Build 60x slower with sphinx v3.2 compared to v2.4 when using autodoc_mock_imports

[ghost]

[EDIT: updated with reproducible example]

Describe the bug

On a large python project, we recently updated our kernel to use a later version of Sphinx, and the build time increased from 44 seconds to over half an hour!

I believe the issue is related to the automodule mock imports: the slowdown only occurs when a mocked-out library is imported with the from ... import ... syntax as described below.

To Reproduce

Example module foo, and a mocked-out external dependency baz:

foo.py:

from baz import BazClass
import IPython

index.rst :

.. autosummary::
   :toctree: _autosummary
   
   foo

config.py:

autodoc_mock_imports = ["baz"]
autosummary_generate = True  
autosummary_generate_overwrite = True
extensions = ['sphinx.ext.autodoc',  'sphinx.ext.autosummary']

Build command:

$ sphinx-build docs docs/html -E -a

This leads to quite a slow build time of ~2 minutes, whereas it ought to build in about 5 seconds.

Environment info

• OS: Linux-3.10
• Python version: 3.8.5
• Sphinx version: 3.2.1
• Sphinx extensions: autodoc, autosummary, napoleon

Additional context

Bizarrely, any of these changes speeds up the build time to 5 seconds:

• Writing import baz.BazClass rather than from baz import BazClass in foo.py
• Commenting out from baz import BazClass in foo.py
• Commenting out import IPython in foo.py
• Making an actual module called baz available on the path and removing it from the autodoc_mock_imports list

I don't know why the IPython import is needed for the bug to appear. I have ipython v7.18.1 . In practice, none of our modules import IPython directly, but it is imported indirectly in several common libraries like ipykernel and ipywidgets.

On my real-life project, the build time is over half an hour. We mock out a lot of dependencies in the build script assigning a list of modules to autodoc_mock_imports. The vast majority of the documentation is automatically built from the module docstrings with autosummary.

Possibly related: Issue #7479 was closed by #7504 , slowdown apparently due to "recursive calls to inspect.unwrap() for Mocked objects".

[ghost]

@tk0miya I wonder if this bug has a similar root cause to #8164 , which I believe you fixed?

[tk0miya]

Sorry for the late response. Finally, I found the reason for this slowdown. I think #8412 will fix your case. Please check it.