cli-plugins: use non-abstract socket on darwin

As macOS does not support the abstract socket namespace, use a temporary
socket in $TMPDIR to connect with the plugin. Ensure this socket is
cleaned up even in the case of crash/ungraceful termination by removing
it after the first connection is accepted.

Co-authored-by: Laura Brehm <laurabrehm@hey.com>
Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

cli-plugins/socket/socket.go

@@ -16,7 +16,7 @@ const EnvKey = "DOCKER_CLI_PLUGIN_SOCKET"
 // SetupConn sets up a Unix socket listener, establishes a goroutine to handle connections
 // and update the conn pointer, and returns the environment variable to pass to the plugin.
 func SetupConn(conn **net.UnixConn) (string, error) {
-	listener, err := listen()
+	listener, err := listen("docker_cli_" + uuid.Generate().String())
 	if err != nil {
 		return "", err
 	}
@@ -26,13 +26,6 @@ func SetupConn(conn **net.UnixConn) (string, error) {
 	return EnvKey + "=" + listener.Addr().String(), nil
 }
 
-func listen() (*net.UnixListener, error) {
-	return net.ListenUnix("unix", &net.UnixAddr{
-		Name: "@docker_cli_" + uuid.Generate().String(),
-		Net:  "unix",
-	})
-}
-
 func accept(listener *net.UnixListener, conn **net.UnixConn) {
 	defer listener.Close()
 
@@ -41,6 +34,8 @@ func accept(listener *net.UnixListener, conn **net.UnixConn) {
 			// ignore error here, if we failed to accept a connection,
 			// conn is nil and we fallback to previous behavior
 			*conn, _ = listener.AcceptUnix()
+			// perform any platform-specific actions on accept (e.g. unlink non-abstract sockets)
+			onAccept(*conn, listener)
 		}
 	}()
 }

cli-plugins/socket/socket_darwin.go

@@ -0,0 +1,19 @@
+package socket
+
+import (
+	"net"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+func listen(socketname string) (*net.UnixListener, error) {
+	return net.ListenUnix("unix", &net.UnixAddr{
+		Name: filepath.Join(os.TempDir(), socketname),
+		Net:  "unix",
+	})
+}
+
+func onAccept(conn *net.UnixConn, listener *net.UnixListener) {
+	syscall.Unlink(listener.Addr().String())
+}

cli-plugins/socket/socket_nodarwin.go

@@ -0,0 +1,19 @@
+//go:build !darwin
+
+package socket
+
+import (
+	"net"
+)
+
+func listen(socketname string) (*net.UnixListener, error) {
+	return net.ListenUnix("unix", &net.UnixAddr{
+		Name: "@" + socketname,
+		Net:  "unix",
+	})
+}
+
+func onAccept(conn *net.UnixConn, listener *net.UnixListener) {
+	// do nothing
+	// while on darwin we would unlink here; on non-darwin the socket is abstract and not present on the filesystem
+}