replace API server with transparent gRPC proxy that routes requests to local machine API

internal/machine/api/proxy/local.go

@@ -0,0 +1,68 @@
+package proxy
+
+import (
+	"context"
+	"github.com/siderolabs/grpc-proxy/proxy"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/metadata"
+	"sync"
+)
+
+// LocalBackend is a proxy.Backend implementation that proxies to a local gRPC server listening on a Unix socket.
+type LocalBackend struct {
+	sockPath string
+
+	mu   sync.RWMutex
+	conn *grpc.ClientConn
+}
+
+var _ proxy.Backend = (*LocalBackend)(nil)
+
+// NewLocalBackend returns a new LocalBackend for the given Unix socket path.
+func NewLocalBackend(sockPath string) *LocalBackend {
+	return &LocalBackend{
+		sockPath: sockPath,
+	}
+}
+
+func (l *LocalBackend) String() string {
+	return "local"
+}
+
+// GetConnection returns a gRPC connection to the local server listening on the Unix socket.
+func (l *LocalBackend) GetConnection(ctx context.Context, _ string) (context.Context, *grpc.ClientConn, error) {
+	md, _ := metadata.FromIncomingContext(ctx)
+	outCtx := metadata.NewOutgoingContext(ctx, md)
+
+	l.mu.RLock()
+	if l.conn != nil {
+		l.mu.RUnlock()
+		return outCtx, l.conn, nil
+	}
+	l.mu.RUnlock()
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	var err error
+	l.conn, err = grpc.NewClient(
+		"unix://"+l.sockPath,
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+		grpc.WithDefaultCallOptions(
+			grpc.ForceCodecV2(proxy.Codec()),
+		),
+	)
+
+	return outCtx, l.conn, err
+}
+
+// AppendInfo is called to enhance response from the backend with additional data.
+func (l *LocalBackend) AppendInfo(_ bool, resp []byte) ([]byte, error) {
+	return resp, nil
+}
+
+// BuildError is called to convert error from upstream into response field.
+func (l *LocalBackend) BuildError(bool, error) ([]byte, error) {
+	return nil, nil
+}