Improve fuzz_submodule.py coverage & efficacy

The fuzzer was having trouble analyzing `fuzz_submodule.py` when using
the `atheris.instrument_imports()` context manager. Switching to
`atheris.instrument_all()` instead slightly increases the startup time
for the fuzzer, but significantly improves the fuzzing engines ability
to identify new coverage.

The changes here also disable warnings that are logged to `stdout` from
the SUT. These warnings are expected to happen with some inputs and
clutter the fuzzer output logs. They can be optionally re-enabled for
debugging by passing a flag o the Python interpreter command line or
setting the `PYTHONWARNINGS` environment variable.

fuzzing/fuzz-targets/fuzz_submodule.py

@@ -4,13 +4,22 @@
 import tempfile
 from configparser import ParsingError
 from utils import is_expected_exception_message, get_max_filename_length
+from git import Repo, GitCommandError, InvalidGitRepositoryError
 
-if getattr(sys, "frozen", False) and hasattr(sys, "_MEIPASS"):
+if getattr(sys, "frozen", False) and hasattr(sys, "_MEIPASS"):  # pragma: no cover
     path_to_bundled_git_binary = os.path.abspath(os.path.join(os.path.dirname(__file__), "git"))
     os.environ["GIT_PYTHON_GIT_EXECUTABLE"] = path_to_bundled_git_binary
 
-with atheris.instrument_imports():
-    from git import Repo, GitCommandError, InvalidGitRepositoryError
+if not sys.warnoptions:  # pragma: no cover
+    # The warnings filter below can be overridden by passing the -W option
+    # to the Python interpreter command line or setting the `PYTHONWARNINGS` environment variable.
+    import warnings
+    import logging
+
+    # Fuzzing data causes some plugins to generate a large number of warnings
+    # which are not usually interesting and make the test output hard to read, so we ignore them.
+    warnings.simplefilter("ignore")
+    logging.getLogger().setLevel(logging.ERROR)
 
 
 def TestOneInput(data):
@@ -92,6 +101,7 @@ def TestOneInput(data):
 
 
 def main():
+    atheris.instrument_all()
     atheris.Setup(sys.argv, TestOneInput)
     atheris.Fuzz()
 

fuzzing/fuzz-targets/utils.py

@@ -1,5 +1,5 @@
 import atheris  # pragma: no cover
-import os
+import os  # pragma: no cover
 from typing import List  # pragma: no cover
 
 
@@ -24,7 +24,7 @@ def is_expected_exception_message(exception: Exception, error_message_list: List
 
 
 @atheris.instrument_func
-def get_max_filename_length(path: str) -> int:
+def get_max_filename_length(path: str) -> int:  # pragma: no cover
     """
     Get the maximum filename length for the filesystem containing the given path.