Support reentrant locking on lock file path via optional singleton instance (#283)

Author: nefrob

.pre-commit-config.yaml

@@ -5,12 +5,12 @@ repos:
       - id: end-of-file-fixer
       - id: trailing-whitespace
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: "v0.1.1"
+    rev: "v0.1.3"
     hooks:
       - id: ruff
         args: [--fix, --exit-non-zero-on-fix]
   - repo: https://github.com/psf/black
-    rev: 23.10.0
+    rev: 23.10.1
     hooks:
       - id: black
   - repo: https://github.com/tox-dev/tox-ini-fmt
@@ -19,10 +19,10 @@ repos:
       - id: tox-ini-fmt
         args: ["-p", "fix"]
   - repo: https://github.com/tox-dev/pyproject-fmt
-    rev: "1.2.0"
+    rev: "1.3.0"
     hooks:
       - id: pyproject-fmt
-        additional_dependencies: ["tox>=4.8"]
+        additional_dependencies: ["tox>=4.11.3"]
   - repo: https://github.com/pre-commit/mirrors-prettier
     rev: "v3.0.3"
     hooks:

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing
+
+This page lists the steps needed to set up a development environment and contribute to the project.
+
+1. Fork and clone this repo.
+
+2. [Install tox](https://tox.wiki/en/latest/installation.html#via-pipx).
+
+3. Run tests:
+
+   ```shell
+   tox run
+   ```
+
+   or for a specific python version
+
+   ```shell
+   tox run -f py311
+   ```
+
+4. Running other tox commands (eg. linting):
+
+   ```shell
+   tox -e fix
+   ```

pyproject.toml

@@ -39,21 +39,21 @@ dynamic = [
   "version",
 ]
 optional-dependencies.docs = [
-  "furo>=2023.7.26",
-  "sphinx>=7.1.2",
+  "furo>=2023.9.10",
+  "sphinx>=7.2.6",
   "sphinx-autodoc-typehints!=1.23.4,>=1.24",
 ]
 optional-dependencies.testing = [
   "covdefaults>=2.3",
-  "coverage>=7.3",
-  "diff-cover>=7.7",
-  "pytest>=7.4",
+  "coverage>=7.3.2",
+  "diff-cover>=8",
+  "pytest>=7.4.3",
   "pytest-cov>=4.1",
-  "pytest-mock>=3.11.1",
-  "pytest-timeout>=2.1",
+  "pytest-mock>=3.12",
+  "pytest-timeout>=2.2",
 ]
 optional-dependencies.typing = [
-  'typing-extensions>=4.7.1; python_version < "3.11"',
+  'typing-extensions>=4.8; python_version < "3.11"',
 ]
 urls.Documentation = "https://py-filelock.readthedocs.io"
 urls.Homepage = "https://github.com/tox-dev/py-filelock"

src/filelock/__init__.py

@@ -32,7 +32,7 @@
         if warnings is not None:
             warnings.warn("only soft file lock is available", stacklevel=2)
 
-if TYPE_CHECKING:  # noqa: SIM108
+if TYPE_CHECKING:
     FileLock = SoftFileLock
 else:
     #: Alias for the lock, which should be used for the current platform.

src/filelock/_api.py

@@ -8,7 +8,8 @@
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
 from threading import local
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, ClassVar
+from weakref import WeakValueDictionary
 
 from ._error import Timeout
 
@@ -76,25 +77,53 @@ class ThreadLocalFileContext(FileLockContext, local):
 class BaseFileLock(ABC, contextlib.ContextDecorator):
     """Abstract base class for a file lock object."""
 
-    def __init__(
+    _instances: ClassVar[WeakValueDictionary[str, BaseFileLock]] = WeakValueDictionary()
+
+    def __new__(  # noqa: PLR0913
+        cls,
+        lock_file: str | os.PathLike[str],
+        timeout: float = -1,  # noqa: ARG003
+        mode: int = 0o644,  # noqa: ARG003
+        thread_local: bool = True,  # noqa: ARG003, FBT001, FBT002
+        *,
+        is_singleton: bool = False,
+    ) -> Self:
+        """Create a new lock object or if specified return the singleton instance for the lock file."""
+        if not is_singleton:
+            return super().__new__(cls)
+
+        instance = cls._instances.get(str(lock_file))
+        if not instance:
+            instance = super().__new__(cls)
+            cls._instances[str(lock_file)] = instance
+
+        return instance  # type: ignore[return-value] # https://github.com/python/mypy/issues/15322
+
+    def __init__(  # noqa: PLR0913
         self,
         lock_file: str | os.PathLike[str],
         timeout: float = -1,
         mode: int = 0o644,
         thread_local: bool = True,  # noqa: FBT001, FBT002
+        *,
+        is_singleton: bool = False,
     ) -> None:
         """
         Create a new lock object.
 
         :param lock_file: path to the file
-        :param timeout: default timeout when acquiring the lock, in seconds. It will be used as fallback value in
-        the acquire method, if no timeout value (``None``) is given. If you want to disable the timeout, set it
-        to a negative value. A timeout of 0 means, that there is exactly one attempt to acquire the file lock.
-        :param mode: file permissions for the lockfile.
-        :param thread_local: Whether this object's internal context should be thread local or not.
-        If this is set to ``False`` then the lock will be reentrant across threads.
+        :param timeout: default timeout when acquiring the lock, in seconds. It will be used as fallback value in \
+            the acquire method, if no timeout value (``None``) is given. If you want to disable the timeout, set it \
+            to a negative value. A timeout of 0 means, that there is exactly one attempt to acquire the file lock.
+        :param mode: file permissions for the lockfile
+        :param thread_local: Whether this object's internal context should be thread local or not. If this is set to \
+            ``False`` then the lock will be reentrant across threads.
+        :param is_singleton: If this is set to ``True`` then only one instance of this class will be created \
+            per lock file. This is useful if you want to use the lock object for reentrant locking without needing \
+            to pass the same object around.
         """
         self._is_thread_local = thread_local
+        self._is_singleton = is_singleton
 
         # Create the context. Note that external code should not work with the context directly  and should instead use
         # properties of this class.
@@ -109,6 +138,11 @@ def is_thread_local(self) -> bool:
         """:return: a flag indicating if this lock is thread local or not"""
         return self._is_thread_local
 
+    @property
+    def is_singleton(self) -> bool:
+        """:return: a flag indicating if this lock is singleton or not"""
+        return self._is_singleton
+
     @property
     def lock_file(self) -> str:
         """:return: path to the lock file"""

tests/test_filelock.py

@@ -611,3 +611,47 @@ def test_lock_can_be_non_thread_local(
     assert lock.lock_counter == 2
 
     lock.release(force=True)
+
+
+@pytest.mark.parametrize("lock_type", [FileLock, SoftFileLock])
+def test_singleton_and_non_singleton_locks_are_distinct(lock_type: type[BaseFileLock], tmp_path: Path) -> None:
+    lock_path = tmp_path / "a"
+    lock_1 = lock_type(str(lock_path), is_singleton=False)
+    assert lock_1.is_singleton is False
+
+    lock_2 = lock_type(str(lock_path), is_singleton=True)
+    assert lock_2.is_singleton is True
+    assert lock_2 is not lock_1
+
+
+@pytest.mark.parametrize("lock_type", [FileLock, SoftFileLock])
+def test_singleton_locks_are_the_same(lock_type: type[BaseFileLock], tmp_path: Path) -> None:
+    lock_path = tmp_path / "a"
+    lock_1 = lock_type(str(lock_path), is_singleton=True)
+
+    lock_2 = lock_type(str(lock_path), is_singleton=True)
+    assert lock_2 is lock_1
+
+
+@pytest.mark.parametrize("lock_type", [FileLock, SoftFileLock])
+def test_singleton_locks_are_distinct_per_lock_file(lock_type: type[BaseFileLock], tmp_path: Path) -> None:
+    lock_path_1 = tmp_path / "a"
+    lock_1 = lock_type(str(lock_path_1), is_singleton=True)
+
+    lock_path_2 = tmp_path / "b"
+    lock_2 = lock_type(str(lock_path_2), is_singleton=True)
+    assert lock_1 is not lock_2
+
+
+@pytest.mark.skipif(hasattr(sys, "pypy_version_info"), reason="del() does not trigger GC in PyPy")
+@pytest.mark.parametrize("lock_type", [FileLock, SoftFileLock])
+def test_singleton_locks_are_deleted_when_no_external_references_exist(
+    lock_type: type[BaseFileLock],
+    tmp_path: Path,
+) -> None:
+    lock_path = tmp_path / "a"
+    lock = lock_type(str(lock_path), is_singleton=True)
+
+    assert lock_type._instances == {str(lock_path): lock}  # noqa: SLF001
+    del lock
+    assert lock_type._instances == {}  # noqa: SLF001

tox.ini

@@ -38,15 +38,15 @@ description = format the code base to adhere to our styles, and complain about w
 base_python = python3.10
 skip_install = true
 deps =
-    pre-commit>=3.3.3
+    pre-commit>=3.5
 commands =
     pre-commit run --all-files --show-diff-on-failure
     python -c 'import pathlib; print("hint: run \{\} install to add checks as pre-commit hook".format(pathlib.Path(r"{envdir}") / "bin" / "pre-commit"))'
 
 [testenv:type]
 description = run type check on code base
 deps =
-    mypy==1.5
+    mypy==1.6.1
 set_env =
     {tty:MYPY_FORCE_COLOR = 1}
 commands =
@@ -58,8 +58,8 @@ description = combine coverage files and generate diff (against DIFF_AGAINST def
 skip_install = true
 deps =
     covdefaults>=2.3
-    coverage[toml]>=7.3
-    diff-cover>=7.7
+    coverage[toml]>=7.3.2
+    diff-cover>=8
 extras =
 parallel_show_output = true
 pass_env =
@@ -91,7 +91,7 @@ commands =
 description = check that the long description is valid (need for PyPI)
 skip_install = true
 deps =
-    build[virtualenv]>=0.10
+    build[virtualenv]>=1.0.3
     twine>=4.0.2
 extras =
 commands =