implement admin client for Corrosion admin API

internal/corrosion/admin.go

@@ -0,0 +1,83 @@
+package corrosion
+
+import (
+	"fmt"
+	"gvisor.dev/gvisor/pkg/binary"
+	"io"
+	"net"
+)
+
+// AdminClient is a client for the Corrosion admin API.
+type AdminClient struct {
+	conn net.Conn
+}
+
+func NewAdminClient(sockPath string) (*AdminClient, error) {
+	conn, err := net.Dial("unix", sockPath)
+	if err != nil {
+		return nil, fmt.Errorf("connect to admin socket: %w", err)
+	}
+	return &AdminClient{conn: conn}, nil
+}
+
+func (c *AdminClient) Close() error {
+	return c.conn.Close()
+}
+
+func (c *AdminClient) SendCommand(cmd []byte) (<-chan []byte, error) {
+	if _, err := c.conn.Write(encodeFrame(cmd)); err != nil {
+		return nil, fmt.Errorf("send command: %w", err)
+	}
+
+	// Read the first frame immediately to check for errors.
+	data, err := c.readFrame()
+	if err != nil {
+		return nil, fmt.Errorf("read response: %w", err)
+	}
+
+	ch := make(chan []byte)
+	go func() {
+		defer close(ch)
+		ch <- data
+
+		for {
+			data, err = c.readFrame()
+			if err != nil {
+				return
+			}
+			ch <- data
+		}
+	}()
+
+	return ch, nil
+}
+
+// encodeFrame encodes a length_delimited Tokio frame by prefacing frame data with a frame head that specifies
+// the length of the frame.
+func encodeFrame(data []byte) []byte {
+	encoded := make([]byte, 4+len(data))
+	binary.BigEndian.PutUint32(encoded, uint32(len(data)))
+	copy(encoded[4:], data)
+	return encoded
+}
+
+// readFrame reads a length_delimited Tokio frame by extracting the frame data that follows the frame head.
+func (c *AdminClient) readFrame() ([]byte, error) {
+	// Read the frame head (4 bytes).
+	head := make([]byte, 4)
+	if _, err := io.ReadFull(c.conn, head); err != nil {
+		return nil, fmt.Errorf("read frame head: %w", err)
+	}
+	// Read the frame data (length specified in the frame head).
+	length := binary.BigEndian.Uint32(head)
+	data := make([]byte, length)
+	if _, err := io.ReadFull(c.conn, data); err != nil {
+		return nil, fmt.Errorf("read frame data: %w", err)
+	}
+
+	return data, nil
+}
+
+func (c *AdminClient) ClusterMembershipStates() (<-chan []byte, error) {
+	return c.SendCommand([]byte("{\"Cluster\":\"MembershipStates\"}"))
+}