RangeTransfer: fix exception handling.

If a normal exception occurs while reading an HTTP response, we want
to read the remaining data so that the connection can be reused for
another transfer.

If the RangeTransfer is deleted without calling close() or __exit__(),
the response object must still be explicitly closed so that it no
longer counts against the connection pool limit.  This doesn't happen
automatically when a Response is garbage-collected, which is probably
a bug in python-requests.

Author: Benjamin Moody

wfdb/io/_url.py

@@ -151,10 +151,11 @@ def __init__(self, url, start, end):
             }
 
         session = _get_session()
-        response = session.request(method, url, headers=headers, stream=True)
-        self._content_iter = response.iter_content(4096)
+        self._response = session.request(method, url, headers=headers,
+                                         stream=True)
+        self._content_iter = self._response.iter_content(4096)
         try:
-            self._parse_headers(method, response)
+            self._parse_headers(method, self._response)
         except Exception:
             self.close()
             raise
@@ -264,7 +265,7 @@ def close(self):
                 pass
         except Exception:
             pass
-        self._content_iter = iter([])
+        self._response.close()
 
     def __enter__(self):
         return self
@@ -273,9 +274,16 @@ def __exit__(self, exc_type, exc_val, exc_tb):
         # When exiting with a normal exception, shut down cleanly by
         # reading leftover response data.  When exiting abnormally
         # (SystemExit, KeyboardInterrupt), do nothing.
-        if not exc_type or isinstance(exc_type, Exception):
+        if not exc_type or issubclass(exc_type, Exception):
             self.close()
 
+    def __del__(self):
+        # If the object is deleted without calling close(), forcibly
+        # close the existing connection.
+        response = getattr(self, '_response', None)
+        if response:
+            response.close()
+
     def iter_chunks(self):
         """
         Iterate over the response body as a sequence of chunks.