Catch and wrap ClosedPoolError

Partially resolves #1572: "urllib3 exceptions passing through requests
API".

Inspired from Lukasa's 2605be11d82d42438ac7c3993810c955bde74cef.

requests/adapters.py

@@ -19,6 +19,7 @@ from .compat import urlparse, basestring
 from .utils import (DEFAULT_CA_BUNDLE_PATH, get_encoding_from_headers,
                     prepend_scheme_if_needed, get_auth_from_url, urldefragauth)
 from .structures import CaseInsensitiveDict
+from .packages.urllib3.exceptions import ClosedPoolError
 from .packages.urllib3.exceptions import ConnectTimeoutError
 from .packages.urllib3.exceptions import HTTPError as _HTTPError
 from .packages.urllib3.exceptions import MaxRetryError
@@ -421,6 +422,9 @@ class HTTPAdapter(BaseAdapter):
 
             raise ConnectionError(e, request=request)
 
+        except ClosedPoolError as e:
+            raise ConnectionError(e, request=request)
+
         except _ProxyError as e:
             raise ProxyError(e)
 

test_requests.py

@@ -1655,6 +1655,16 @@ def test_urllib3_retries():
     with pytest.raises(RetryError):
         s.get(httpbin('status/500'))
 
+
+def test_urllib3_pool_connection_closed():
+    s = requests.Session()
+    s.mount('http://', HTTPAdapter(pool_connections=0, pool_maxsize=0))
+
+    try:
+        s.get(httpbin('status/200'))
+    except ConnectionError as e:
+        assert u"HTTPConnectionPool(host='httpbin.org', port=80): Pool is closed." in str(e.message)
+
 def test_vendor_aliases():
     from requests.packages import urllib3
     from requests.packages import chardet