Python Reentrant Read Write Lock: Allowing Multithreaded Read Access While Maintaining a Write Lock

read_write_lock.py

import threading
from typing import List


class ReentrantRWLock:
    """
    A lock object that allows many simultaneous "read locks", but only one "write lock."
    it also allows multiple write locks from the same thread
    """

    def __init__(self):
        self._writer = None  # current writer
        self._readers: List[int] = []  # list of unique readers
        self._read_ready = threading.Condition(threading.Lock())
        self._with_ops_write = []  # stack for 'with' keyword for write or read operations, 0 for read 1 for write
        self._ops_arr_lock = threading.Lock() # lock for previous list

    def acquire_read(self):
        """
        Acquire a read lock. Blocks only if a another thread has acquired the write lock.
        """
        ident = threading.current_thread().ident
        if self._writer == ident or ident in self._readers:
            return
        with self._read_ready:
            self._readers.append(ident)

    def release_read(self):
        """
        Release a read lock if exists from this thread
        """
        ident = threading.current_thread().ident
        if self._writer == ident or ident not in self._readers:
            return
        with self._read_ready:
            self._readers.remove(ident)
            if len(self._readers) == 0:
                self._read_ready.notifyAll()

    def acquire_write(self):
        """
        Acquire a write lock. Blocks until there are no acquired read or write locks from another thread.
        """
        ident = threading.current_thread().ident
        if self._writer == ident:
            return
        self._read_ready.acquire()
        me_included = 1 if ident in self._readers else 0
        while len(self._readers) - me_included > 0:
            self._read_ready.wait()
        self._writer = ident

    def release_write(self):
        """
        Release a write lock if exists from this thread.
        """
        if not self._writer or not self._writer == threading.current_thread().ident:
            return
        self._writer = None
        self._read_ready.release()

    def __enter__(self):
        with self._ops_arr_lock:
            if len(self._with_ops_write) == 0:
                raise RuntimeError("ReentrantRWLock: used 'with' block without call to for_read or for_write")
            write = self._with_ops_write[-1]
        if write:
            self.acquire_write()
        else:
            self.acquire_read()

    def __exit__(self, exc_type, exc_value, tb):
        with self._ops_arr_lock:
            write = self._with_ops_write.pop()
        if write:
            self.release_write()
        else:
            self.release_read()
        if exc_type is not None:
            return False  # exception happened
        return True

    def for_read(self):
        """
        used for 'with' block
        """
        with self._ops_arr_lock:
            self._with_ops_write.append(0)
        return self

    def for_write(self):
        """
        used for 'with' block
        """
        with self._ops_arr_lock:
            self._with_ops_write.append(1)
        return self

I forked your repo and tried my hand at a re-implementation using context managers to fix the problem of "not knowing if we are writing or not".
This at least seems to fix my test case above.
Also, it has the advantage of not allowing anyone to enter the context without explicitly calling either the for_read or for_write methods.

Update:
I also found with the original code that the reentrant part of the lock is brocken.
If you aquire the same lock multiple times and then release once, then thread will no longer hold the lock even though it should still hold it (multiple times). I have a counting/fixed version here .