[Worker support] Error handling and warnings

[antocuni]

This is a follow-up of #1333 (once it's merged).

It seems that for some reason, error and warning banners don't appear when we run in a worker. There are 3 tests which are skipped because of that:

pyscript/pyscriptjs/tests/integration/test_01_basic.py

Lines 152 to 170 in ef0e838

	@skip_worker("FIXME: the banner doesn't appear")
	def test_non_existent_package(self):
	self.pyscript_run(
	"""
	<py-config>
	packages = ["nonexistendright"]
	</py-config>
	""",
	wait_for_pyscript=False,
	)
	expected_alert_banner_msg = (
	"(PY1001): Unable to install package(s) 'nonexistendright'. "
	"Unable to find package in PyPI. Please make sure you have "
	"entered a correct package name."
	)
	alert_banner = self.page.wait_for_selector(".alert-banner")
	assert expected_alert_banner_msg in alert_banner.inner_text()

pyscript/pyscriptjs/tests/integration/test_01_basic.py

Lines 172 to 189 in ef0e838

	@skip_worker("FIXME: the banner doesn't appear")
	def test_no_python_wheel(self):
	self.pyscript_run(
	"""
	<py-config>
	packages = ["opsdroid"]
	</py-config>
	""",
	wait_for_pyscript=False,
	)
	expected_alert_banner_msg = (
	"(PY1001): Unable to install package(s) 'opsdroid'. "
	"Reason: Can't find a pure Python 3 Wheel for package(s) 'opsdroid'"
	)
	alert_banner = self.page.wait_for_selector(".alert-banner")
	assert expected_alert_banner_msg in alert_banner.inner_text()

pyscript/pyscriptjs/tests/integration/test_01_basic.py

Lines 274 to 289 in ef0e838

	@skip_worker("FIXME: showWarning()")
	def test_assert_no_banners(self):
	"""
	Test that the DOM doesn't contain error/warning banners
	"""
	self.pyscript_run(
	"""
	<py-script>
	from _pyscript_js import showWarning
	showWarning("hello")
	showWarning("world")
	</py-script>
	"""
	)
	with pytest.raises(AssertionError, match="Found 2 alert banners"):
	self.assert_no_banners()

[antocuni]

I have started to investigate the issue.
In theory the execution flows like this:

1. We are in the main thread: PyScriptApp.installPackage calls interpreter._remote.installPackage: interpreter._remote is a Synclink proxy, so the call happens through synclink:

   pyscript/pyscriptjs/src/main.ts

   Line 315 in d3bcd87

   await this.interpreter._remote.installPackage(this.config.packages);

2. We are in the worker: installPackage throws InstallError, which is a subclass of UserError:

   pyscript/pyscriptjs/src/remote_interpreter.ts

   Line 238 in d3bcd87

   throw new InstallError(ErrorCode.MICROPIP_INSTALL_ERROR, exceptionMessage);

3. in theory, the main() function in the main thread catches the UserError and displays the appropriate banner:

   pyscript/pyscriptjs/src/main.ts

   Lines 103 to 107 in d3bcd87

   	try {
   	await this._realMain();
   	} catch (error) {
   	await this._handleUserErrorMaybe(error);
   	}

It seems that synclink propagates the UserError correctly in the main thread case, but it doesn't in the worker case.

As an additional data point, I also tried to run the test with this patch:

diff --git a/pyscriptjs/src/remote_interpreter.ts b/pyscriptjs/src/remote_interpreter.ts
index a245aaa..7fd2f94 100644
--- a/pyscriptjs/src/remote_interpreter.ts
+++ b/pyscriptjs/src/remote_interpreter.ts
@@ -2,7 +2,7 @@ import type { AppConfig } from './pyconfig';
 import { version } from './version';
 import { getLogger } from './logger';
 import { Stdio } from './stdio';
-import { InstallError, ErrorCode } from './exceptions';
+import { InstallError, ErrorCode, UserError } from './exceptions';
 import { robustFetch } from './fetch';
 import type { loadPyodide as loadPyodideDeclaration, PyodideInterface, PyProxy, PyProxyDict } from 'pyodide';
 import type { ProxyMarked } from 'synclink';
@@ -114,6 +114,9 @@ export class RemoteInterpreter extends Object {
                 fullStdLib: false,
             }),
         );
+
+        throw new UserError("hello UserError");
+
         this.interface.registerComlink(Synclink);
         // eslint-disable-next-line @typescript-eslint/no-unsafe-assignment
         this.FS = this.interface.FS;

And I confirm that with pytest -m main the hello UserError banner appears correctly, while in the -m worker case it doesn't.

In the worker case, the console shows this log:

transfer_handlers.ts:134 Uncaught (in promise) UserError: (hello UserError): undefined
    at RemoteInterpreter.loadInterpreter (interpreter_worker.js:3046:13)
deserialize @ transfer_handlers.ts:134
fromWireValue @ transfer_handlers.ts:208
(anonymous) @ task.ts:195
fulfilled @ pyconfig.ts:266

I guess it's an upstream bug of Synclink.

/cc @hoodmane