Add Submodules Fuzz Target

Fuzz Introspector heuristics suggest the Submodule API code represent
"optimal analysis targets" that should yield a meaningful increase in
code coverage. The changes here introduce a first pass at implementing a
fuzz harness that cover the primary APIs/methods related to Submodules.
Of particular interest to me is the `Submodule.config_writer()`
coverage.

Please note however, there is likely plenty of room for improvement in
this harness in terms of both code coverage as well as performance; the
latter of which will see significant benefit from a well curated seed
corpus of `.gitmodules` file like inputs. The `ParsingError` raised by
the fuzzer without a good seed corpus hinders test efficacy
significantly.

fuzzing/fuzz-targets/fuzz_submodule.py

@@ -0,0 +1,92 @@
+import atheris
+import sys
+import os
+import tempfile
+from configparser import ParsingError
+from utils import is_expected_exception_message
+
+if getattr(sys, "frozen", False) and hasattr(sys, "_MEIPASS"):
+    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
+
+
+def TestOneInput(data):
+    fdp = atheris.FuzzedDataProvider(data)
+
+    with tempfile.TemporaryDirectory() as repo_temp_dir:
+        repo = Repo.init(path=repo_temp_dir)
+        repo.index.commit("Initial commit")
+
+        try:
+            with tempfile.TemporaryDirectory() as submodule_temp_dir:
+                sub_repo = Repo.init(submodule_temp_dir, bare=fdp.ConsumeBool())
+                sub_repo.index.commit(fdp.ConsumeUnicodeNoSurrogates(fdp.ConsumeIntInRange(1, 512)))
+
+                submodule_name = f"submodule_{fdp.ConsumeUnicodeNoSurrogates(fdp.ConsumeIntInRange(1, 512))}"
+                submodule_path = os.path.join(repo.working_tree_dir, submodule_name)
+                submodule_url = sub_repo.git_dir
+
+                submodule = repo.create_submodule(submodule_name, submodule_path, url=submodule_url)
+                repo.index.commit(f"Added submodule {submodule_name}")
+
+                with submodule.config_writer() as writer:
+                    key_length = fdp.ConsumeIntInRange(1, max(1, fdp.remaining_bytes()))
+                    value_length = fdp.ConsumeIntInRange(1, max(1, fdp.remaining_bytes()))
+
+                    writer.set_value(
+                        fdp.ConsumeUnicodeNoSurrogates(key_length), fdp.ConsumeUnicodeNoSurrogates(value_length)
+                    )
+                    writer.release()
+
+                submodule.update(init=fdp.ConsumeBool(), dry_run=fdp.ConsumeBool(), force=fdp.ConsumeBool())
+
+                submodule_repo = submodule.module()
+                new_file_path = os.path.join(
+                    submodule_repo.working_tree_dir,
+                    f"new_file_{fdp.ConsumeUnicodeNoSurrogates(fdp.ConsumeIntInRange(1, 512))}",
+                )
+                with open(new_file_path, "wb") as new_file:
+                    new_file.write(fdp.ConsumeBytes(fdp.ConsumeIntInRange(1, 512)))
+                submodule_repo.index.add([new_file_path])
+                submodule_repo.index.commit("Added new file to submodule")
+
+                repo.submodule_update(recursive=fdp.ConsumeBool())
+                submodule_repo.head.reset(commit="HEAD~1", working_tree=fdp.ConsumeBool(), head=fdp.ConsumeBool())
+                # Use fdp.PickValueInList to ensure at least one of 'module' or 'configuration' is True
+                module_option_value, configuration_option_value = fdp.PickValueInList(
+                    [(True, False), (False, True), (True, True)]
+                )
+                submodule.remove(
+                    module=module_option_value,
+                    configuration=configuration_option_value,
+                    dry_run=fdp.ConsumeBool(),
+                    force=fdp.ConsumeBool(),
+                )
+                repo.index.commit(f"Removed submodule {submodule_name}")
+
+        except (ParsingError, GitCommandError, InvalidGitRepositoryError, FileNotFoundError, BrokenPipeError):
+            return -1
+        except ValueError as e:
+            expected_messages = [
+                "SHA is empty",
+                "Reference at",
+                "embedded null byte",
+                "This submodule instance does not exist anymore",
+                "cmd stdin was empty",
+            ]
+            if is_expected_exception_message(e, expected_messages):
+                return -1
+            else:
+                raise e
+
+
+def main():
+    atheris.Setup(sys.argv, TestOneInput)
+    atheris.Fuzz()
+
+
+if __name__ == "__main__":
+    main()

fuzzing/fuzz-targets/utils.py

@@ -0,0 +1,22 @@
+import atheris  # pragma: no cover
+from typing import List  # pragma: no cover
+
+
+@atheris.instrument_func
+def is_expected_exception_message(exception: Exception, error_message_list: List[str]) -> bool:  # pragma: no cover
+    """
+    Checks if the message of a given exception matches any of the expected error messages, case-insensitively.
+
+    Args:
+         exception (Exception): The exception object raised during execution.
+         error_message_list (List[str]): A list of error message substrings to check against the exception's message.
+
+    Returns:
+      bool: True if the exception's message contains any of the substrings from the error_message_list,
+      case-insensitively, otherwise False.
+    """
+    exception_message = str(exception).lower()
+    for error in error_message_list:
+        if error.lower() in exception_message:
+            return True
+    return False