[Jaeger-Agent] Use RawConn.Control to get fd instead of Fd() (#4449)

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## Which problem is this PR solving?
Resolves #4448

## Short description of the changes
Jaeger agent gets stuck when closing with SocketBufferSize set. This is
because `Close()` of `net.UDPConn` will be blocked if `Fd()` is used to
get the file descriptor.
Use `RawConn.Control` instead to get fd to set the socket buffer.

Same issue was discussed here: golang/go#29277
The fix refers to here: brucespang/go-tcpinfo#3

Signed-off-by: Chen Xu <[email protected]>

cmd/agent/app/agent_test.go

@@ -184,6 +184,36 @@ func TestStartStopRace(t *testing.T) {
 	t.Fatal("Expecting server exit log")
 }
 
+func TestStartStopWithSocketBufferSet(t *testing.T) {
+	resetDefaultPrometheusRegistry()
+	cfg := Builder{
+		Processors: []ProcessorConfiguration{
+			{
+				Model:    jaegerModel,
+				Protocol: compactProtocol,
+				Workers:  1,
+				Server: ServerConfiguration{
+					HostPort:         "127.0.0.1:0",
+					SocketBufferSize: 10,
+				},
+			},
+		},
+	}
+	mBldr := &metricsbuilder.Builder{HTTPRoute: "/metrics", Backend: "prometheus"}
+	metricsFactory, err := mBldr.CreateMetricsFactory("jaeger")
+	mFactory := fork.New("internal", metrics.NullFactory, metricsFactory)
+	require.NoError(t, err)
+	agent, err := cfg.CreateAgent(fakeCollectorProxy{}, zap.NewNop(), mFactory)
+	require.NoError(t, err)
+
+	if err := agent.Run(); err != nil {
+		t.Fatalf("error from agent.Run(): %s", err)
+	}
+
+	t.Log("stopping agent")
+	agent.Stop()
+}
+
 func resetDefaultPrometheusRegistry() {
 	// Create and assign a new Prometheus Registerer/Gatherer for each test
 	registry := prometheus.NewRegistry()

cmd/agent/app/servers/thriftudp/socket_buffer.go

@@ -18,15 +18,27 @@
 package thriftudp
 
 import (
+	"fmt"
 	"net"
 	"syscall"
 )
 
 func setSocketBuffer(conn *net.UDPConn, bufferSize int) error {
-	file, err := conn.File()
+	rawConn, err := conn.SyscallConn()
 	if err != nil {
-		return err
+		return fmt.Errorf("failed to get raw connection: %w", err)
 	}
 
-	return syscall.SetsockoptInt(int(file.Fd()), syscall.SOL_SOCKET, syscall.SO_RCVBUF, bufferSize)
+	var syscallErr error
+	controlErr := rawConn.Control(func(fd uintptr) {
+		syscallErr = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_RCVBUF, bufferSize)
+	})
+	if controlErr != nil {
+		return fmt.Errorf("rawconn control failed: %w", controlErr)
+	}
+	if syscallErr != nil {
+		return fmt.Errorf("syscall failed: %w", syscallErr)
+	}
+
+	return nil
 }