Add an 'OSS-Fuzz for CPython' overview (#1801)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: StanFromIreland

testing/index.rst

@@ -10,5 +10,6 @@ Testing and buildbots
    run-write-tests
    silence-warnings
    coverage
+   oss-fuzz
    buildbots
    new-buildbot-worker

testing/oss-fuzz.rst

@@ -0,0 +1,55 @@
+OSS-Fuzz for CPython
+====================
+
+CPython uses `OSS-Fuzz <https://google.github.io/oss-fuzz/>`__, Google's
+continuous fuzzing service for open-source projects, to find bugs and
+security vulnerabilities by feeding semi-random data to various APIs.
+
+CPython has two OSS-Fuzz projects:
+
+* `cpython3 <https://github.com/google/oss-fuzz/tree/master/projects/cpython3>`__:
+  The fuzz targets, seed corpora, and dictionaries can be found in the
+  :cpy-file:`Modules/_xxtestfuzz/` directory of CPython. This project
+  is maintained for existing fuzz targets; add new targets to
+  ``python3-libraries``.
+
+* `python3-libraries <https://github.com/google/oss-fuzz/tree/master/projects/python3-libraries>`__:
+  The fuzz targets, seed corpora, and dictionaries can be found in the
+  :github:`python/library-fuzzers` repository. Access to the repository is
+  managed through the `@python/fuzzers
+  <https://github.com/orgs/python/teams/fuzzers>`__ team on GitHub.
+
+OSS-Fuzz bug reports are private when filed, so access to crash details and
+reproducer test cases is limited to those listed in the ``auto_ccs`` fields of
+the OSS-Fuzz project configuration files. Those listed can log into
+https://oss-fuzz.com/ with their Google account to view crash details,
+reproducer test cases, and project statistics.
+If you need access, contact the ``@python/fuzzers`` team.
+Completed issues, and issues that remain unresolved after 90 days, are publicly
+visible in the `OSS-Fuzz issue tracker
+<https://issues.oss-fuzz.com/issues?q=(python3-libraries%20%7C%20cpython3)>`__.
+
+Coverage and target statistics are available in the OSS-Fuzz Introspector
+project profiles for `cpython3 <https://introspector.oss-fuzz.com/project-profile?project=cpython3>`__ and
+`python3-libraries <https://introspector.oss-fuzz.com/project-profile?project=python3-libraries>`__.
+
+In addition, `CIFuzz <https://google.github.io/oss-fuzz/getting-started/continuous-integration/>`__
+runs the fuzz targets on GitHub Actions for PRs to the ``main`` branch changing
+relevant files.
+
+.. seealso::
+
+   The `libFuzzer <https://llvm.org/docs/LibFuzzer.html>`__ documentation for
+   details about the fuzzing engine used by OSS-Fuzz.
+
+
+Adding new targets
+------------------
+
+Add new targets to the ``python3-libraries`` project. For more
+information, see the documentation in the :github:`python/library-fuzzers`
+repository.
+
+If the new target covers a standard library module, update the relevant CIFuzz
+path configuration so pull requests touching that module trigger fuzzing. See
+the ``LIBRARY_FUZZER_PATHS`` set in :cpy-file:`Tools/build/compute-changes.py`.