workers: basic event loop (#3828)

* establish basic event loop for workers
* make "self.close()" inside worker
* remove "runWorkerMessageLoop() - instead manually call global function 
  in Rust when message arrives. This is done in preparation for structured clone
* refactor "WorkerChannel" and use distinct structs for internal 
  and external channels;  "WorkerChannelsInternal" and "WorkerHandle"
* move "State.worker_channels_internal" to "Worker.internal_channels"
* add "WorkerEvent" enum for child->host communication; 
  currently "Message(Buf)" and  "Error(ErrBox)" variants are supported
* add tests for nested workers
* add tests for worker throwing error on startup

1 files changed, 19 insertions, 0 deletions

diff --git a/cli/tests/subdir/nested_worker.js b/cli/tests/subdir/nested_worker.js
new file mode 100644
index 000000000..b0acd70d7
--- /dev/null
+++ b/cli/tests/subdir/nested_worker.js
@@ -0,0 +1,19 @@
+// Specifier should be resolved relative to current file
+const jsWorker = new Worker("./sibling_worker.js", {
+  type: "module",
+  name: "sibling"
+});
+
+jsWorker.onerror = _e => {
+  postMessage({ type: "error" });
+};
+
+jsWorker.onmessage = e => {
+  console.log("js worker on message");
+  postMessage({ type: "msg", text: e });
+  close();
+};
+
+onmessage = function(e) {
+  jsWorker.postMessage(e.data);
+};