THRIFT-5248: Python: Make TSocket.isOpen check if server still connected

Client: py

This is inspired by changes to the Go library (THRIFT-5214) and, by
proxy, this blog post[1]. The idea is that if the other end of the
socket has closed their end of the connection, we can figure that out by
doing a non-blocking read on our socket before we waste time serializing
and sending a message just to find out the socket is closed when we try
to read the response.

[1]: https://github.blog/2020-05-20-three-bugs-in-the-go-mysql-driver/

3 files changed, 83 insertions, 1 deletions

diff --git a/lib/py/Makefile.am b/lib/py/Makefile.am
index c6c5ff38c..b16305790 100644
--- a/lib/py/Makefile.am
+++ b/lib/py/Makefile.am
@@ -50,6 +50,7 @@ check-local: all py3-test
 	$(PYTHON) test/thrift_json.py
 	$(PYTHON) test/thrift_transport.py
 	$(PYTHON) test/test_sslsocket.py
+	$(PYTHON) test/test_socket.py
 	$(PYTHON) test/thrift_TBinaryProtocol.py
 	$(PYTHON) test/thrift_TZlibTransport.py
 	$(PYTHON) test/thrift_TCompactProtocol.py
diff --git a/lib/py/src/transport/TSocket.py b/lib/py/src/transport/TSocket.py
index 9886fa2f8..3c7a3ca7d 100644
--- a/lib/py/src/transport/TSocket.py
+++ b/lib/py/src/transport/TSocket.py
@@ -74,7 +74,31 @@ class TSocket(TSocketBase):
         self.handle = h
 
     def isOpen(self):
-        return self.handle is not None
+        if self.handle is None:
+            return False
+
+        # this lets us cheaply see if the other end of the socket is still
+        # connected. if disconnected, we'll get EOF back (expressed as zero
+        # bytes of data) otherwise we'll get one byte or an error indicating
+        # we'd have to block for data.
+        #
+        # note that we're not doing this with socket.MSG_DONTWAIT because 1)
+        # it's linux-specific and 2) gevent-patched sockets hide EAGAIN from us
+        # when timeout is non-zero.
+        original_timeout = self.handle.gettimeout()
+        try:
+            self.handle.settimeout(0)
+            try:
+                peeked_bytes = self.handle.recv(1, socket.MSG_PEEK)
+            except (socket.error, OSError) as exc:  # on modern python this is just BlockingIOError
+                if exc.errno in (errno.EWOULDBLOCK, errno.EAGAIN):
+                    return True
+                return False
+        finally:
+            self.handle.settimeout(original_timeout)
+
+        # the length will be zero if we got EOF (indicating connection closed)
+        return len(peeked_bytes) == 1
 
     def setTimeout(self, ms):
         if ms is None:
diff --git a/lib/py/test/test_socket.py b/lib/py/test/test_socket.py
new file mode 100644
index 000000000..95124dcbe
--- /dev/null
+++ b/lib/py/test/test_socket.py
@@ -0,0 +1,57 @@
+import errno
+import unittest
+
+from test_sslsocket import ServerAcceptor
+
+import _import_local_thrift  # noqa
+
+from thrift.transport.TSocket import TServerSocket
+from thrift.transport.TSocket import TSocket
+from thrift.transport.TTransport import TTransportException
+
+
+class TSocketTest(unittest.TestCase):
+    def test_isOpen_checks_for_readability(self):
+        # https://docs.python.org/3/library/socket.html#notes-on-socket-timeouts
+        # https://docs.python.org/3/library/socket.html#socket.socket.settimeout
+        timeouts = [
+            None,  # blocking mode
+            0,  # non-blocking mode
+            1.0,  # timeout mode
+        ]
+
+        for timeout in timeouts:
+            acc = ServerAcceptor(TServerSocket(port=0))
+            acc.start()
+
+            sock = TSocket(host="localhost", port=acc.port)
+            sock.open()
+            sock.setTimeout(timeout)
+
+            # the socket shows as open immediately after connecting
+            self.assertTrue(sock.isOpen())
+
+            # and remains open during usage
+            sock.write(b"hello")
+            self.assertTrue(sock.isOpen())
+            while True:
+                try:
+                    sock.read(5)
+                except TTransportException as exc:
+                    if exc.inner.errno == errno.EAGAIN:
+                        # try again when we're in non-blocking mode
+                        continue
+                    raise
+                break
+            self.assertTrue(sock.isOpen())
+
+            # once the server side closes, it no longer shows open
+            acc.client.close()  # this also blocks until the other thread is done
+            acc.close()
+            self.assertFalse(sock.isOpen())
+
+            sock.close()
+
+
+if __name__ == "__main__":
+    unittest.main()