webapi: don't fire error event when a worker script fetch is aborted

terminate() can cancel a still-inflight script fetch. That's a cancellation, not
a failure: a terminated worker must fire no further events, and Chrome is silent
here. We logged an error and fired an ErrorEvent — observable by Cloudflare
challenge scripts, which create a blob-URL worker and terminate it in the same
tick.

On error.TransferCanceled, skip the log and the error event; same log skip in
SharedWorkerGlobalScope.

This was tried in https://github.com/lightpanda-io/browser/pull/3189, but is
now ambiguous thanks to https://github.com/lightpanda-io/browser/pull/3368
which introduced a distinct `error.TransferCanceled`.

src/browser/tests/worker/worker.html

@@ -553,3 +553,42 @@
     });
   }
 </script>
+
+<script id="worker_terminate_during_fetch_fires_no_error" type=module>
+  // terminate() cancels the still-inflight script fetch. That's a
+  // cancellation, not a load failure: a terminated worker fires no further
+  // events, error included. Challenge scripts create a blob worker and
+  // terminate it in the same tick, and probe for exactly this.
+  {
+    const state = await testing.async();
+    const blobUrl = URL.createObjectURL(new Blob(["postMessage('loaded');"], { type: 'text/javascript' }));
+    const worker = new Worker(blobUrl);
+    let fired = null;
+    worker.onerror = () => { fired = 'error'; };
+    worker.onmessage = () => { fired = 'message'; };
+    worker.terminate();
+
+    setTimeout(() => state.resolve(fired), 100);
+    await state.done((fired) => {
+      testing.expectEqual(null, fired);
+      URL.revokeObjectURL(blobUrl);
+    });
+  }
+</script>
+
+<script id="worker_missing_script_fires_error" type=module>
+  // A non-2xx script response is a load failure, and must keep firing the
+  // error event: it travels the same error callback as a cancellation.
+  {
+    const state = await testing.async();
+    const worker = new Worker('./does-not-exist.js');
+    worker.onerror = (e) => {
+      e.preventDefault();
+      state.resolve(e);
+    };
+
+    await state.done((e) => {
+      testing.expectTrue(e instanceof ErrorEvent);
+    });
+  }
+</script>

src/browser/webapi/SharedWorkerGlobalScope.zig

@@ -250,10 +250,14 @@ fn httpErrorCallback(ctx: *anyopaque, err: anyerror) void {
     self._http_transfer = null;
     self.releaseScriptArena();
 
-    log.err(.browser, "shared worker fetch error", .{
-        .url = self._url,
-        .err = err,
-    });
+    // TransferCanceled is teardown cancelling a still-inflight script fetch,
+    // not a load failure.
+    if (err != error.TransferCanceled) {
+        log.err(.browser, "shared worker fetch error", .{
+            .url = self._url,
+            .err = err,
+        });
+    }
 
     // The worker will never load and onconnect will never be registered.
     // Drain the buffered connects so they get dispatched (and dropped at the

src/browser/webapi/Worker.zig

@@ -268,10 +268,16 @@ fn httpErrorCallback(ctx: *anyopaque, err: anyerror) void {
     self._http_transfer = null;
     self.releaseScriptArena();
 
-    log.err(.browser, "worker fetch error", .{
-        .url = self._url,
-        .err = err,
-    });
+    // TransferCanceled is not a load failure: terminate() (or worker teardown)
+    // cancelled a still-inflight script fetch. We shouldn't fireErrorEvent
+    // (or bother logging)
+    const canceled = err == error.TransferCanceled;
+    if (!canceled) {
+        log.err(.browser, "worker fetch error", .{
+            .url = self._url,
+            .err = err,
+        });
+    }
 
     // The worker will never load and onmessage will never be registered.
     // Drain any buffered messages so they get dispatched (and silently
@@ -280,7 +286,9 @@ fn httpErrorCallback(ctx: *anyopaque, err: anyerror) void {
     self._script_loaded = true;
     self._worker_scope.drainPendingMessages();
 
-    self.fireErrorEvent(@errorName(err), null);
+    if (!canceled) {
+        self.fireErrorEvent(@errorName(err), null);
+    }
 }
 
 fn releaseScriptArena(self: *Worker) void {
@@ -481,7 +489,7 @@ pub const JsApi = struct {
 
 const testing = @import("../../testing.zig");
 test "WebApi: Worker" {
-    testing.silenceLog(&.{.http});
+    testing.silenceLog(&.{.http, .browser, .browser});
 
     // Worker tests chain a worker-script fetch with a dynamic-import fetch
     // and a cross-context postMessage. The default 2 s assertion budget can