feat: add client agent handshake transport

internal/client/agent/handshake.go

@@ -0,0 +1,61 @@
+// Package agent implements the client side of the RVBox Agent protocol.
+package agent
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	rvboxv1 "github.com/rvbox/rvbox/gen/go/rvbox/v1"
+	"github.com/rvbox/rvbox/internal/agentproto"
+	"google.golang.org/protobuf/proto"
+)
+
+var (
+	ErrProtocolHandshake = errors.New("invalid agent session handshake")
+	ErrSessionClosed     = errors.New("agent session transport closed")
+)
+
+// Transport has exactly the binary-message operations the protocol needs. A
+// WebSocket implementation owns its deadline/ping details outside this layer.
+type Transport interface {
+	Write(context.Context, []byte) error
+	Read(context.Context) ([]byte, error)
+	Close() error
+}
+
+type Session struct {
+	ID         string
+	Generation uint64
+	Protocol   *rvboxv1.ProtocolVersion
+}
+
+func Handshake(ctx context.Context, transport Transport, hello *rvboxv1.ClientHello, limits agentproto.Limits) (Session, error) {
+	if transport == nil || hello == nil {
+		return Session{}, ErrProtocolHandshake
+	}
+	request := &rvboxv1.AgentEnvelope{Payload: &rvboxv1.AgentEnvelope_ClientHello{ClientHello: hello}}
+	if err := agentproto.ValidateEnvelope(request, limits, rvboxv1.Platform_PLATFORM_WINDOWS); err != nil {
+		return Session{}, fmt.Errorf("validate ClientHello: %w", err)
+	}
+	encoded, err := proto.Marshal(request)
+	if err != nil {
+		return Session{}, err
+	}
+	if err := transport.Write(ctx, encoded); err != nil {
+		return Session{}, err
+	}
+	received, err := transport.Read(ctx)
+	if err != nil {
+		return Session{}, err
+	}
+	welcome, err := agentproto.DecodeEnvelope(received, limits, rvboxv1.Platform_PLATFORM_WINDOWS)
+	if err != nil {
+		return Session{}, fmt.Errorf("decode ServerWelcome: %w", err)
+	}
+	payload := welcome.GetServerWelcome()
+	if payload == nil || welcome.GetSessionId() == "" || welcome.GetSessionGeneration() == 0 || payload.GetSelectedProtocol() == nil {
+		return Session{}, ErrProtocolHandshake
+	}
+	return Session{ID: welcome.GetSessionId(), Generation: welcome.GetSessionGeneration(), Protocol: proto.Clone(payload.GetSelectedProtocol()).(*rvboxv1.ProtocolVersion)}, nil
+}