tree ec6749ce0d4d02b717f0fd45ee1fa430be4391b8
parent c0081cef3f690aacc098c3ef724637c2c23f117d
author Jonathan Lee <jonathan-j-lee@users.noreply.github.com> 1694451439 -0700
committer GitHub <noreply@github.com> 1694451439 -0700
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[wptrunner] Speed up `wptserve` startup (#41897)

`serve` constructs a list of routes and their handlers and passes them
to each server, which runs in a subprocess. However, when `start()`ing
each subprocess [0], `multiprocessing` implicitly pickles and writes all
handlers to an underlying OS pipe for transport [1]. This can block each
`start()` for a long time (~0.5s) because `StringHandler`s hold file
contents that, collectively, can easily exceed the pipe's initial size
[2].

The worst offenders are:
* /_pdf_js/pdf.worker.js: 1.7 MB
* /_pdf_js/pdf.js: 0.7 MB
* /resources/testdriver.js: 53 kB

Change the static handlers to lazily read their files so that only the
filename needs to be transported over the pipe. Unfortunately,
`/resources/testdriver.js` needs a bespoke handler because it's composed
of two files concatenated together that are not templates. Empirically,
this reduces the time until the tests start running from ~10s to ~5s.

See also: https://crbug.com/1479850

[0]: https://github.com/web-platform-tests/wpt/blob/a4d75218/tools/serve/serve.py#L629
[1]: https://github.com/python/cpython/blob/3.11/Lib/multiprocessing/popen_spawn_posix.py#L62
[2]: https://man7.org/linux/man-pages/man7/pipe.7.html says 64 KiB for Linux.