client.go: HybridVSockDialer: Change Read EOT to recv peek

Rust agent(grpc-rs) cannot work with firecacker because the first package
will be dropped or truncated by HybridVSockDialer read EOT code.
This issue was reported in
kata-containers/kata-containers#79

Golang agent(grpc-golang) is also affected by this issue.  But it is first
package is 9 bytes FRAME_SETTINGS.  Drop it will not affect subsequent
communications.

This commit change to use recv peek that will do same check with read
EOT but not drop any package.

Fixes: kata-containers#695

Signed-off-by: Hui Zhu <teawater@antfin.com>

protocols/client/client.go

@@ -13,12 +13,14 @@ import (
 	"net/url"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/grpc-ecosystem/grpc-opentracing/go/otgrpc"
 	"github.com/hashicorp/yamux"
 	"github.com/mdlayher/vsock"
 	opentracing "github.com/opentracing/opentracing-go"
+	"golang.org/x/sys/unix"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
 	grpcStatus "google.golang.org/grpc/status"
@@ -407,12 +409,19 @@ func HybridVSockDialer(sock string, timeout time.Duration) (net.Conn, error) {
 			return nil, err
 		}
 
-		// Read EOT (End of transmission) byte
-		eot := make([]byte, 32)
-		if _, err = conn.Read(eot); err != nil {
-			// Just close the connection, gRPC will dial again
-			// without errors
-			conn.Close()
+		// Receive the packet from the connection without removing it from
+		// the receive queue (MSG_PEEK), ensuring the connection is usable.
+		if uc, ok := conn.(*net.UnixConn); ok {
+			file, err := uc.File()
+			if err != nil {
+				conn.Close()
+				return nil, err
+			}
+			eot := make([]byte, 1)
+			if _, _, err = unix.Recvfrom(int(file.Fd()), eot, syscall.MSG_PEEK); err != nil {
+				conn.Close()
+				return nil, err
+			}
 		}
 
 		return conn, nil