Support port-forwarding directly to Tart Guest Agent (#489)

internal/worker/socketalias/socketalias.go

@@ -0,0 +1,67 @@
+package socketalias
+
+import (
+	"context"
+	"errors"
+	"net"
+	"os"
+	"path/filepath"
+)
+
+// baseDirectory is the parent directory for transient socket aliases.
+//
+// To keep the Unix socket path short enough and fit the platform limit,
+// we're specifically requesting "/var/tmp" instead of "/var/folders/.../T/".
+const baseDirectory = "/var/tmp"
+
+// Create creates a short Unix socket alias in runtimeDir that points to socketPath.
+//
+// The alias remains valid until runtimeDir is removed.
+func Create(runtimeDir string, socketPath string) (string, error) {
+	// Make the symlink target absolute because a relative TART_HOME would
+	// otherwise be resolved from runtimeDir, breaking the symlink
+	absoluteSocketPath, err := filepath.Abs(socketPath)
+	if err != nil {
+		return "", err
+	}
+
+	// Place the VM's control socket alias in the runtime directory
+	aliasSocketPath := filepath.Join(runtimeDir, "vm.sock")
+	if err := os.Symlink(absoluteSocketPath, aliasSocketPath); err != nil {
+		return "", err
+	}
+
+	return aliasSocketPath, nil
+}
+
+// DialContext connects to socketPath through a short transient alias.
+func DialContext(ctx context.Context, socketPath string) (net.Conn, error) {
+	// Create a directory where the transient Unix socket alias will live
+	runtimeDir, err := os.MkdirTemp(baseDirectory, "orchard-socket-")
+	if err != nil {
+		return nil, err
+	}
+
+	// Route the socket through the short runtime directory
+	aliasSocketPath, err := Create(runtimeDir, socketPath)
+	if err != nil {
+		return nil, errors.Join(err, os.RemoveAll(runtimeDir))
+	}
+
+	// Connect through the short alias
+	var dialer net.Dialer
+	conn, err := dialer.DialContext(ctx, "unix", aliasSocketPath)
+
+	// Remove the transient alias after the connection attempt
+	cleanupErr := os.RemoveAll(runtimeDir)
+	if err != nil {
+		return nil, errors.Join(err, cleanupErr)
+	}
+
+	// Avoid returning a live connection when its alias cannot be removed
+	if cleanupErr != nil {
+		return nil, errors.Join(cleanupErr, conn.Close())
+	}
+
+	return conn, nil
+}