[BUG] redirect-from sphinx extension is not parallel safe #24058
Description
Summary
CI builds the docs in parallel since #20718.
This doesn't work correctly with redirects: in #24054 it raises
Traceback (most recent call last):
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/cmd/build.py", line 279, in build_main
app.build(args.force_all, filenames)
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/application.py", line 350, in build
self.builder.build_update()
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 302, in build_update
self.build(to_build,
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 318, in build
updated_docnames = set(self.read())
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 423, in read
self._read_parallel(docnames, nproc=self.app.parallel)
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/builders/__init__.py", line 479, in _read_parallel
tasks.join()
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/util/parallel.py", line 105, in join
if not self._join_one():
File "/home/circleci/.local/lib/python3.8/site-packages/sphinx/util/parallel.py", line 126, in _join_one
raise SphinxParallelError(*result)
sphinx.errors.SphinxParallelError: ValueError: gallery/pyplots/dollar_ticks is already noted as redirecting to gallery/ticks/dollar_ticks
Sphinx parallel build error:
ValueError: gallery/pyplots/dollar_ticks is already noted as redirecting to gallery/ticks/dollar_ticks
(see also #21876)
Proposed fix
Revert afef2e6.