2 Brython webworkers blocking code - the first blocks Brython Execution of the second #2441

shlomil · 2024-05-17T16:16:02Z

shlomil
May 17, 2024

Hi!
I have a Brython program starting a webworker. The webworker is written in a "synchronous" style and would sometimes block itself on a SharedArrayBuffer. For example, calling sleep using the following code:

def m_sleep(milliseconds: int=0):
    sab = browser.self.SharedArrayBuffer.new(browser.self.Int32Array.BYTES_PER_ELEMENT)
    buffer = browser.self.Int32Array.new(sab)
    browser.self.Atomics.wait(buffer, 0, 0, milliseconds)

I notice that when I open 2 tabs of the same program (using Brython built from the latest git commit), Brython will block the Python code execution of the second tab opened until I close the first tab (or when the sleep ends). I tried to debug but it's not really trivial to debug webworkers. For now my guess is that it's related to Brython's indexedDB operations performed in tasks or mictotasks. IndexedDB will block open calls with higher version number as long as someone else is keeping an open context on an older version. I guess that my sleep call froze that operation on the first tab before it could be closed properly and that's why the second one was blocked.

I have two questions regarding this behaviour:

Should that be considered a bug? I guess not since blocking and writing synchronous programs on the web is "unorthodox".
How would I wait for the Brython indexedDB operations to end and release the opened context before calling the blocking code in my worker? Do you have any other solution besides "Just don't block" your thread?

Thanks,
Shlomi

shlomil · 2024-05-17T20:00:08Z

shlomil
May 17, 2024
Author

I found a temporary solution for my issue: I import and execute the blocking code with a timer.set_timeout(blocking_func, 0).
This way all the microtasks are probably executed before the timeout task and it seems to be enough to free whatever was blocking the execution of the second tab.

0 replies

shlomil · 2024-05-18T04:47:10Z

shlomil
May 18, 2024
Author

Minimal reproduction code:

<!doctype html>
<html>
<head>
<meta charset="utf-8">

<script src="https://raw.githack.com/brython-dev/brython/master/www/src/brython.js"
        crossorigin="anonymous">
</script>
<script src="https://raw.githack.com/brython-dev/brython/master/www/src/brython_stdlib.js"
        crossorigin="anonymous">
</script>
<brython-options debug="1" cache="true"></brython-options>
</head>
<body>
<script type="text/python" class="webworker" id="worker">
print('worker started running...')
from browser import bind, self

def m_sleep(milliseconds: int=0):
    sab = self.SharedArrayBuffer.new(self.Int32Array.BYTES_PER_ELEMENT)
    buffer = self.Int32Array.new(sab)
    self.Atomics.wait(buffer, 0, 0, milliseconds)


@bind(self, "message")
def message(evt):
    print(evt.data)
    result = 'pong' if evt.data == 'ping' else evt.data
    self.send(result)

m_sleep(1000*20)

</script>

<script type="text/python">
print('brython started running...')
from browser import worker

def ready(w):
  w.send('ping')

def message(ev):
  print(ev.data)

worker.create_worker("worker", onready=ready, onmessage=message)
</script>
</body>
</html>

Second simple script:

<!doctype html>
<html>
<head>
<meta charset="utf-8">

<script src="https://raw.githack.com/brython-dev/brython/master/www/src/brython.js"
        crossorigin="anonymous">
</script>
<script src="https://raw.githack.com/brython-dev/brython/master/www/src/brython_stdlib.js"
        crossorigin="anonymous">
</script>
<brython-options debug="1" cache="true"></brython-options>
</head>
<body>
<script>
console.log('hello from js')
</script>
<script type="text/python">
print('hello from brython')
</script>
</body>
</html>

Have the server send with these headers for SharedArrayBuffer support:

{"Cross-Origin-Opener-Policy": "same-origin",
 "Cross-Origin-Embedder-Policy": "require-corp"

Start the first script then start the second in another window/tab.
You will notice that "hello from js" is printed but the "hello from brython" is blocked until the first script will finish sleeping.

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2 Brython webworkers blocking code - the first blocks Brython Execution of the second #2441

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2 Brython webworkers blocking code - the first blocks Brython Execution of the second #2441

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