need to be guarded by a mutex.
+	cc grpc.ClientConnInterface
+
+	mu     sync.Mutex
+	cancel func()
+}
+
+// registerClientSideHealthCheckListener accepts a listener to provide server
+// health state via the health service.
+func registerClientSideHealthCheckListener(ctx context.Context, sc balancer.SubConn, serviceName string, listener func(balancer.SubConnState)) func() {
+	pr, closeFn := sc.GetOrBuildProducer(producerBuilderSingleton)
+	p := pr.(*healthServiceProducer)
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.cancel()
+	if listener == nil {
+		return closeFn
+	}
+
+	ctx, cancel := context.WithCancel(ctx)
+	p.cancel = cancel
+
+	go p.startHealthCheck(ctx, sc, serviceName, listener)
+	return closeFn
+}
+
+func (p *healthServiceProducer) startHealthCheck(ctx context.Context, sc balancer.SubConn, serviceName string, listener func(balancer.SubConnState)) {
+	newStream := func(method string) (any, error) {
+		return p.cc.NewStream(ctx, &grpc.StreamDesc{ServerStreams: true}, method)
+	}
+
+	setConnectivityState := func(state connectivity.State, err error) {
+		listener(balancer.SubConnState{
+			ConnectivityState: state,
+			ConnectionError:   err,
+		})
+	}
+
+	// Call the function through the internal variable as tests use it for
+	// mocking.
+	err := internal.HealthCheckFunc(ctx, newStream, setConnectivityState, serviceName)
+	if err == nil {
+		return
+	}
+	if status.Code(err) == codes.Unimplemented {
+		logger.Errorf("Subchannel health check is unimplemented at server side, thus health check is disabled for SubConn %p", sc)
+	} else {
+		logger.Errorf("Health checking failed for SubConn %p: %v", sc, err)
+	}
+}