Can't attach to debugger on Dart VM running frontend_server

[jonahwilliams]

I'm not sure the exact scenario required to trigger this, but attempting to connect to the VM Service of a frontend_server snapshot was sufficient to reproduce it. Debugging from the Observatory works.

To get the Observatory to print out:

diff --git a/packages/flutter_tools/lib/src/compile.dart b/packages/flutter_tools/lib/src/compile.dart
index 76ba6793b8..bc681ee8c9 100644
--- a/packages/flutter_tools/lib/src/compile.dart
+++ b/packages/flutter_tools/lib/src/compile.dart
@@ -696,11 +696,13 @@ class DefaultResidentCompiler implements ResidentCompiler {
     );
     final List<String> command = <String>[
       _artifacts.getHostArtifact(HostArtifact.engineDartBinary).path,
+      '--observe',
       '--disable-dart-dev',
       frontendServer,
       '--sdk-root',
       sdkRoot,
       '--incremental',
+      '--verbose',
       if (testCompilation)
         '--no-print-incremental-dependencies',
       '--target=$targetModel',

To trigger the crash I'm trying to debug (requires running a flutter web app)

diff --git a/packages/flutter_tools/lib/src/resident_runner.dart b/packages/flutter_tools/lib/src/resident_runner.dart
index 0342a56138..786db092cd 100644
--- a/packages/flutter_tools/lib/src/resident_runner.dart
+++ b/packages/flutter_tools/lib/src/resident_runner.dart
@@ -116,6 +116,7 @@ class FlutterDevice {
       } else {
         assert(false);
       }
+      extraFrontEndOptions.add('--enable-experiment=alternative-invalidation-strategy');

       generator = ResidentCompiler(
         globals.artifacts.getHostArtifact(HostArtifact.flutterWebSdk).path,

Note that it says it is paused in the debugger, but there is no debugger pane and hitting step doesn't seem to do anything.

[jonahwilliams]

Now that I say that, the observatory seems to be having problems too. I wonder if this is an issue trying to debug app jit snapshots?

[kenzieschmoll]

@bkonyi @andrewkolos can either of you provide repro steps on how to run the frontend_server from Flutter Tools today? Or based on @jonahwilliams's last comment, perhaps repro steps on how to run an app jit snapshot to attempt to connect it to DevTools?

[bkonyi]

The frontend server shipped with the Flutter SDK is an AOT snapshot and probably isn't debuggable at this point.

If you want to try DevTools with a generic AppJIT snapshot, you can just do the following:

$ dart compile jit-snapshot foo.dart
Compiling /Users/bkonyi/dart-sdk/sdk/foo.dart to jit-snapshot file /Users/bkonyi/dart-sdk/sdk/foo.jit.
...
$ dart run --observe foo.jit

From my quick tests, the debugger isn't available. I'm guessing that we don't provide sources in the AppJIT snapshots so we disable the debugger.

[srawlins]

As noted in the discussion, the frontend_server shipped with the Flutter SDK is now an AOT snapshot and is not debuggable. Furthermore, generic AppJIT snapshots do not embed Dart sources, so the VM Service cannot provide source files to the debugger (which caused the same behavior in Observatory).

Because frontend_server is not intended to be debugged via DevTools in this manner and the snapshot format does not provide the required source mappings, we are closing this issue. If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!