Make the DAP server's pytb_file CLI argument optional

Summary:
The `tintype_dap_server` CLI's positional `pytb_file` argument was a redundant pre-flight check duplicating logic that already lives in the DAP `launch` handler. The path the server actually opens is the one in the `launch` request body (`arguments.pytbPath`), which `SnapshotDebugSession.handle_launch` validates (`os.path.isfile`) and hands to `SnapshotReader` — the CLI's copy is discarded after the `os.path.isfile` fast-fail.

This diff relaxes the positional to `nargs="?"` and gates the existence check on a non-`None` value. Callers that still pass the path get the same fast-fail UX as before; callers that don't (the VS Code extension is about to stop doing so — see the next diff in the stack) let `handle_launch` validate.

Everything downstream is unchanged: `serve()` / `run_session_on_stdio()` never received the path, `handle_launch` continues to require `arguments.pytbPath`, the outer Meta wrapper (`fbcode/tintype/fb/tintype_dap_server.py`) and the `--show-runtime-path(s)` short-circuit don't inspect `pytb_file`.

Reviewed By: aperez

Differential Revision: D106546434

fbshipit-source-id: c2c0d59aef1d097c79ffda0f2eb3291c761f2411

Author: pdepetro

dap/cli.py

@@ -91,7 +91,14 @@ def _parse_args(argv: list[str] | None = None) -> argparse.Namespace:
     )
     parser.add_argument(
         "pytb_file",
-        help="Path to the .pytb snapshot file.",
+        nargs="?",
+        default=None,
+        help=(
+            "Optional path to the .pytb snapshot file. When supplied, the CLI "
+            "fast-fails with INVALID_PYTB if the path is missing or not a "
+            "regular file. When omitted, all path validation is deferred to "
+            "the DAP ``launch`` request's ``pytbPath`` argument."
+        ),
     )
     parser.add_argument(
         "--listen",
@@ -114,23 +121,26 @@ def _parse_args(argv: list[str] | None = None) -> argparse.Namespace:
 def main(argv: list[str] | None = None) -> LauncherExitCode:
     """Run the DAP server over stdio (default) or TCP (``--listen``).
 
-    Fast-fails on a missing ``.pytb`` path with
-    :attr:`LauncherExitCode.INVALID_PYTB`; full snapshot validity is
-    left to the DAP ``launch`` handler, which surfaces failures as a
-    DAP error the client can render cleanly.
+    When ``pytb_file`` is supplied as a positional argument, fast-fails
+    on a missing path with :attr:`LauncherExitCode.INVALID_PYTB` before
+    the transport starts. When omitted, all path validation is
+    deferred to the DAP ``launch`` handler — which surfaces failures
+    as DAP errors the client can render cleanly via
+    ``arguments.pytbPath`` — so the path can live entirely in the
+    launch request body.
 
     Returns:
         :attr:`LauncherExitCode.OK` on clean shutdown.
         :attr:`LauncherExitCode.ERROR` when the session loop returns a
         non-zero status.
-        :attr:`LauncherExitCode.INVALID_PYTB` when ``pytb_file`` does
-        not exist or is not a regular file.
+        :attr:`LauncherExitCode.INVALID_PYTB` when a ``pytb_file`` was
+        supplied but does not exist or is not a regular file.
         :attr:`LauncherExitCode.KEYBOARD_INTERRUPT` on SIGINT during the
         serve loop.
     """
     args = _parse_args(argv)
 
-    if not os.path.isfile(args.pytb_file):
+    if args.pytb_file is not None and not os.path.isfile(args.pytb_file):
         print(
             f"tintype_dap_server: pytb path does not exist or is not a "
             f"file: {args.pytb_file}",

dap/tests/test_cli.py

@@ -46,6 +46,13 @@ def test_rejects_malformed_listen(self) -> None:
         with self.assertRaises(SystemExit):
             module._parse_args(["/tmp/snap.pytb", "--listen", "not-a-port"])
 
+    def test_pytb_file_optional(self) -> None:
+        """``pytb_file`` is optional — when omitted, validation defers to
+        the DAP ``launch`` handler. Argparse must not raise."""
+        args = module._parse_args([])
+        self.assertIsNone(args.pytb_file)
+        self.assertIsNone(args.listen)
+
 
 class MainFileValidationTest(unittest.TestCase):
     """The CLI fast-fails on obvious path mistakes before touching transports."""
@@ -114,6 +121,33 @@ def test_keyboard_interrupt_maps_to_exit_code(self) -> None:
         ):
             self.assertEqual(module.main([path]), LauncherExitCode.KEYBOARD_INTERRUPT)
 
+    def test_no_pytb_dispatches_to_stdio(self) -> None:
+        """``pytb_file`` may be omitted entirely; the CLI's existence
+        check is skipped and the transport launches as usual. Path
+        validation is left to ``handle_launch``."""
+        with (
+            unittest.mock.patch(
+                "tintype.dap.cli.run_session_on_stdio", return_value=0
+            ) as stdio_mock,
+            unittest.mock.patch("tintype.dap.cli.serve") as serve_mock,
+        ):
+            rc = module.main([])
+        self.assertEqual(rc, LauncherExitCode.OK)
+        stdio_mock.assert_called_once_with()
+        serve_mock.assert_not_called()
+
+    def test_no_pytb_with_listen_dispatches_to_serve(self) -> None:
+        """Same as ``test_no_pytb_dispatches_to_stdio`` but for the
+        TCP transport branch — ``--listen`` works without a positional."""
+        with (
+            unittest.mock.patch("tintype.dap.cli.serve", return_value=0) as serve_mock,
+            unittest.mock.patch("tintype.dap.cli.run_session_on_stdio") as stdio_mock,
+        ):
+            rc = module.main(["--listen", "0"])
+        self.assertEqual(rc, LauncherExitCode.OK)
+        serve_mock.assert_called_once_with(host="127.0.0.1", port=0)
+        stdio_mock.assert_not_called()
+
 
 class LauncherExitCodeTest(unittest.TestCase):
     """Sanity checks on the enum itself so callers relying on the