Merge #13747: tests: Skip P2PConnection's is_closing() check when not available

64b9f27e0e Skip is_closing() check when not available. (Daniel Kraft)

Pull request description:

  #13715 introduced a new check for `_transport.is_closing()` in mininode's `P2PConnection`'s.  This function is [only available from Python 3.4.4](https://docs.python.org/3.4/library/asyncio-protocol.html#asyncio.BaseTransport.is_closing), though, while Bitcoin Core is supposed to support all Python 3.4 versions.

  In this change, we make the check conditional on `is_closing` being available.  If it is not, then we revert to the behaviour before the check was introduced; this means that #13579 is not fixed for old systems, but at least the tests work as they used to do before.

  This includes a small refactoring from a one-line lambda to an inline function, because this makes the code easier to read with more and more conditions being added.

  Fixes #13745.

Tree-SHA512: 15be03b8b49c40a946c5b354c5974858d14dc46283ad48ee25d9e269377077ce741c6b379b3f6581ab981cb65be799809afbb99da278caaa2d8d870fa4fb748f

test/functional/test_framework/mininode.py

@@ -179,7 +179,17 @@ class P2PConnection(asyncio.Protocol):
             raise IOError('Not connected')
         self._log_message("send", message)
         tmsg = self._build_message(message)
-        NetworkThread.network_event_loop.call_soon_threadsafe(lambda: self._transport and not self._transport.is_closing() and self._transport.write(tmsg))
+
+        def maybe_write():
+            if not self._transport:
+                return
+            # Python <3.4.4 does not have is_closing, so we have to check for
+            # its existence explicitly as long as Bitcoin Core supports all
+            # Python 3.4 versions.
+            if hasattr(self._transport, 'is_closing') and self._transport.is_closing():
+                return
+            self._transport.write(tmsg)
+        NetworkThread.network_event_loop.call_soon_threadsafe(maybe_write)
 
     # Class utility methods