mypy/build: Use _load_json_file in load_tree

[woodruffw]

Description

This is a variant of #6156, albeit not the same exact failure site.

In one of my professional projects, we use mypy to lint a relatively complex Python codebase. For reasons that haven't been fully root-caused, we occasionally see JSON corruption errors that look like this:

Traceback (most recent call last):
  File "/usr/local/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/usr/local/lib/python3.8/dist-packages/mypy/__main__.py", line 11, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "mypy/main.py", line 87, in main
  File "mypy/main.py", line 165, in run_build
  File "mypy/build.py", line 179, in build
  File "mypy/build.py", line 254, in _build
  File "mypy/build.py", line 2697, in dispatch
  File "mypy/build.py", line 3014, in process_graph
  File "mypy/build.py", line 3089, in process_fresh_modules
  File "mypy/build.py", line 1975, in load_tree
  File "/usr/lib/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.8/json/decoder.py", line 340, in decode
    raise JSONDecodeError("Extra data", s, end)
json.decoder.JSONDecodeError: Extra data: line 1 column 5069 (char 5068)

That's with the latest MyPy (0.910), Python 3.8, Ubuntu 20.04. Note that the error here happens in load_tree and not _load_json_file.

We're not 100% sure what causes this error, but we do know how to make the error more comprehensible: by reusing the error handling functionality that's already been added to _load_json_file with #6156. So, that's all this PR does: it reuses that utility function to turn the uncontrolled exception into something more recognizable as a MyPy cache error.

Test Plan

I haven't made any test changes, but I've confirmed that the test suite passes locally with the functional changes that I've applied:

source venv/bin/activate
python3 runtests.py

[woodruffw]

Anything else I can do to help this fix move forwards? 🙂

[JelleZijlstra]

Thanks, sorry for dropping this!

[woodruffw]

No problem at all! Thanks a ton for the merge 🙂