1771428 - .then getted twice when await in devtools console

Status: VERIFIED FIXED

(DevTools :: Console, defect, P3)

Description

[BZZZZ]

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0

Steps to reproduce:

1. open any web page like https://example.net/
2. press Ctrl+Shift+I
3. select "Console" tab
4. type obj={get then(){console.log("get then")}} into console
5. press enter
6. type await obj
7. press enter

Actual results:

get then appeared twice in console

Expected results:

get then appeared once in console (like if you run (async()=>{await obj})())

Comment 1

[BugBot [:suhaib / :marco]]

The Bugbug bot thinks this bug should belong to the 'DevTools::Console' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Comment 2

[Julian Descottes [:jdescottes]]

This seems to be a side-effect of the way we wrap top-level await expressions in the console:
https://searchfox.org/mozilla-central/rev/b462b11e71b500e084f51e61fbd9e19ea0122c78/devtools/client/debugger/src/workers/parser/mapAwaitExpression.js#181

Basically when we execute await obj in the Firefox DevTools webconsole, we actually run:

(async () => {
  return await obj;
})();

Comment 3

[Julian Descottes [:jdescottes]]

To check with Nicolas after PTO

Comment 4

[Julian Descottes [:jdescottes]]

This is most likely going to be a technical limitation for now, but Nicolas might share more details.

Comment 5

[Nicolas Chevobbe [:nchevobbe]]

As Julian pointed out in Comment 2 , await obj is transformed into

(async () => {
  return await obj;
})();

When this is evaluated, await obj does trigger the first call to then
And then, since await obj returns a thenable and given that the wrapping async function returns a Promise, it will also call then.

We should probably tweak our babel mapper to transform the expression into

(async () => {
  return obj;
})();

which should be enough for what we want to achieve (and won't call then twice)

Comment 6

[Nicolas Chevobbe [:nchevobbe]]

Attachment: Bug 1771428 - [devtools] Remove last statement AwaitExpression in mapTopLevelAwait. r=bomsy.

We use to transform expression like await obj into

(async () => {
  return await obj;
})()
``

So we were basically returning the expression, wrapped in an async iife, adding
a `return` statement before the last statement.
But in the case the last statement is an `AwaitExpression`, we can simply return
the argument, stripping the `await` keyword.
This fixes an issue when awaiting for an object with a `then` getter would execute
the getter twice.
A test case is added in mochitest, and unit test are updated to reflect the new
output.

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/fc9a5269e55c
[devtools] Remove last statement AwaitExpression in mapTopLevelAwait. r=bomsy.

Comment 8

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/fc9a5269e55c

Comment 9

[Alin Ilea, Desktop Test Engineering]

Reproduced the issue in release 102.
Verified - Fixed in Beta 103.0b4 and the latest Nightly build 104.0a1 (2022-07-05) using Windows 10 and macOS 12. "get then" appeared only once in console.