Make urllib3 thread safe.

patch.diff

diff --git a/src/urllib3/connectionpool.py b/src/urllib3/connectionpool.py
index 68015357..45f223a7 100644
--- a/src/urllib3/connectionpool.py
+++ b/src/urllib3/connectionpool.py
@@ -4,6 +4,8 @@ import queue
 import socket
 import sys
 import warnings
+import weakref
+
 from http.client import HTTPResponse as _HttplibHTTPResponse
 from socket import timeout as SocketTimeout
 from types import TracebackType
@@ -129,6 +131,39 @@ class ConnectionPool:
 _blocking_errnos = {errno.EAGAIN, errno.EWOULDBLOCK}
 
 
+# This is basically a backport to make it run under python2:
+# https://docs.python.org/3.9/library/weakref.html#finalizer-objects
+class finalize(object):
+    __slots__ = ()
+
+    _active = {}
+
+    class State(object):
+        __slots__ = ('weakref', 'callback')
+
+    def __init__(self, ob, callback):
+        state = self.State()
+        state.weakref = weakref.ref(ob, self)
+        state.callback = callback
+
+        self._active[self] = state
+
+    def __call__(self, _=None):
+        state = self._active.pop(self, None)
+        if state:
+            state.callback()
+
+
+def close_connections(pool):
+    try:
+        while True:
+            conn = pool.get(block=False)
+            if conn:
+                conn.close()
+    except queue.Empty:
+        pass  # Done.
+
+
 class HTTPConnectionPool(ConnectionPool, RequestMethods):
     """
     Thread-safe connection pool for one host.
@@ -236,6 +271,16 @@ class HTTPConnectionPool(ConnectionPool, RequestMethods):
             self.conn_kw["proxy"] = self.proxy
             self.conn_kw["proxy_config"] = self.proxy_config
 
+        # Do not pass 'self' as callback to 'finalize'.
+        # Then the 'finalize' would keep an endless living (leak) to self.
+        # By just passing a reference to the pool allows the garbage collector
+        # to free self if nobody else has a reference to it.
+        pool = self.pool
+
+        # This calls 'close_connections(pools)' just before HTTPConnectionPool
+        # gets gargabe collected.
+        finalize(self, lambda: close_connections(pool))
+
     def _new_conn(self) -> HTTPConnection:
         """
         Return a fresh :class:`HTTPConnection`.
@@ -500,14 +545,7 @@ class HTTPConnectionPool(ConnectionPool, RequestMethods):
         # Disable access to the pool
         old_pool, self.pool = self.pool, None
 
-        try:
-            while True:
-                conn = old_pool.get(block=False)
-                if conn:
-                    conn.close()
-
-        except queue.Empty:
-            pass  # Done.
+        close_connections(old_pool)
 
     def is_same_host(self, url: str) -> bool:
         """
diff --git a/src/urllib3/poolmanager.py b/src/urllib3/poolmanager.py
index a36f96ae..81ba25d8 100644
--- a/src/urllib3/poolmanager.py
+++ b/src/urllib3/poolmanager.py
@@ -214,11 +214,8 @@ class PoolManager(RequestMethods):
         super().__init__(headers)
         self.connection_pool_kw = connection_pool_kw
 
-        def dispose_func(p: Any) -> None:
-            p.close()
-
         self.pools: RecentlyUsedContainer[PoolKey, HTTPConnectionPool]
-        self.pools = RecentlyUsedContainer(num_pools, dispose_func=dispose_func)
+        self.pools = RecentlyUsedContainer(num_pools)
 
         # Locally set the pool classes and keys so other PoolManagers can
         # override them.