feat: Add optional poll-based ready check

pkg/client/nbd.go

@@ -7,7 +7,12 @@ import (
 	"io"
 	"net"
 	"os"
+	"path"
+	"path/filepath"
+	"strconv"
+	"strings"
 	"syscall"
+	"time"
 
 	"github.com/pilebones/go-udev/netlink"
 	"github.com/pojntfx/go-nbd/pkg/ioctl"
@@ -24,10 +29,12 @@ var (
 )
 
 type Options struct {
-	ExportName  string
-	BlockSize   uint32
-	OnConnected func()
-	Timeout     int
+	ExportName             string
+	BlockSize              uint32
+	OnConnected            func()
+	ReadyCheckUdev         bool
+	ReadyCheckPollInterval time.Duration
+	Timeout                int
 }
 
 func negotiateNewstyle(conn net.Conn) error {
@@ -60,6 +67,10 @@ func Connect(conn net.Conn, device *os.File, options *Options) error {
 		options.ExportName = "default"
 	}
 
+	if !options.ReadyCheckUdev && options.ReadyCheckPollInterval <= 0 {
+		options.ReadyCheckPollInterval = time.Millisecond
+	}
+
 	var cfd uintptr
 	switch c := conn.(type) {
 	case *net.TCPConn:
@@ -82,41 +93,83 @@ func Connect(conn net.Conn, device *os.File, options *Options) error {
 
 	fatal := make(chan error)
 	if options.OnConnected != nil {
-		udevConn := new(netlink.UEventConn)
-		if err := udevConn.Connect(netlink.UdevEvent); err != nil {
-			return err
-		}
-		defer udevConn.Close()
-
-		var (
-			udevReadyCh = make(chan netlink.UEvent)
-			udevErrCh   = make(chan error)
-			udevQuit    = udevConn.Monitor(udevReadyCh, udevErrCh, &netlink.RuleDefinitions{
-				Rules: []netlink.RuleDefinition{
-					{
-						Env: map[string]string{
-							"DEVNAME": device.Name(),
+		if options.ReadyCheckUdev {
+			udevConn := new(netlink.UEventConn)
+			if err := udevConn.Connect(netlink.UdevEvent); err != nil {
+				return err
+			}
+			defer udevConn.Close()
+
+			var (
+				udevReadyCh = make(chan netlink.UEvent)
+				udevErrCh   = make(chan error)
+				udevQuit    = udevConn.Monitor(udevReadyCh, udevErrCh, &netlink.RuleDefinitions{
+					Rules: []netlink.RuleDefinition{
+						{
+							Env: map[string]string{
+								"DEVNAME": device.Name(),
+							},
 						},
 					},
-				},
-			})
-		)
-		defer close(udevQuit)
+				})
+			)
+			defer close(udevQuit)
 
-		go func() {
-			select {
-			case <-udevReadyCh:
-				close(udevQuit)
+			go func() {
+				select {
+				case <-udevReadyCh:
+					close(udevQuit)
 
-				options.OnConnected()
+					options.OnConnected()
 
-				return
-			case err := <-udevErrCh:
-				fatal <- err
+					return
+				case err := <-udevErrCh:
+					fatal <- err
 
-				return
-			}
-		}()
+					return
+				}
+			}()
+		} else {
+			go func() {
+				sizeFile, err := os.Open(path.Join("/sys", "block", filepath.Base(device.Name()), "size"))
+				if err != nil {
+					fatal <- err
+
+					return
+				}
+				defer sizeFile.Close()
+
+				for {
+					if _, err := sizeFile.Seek(0, io.SeekStart); err != nil {
+						fatal <- err
+
+						return
+					}
+
+					rsize, err := io.ReadAll(sizeFile)
+					if err != nil {
+						fatal <- err
+
+						return
+					}
+
+					size, err := strconv.ParseInt(strings.TrimSpace(string(rsize)), 10, 64)
+					if err != nil {
+						fatal <- err
+
+						return
+					}
+
+					if size > 0 {
+						options.OnConnected()
+
+						return
+					}
+
+					time.Sleep(options.ReadyCheckPollInterval)
+				}
+			}()
+		}
 	}
 
 	if _, _, err := syscall.Syscall(