[BUG] Firefox webworker's console messages are reported twice

[achingbrain]

Context:

  System:
    OS: macOS 11.2.1
    Memory: 346.17 MB / 32.00 GB
  Binaries:
    Node: 15.12.0 - ~/.nvm/versions/node/v15.12.0/bin/node
    npm: 7.6.3 - ~/.nvm/versions/node/v15.12.0/bin/npm
  Languages:
    Bash: 3.2.57 - /bin/bash
  npmPackages:
    playwright: ^1.10.0 => 1.10.0 

Code Snippet

Repro repo: https://github.com/achingbrain/playwright-ff-webworkers

$ git clone https://github.com/achingbrain/playwright-ff-webworkers.git
$ cd playwright-ff-webworkers
$ npm i
$ npm start

> playwright-ff-webworkers@1.0.0 start
> node .

globalThis in worker is JSHandle@object
globalThis in worker is [object DedicatedWorkerGlobalScope]

Describe the bug

When running webworker code in Firefox, it appears to get run twice, see the linked repo above for more details and a runnable example.

• Only seems to affect Firefox run via playwright, running the same page in standalone Firefox or Chrome works as expected
• Happens when loading a page via file:// or via a webserver

[achingbrain]

It could just be the console event getting fired twice, rather than the whole worker running twice?

[pavelfeldman]

Thanks for the repro!

Yes, that's the same message printed twice: once from the worker itself and once from the page that owns the worker:

pw:protocol ◀ RECV {"method":"Page.dispatchMessageFromWorker","params":{"workerId":"8a646122-5c21-e140-8dfa-d978e751c86b","message":"{\"method\":\"Runtime.console\",\"params\":{\"args\":[{\"value\":\"globalThis in worker is\"},{\"objectId\":\"id-2\",\"type\":\"object\"}],\"type\":\"info\",\"executionContextId\":\"id-1\",\"location\":{\"lineNumber\":1,\"columnNumber\":8,\"url\":\"http://localhost:5000/worker.js\"}}}"},"sessionId":"9ecfb2d5-2c78-8e47-94e4-b407f7156e4a"} +2ms
console message
globalThis in worker is JSHandle@object
pw:protocol ◀ RECV {"method":"Runtime.console","params":{"args":[{"value":"globalThis in worker is"},{"value":"[object DedicatedWorkerGlobalScope]"}],"type":"info","executionContextId":"id-5","location":{"lineNumber":1,"columnNumber":8,"url":"http://localhost:5000/worker.js"}},"sessionId":"9ecfb2d5-2c78-8e47-94e4-b407f7156e4a"} +7ms

Note that workers aren't supposed to work when running off file://.