Promises sometimes don't get called

[rotu]

Specification

• pywebview version: 3.0.2
• platform / version: MacOS Catalina

Description

Initiate a function call from python to javascript multiple times. Observe that the function does get called multiple times but only one value is returned to Javascript via promise.

    for (let i = 0; i < 10; i++) {
        console.log('request ' + i);
        api.echo(i).then((j) => {
            console.log('response ' + i)
        })
    }

class JSApi:
    def echo(self, value):
        print(value)
        return value

Javascript output:

[Log] request 0 (main.js, line 39)
[Log] request 1 (main.js, line 39)
[Log] request 2 (main.js, line 39)
[Log] request 3 (main.js, line 39)
[Log] request 4 (main.js, line 39)
[Log] request 5 (main.js, line 39)
[Log] request 6 (main.js, line 39)
[Log] request 7 (main.js, line 39)
[Log] request 8 (main.js, line 39)
[Log] request 9 (main.js, line 39)
[Log] response 0 (main.js, line 41)

Practicalities

• YES I am willing to work on this issue myself.

• NO I am not prepared to support this issue financially.

[r0x0r]

It looks like a race condition is in question here. As a function level global return value is used, it gets immediately overwritten by subsequent function calls. A solution would be to differentiate function calls by uuid or something along these lines.

[rotu]

I wouldn’t call it a race condition but you’re spot on about the cause. I’m actively working on a fix.

[r0x0r]

I have pushed a tentative fix to the js-api-fix branch. It is mostly untested, tests should be fixed and Windows binaries should be regenerated as well.

[r0x0r]

PR here
#400

[r0x0r]

test_js_api::test_concurrent randomly fails on Windows. After hours of debugging I could not track the cause of the bug down. I won't spend any more time trying to fix it and let it be for the time being. Let's hope it is a bug in the assert_js and not the underlying code.