document memoryview usage, minor frame_writer.write_frame refactor (#384)

* document memory_view usage, refactor frame_writer.write_frame

* improve test for changing frame_max in write_frame

* add integration test for write_frame/send_heartbeat

3 files changed, 42 insertions, 15 deletions

diff --git a/amqp/method_framing.py b/amqp/method_framing.py
index 5fe0505..6c49833 100644
--- a/amqp/method_framing.py
+++ b/amqp/method_framing.py
@@ -94,6 +94,9 @@ class Buffer:
     @buf.setter
     def buf(self, buf):
         self._buf = buf
+        # Using a memoryview allows slicing without copying underlying data.
+        # Slicing this is much faster than slicing the bytearray directly.
+        # More details: https://stackoverflow.com/a/34257357
         self.view = memoryview(buf)
 
 
@@ -107,13 +110,6 @@ def frame_writer(connection, transport,
 
     def write_frame(type_, channel, method_sig, args, content):
         chunk_size = connection.frame_max - 8
-        # frame_max can be updated via connection._on_tune. If
-        # it became larger, then we need to resize the buffer
-        # to prevent overflow.
-        if chunk_size > len(buffer_store.buf):
-            buffer_store.buf = bytearray(chunk_size)
-        buf = buffer_store.buf
-        view = buffer_store.view
         offset = 0
         properties = None
         args = str_to_bytes(args)
@@ -155,6 +151,13 @@ def frame_writer(connection, transport,
                                frame, 0xce))
 
         else:
+            # frame_max can be updated via connection._on_tune. If
+            # it became larger, then we need to resize the buffer
+            # to prevent overflow.
+            if chunk_size > len(buffer_store.buf):
+                buffer_store.buf = bytearray(chunk_size)
+            buf = buffer_store.buf
+
             # ## FAST: pack into buffer and single write
             frame = (b''.join([pack('>HH', *method_sig), args])
                      if type_ == 1 else b'')
@@ -180,7 +183,7 @@ def frame_writer(connection, transport,
                               3, channel, framelen, body, 0xce)
                     offset += 8 + framelen
 
-            write(view[:offset])
+            write(buffer_store.view[:offset])
 
         connection.bytes_sent += 1
     return write_frame
diff --git a/t/integration/test_integration.py b/t/integration/test_integration.py
index d441488..d0b67b9 100644
--- a/t/integration/test_integration.py
+++ b/t/integration/test_integration.py
@@ -416,6 +416,17 @@ class test_connection:
                 )
                 callback_mock.assert_called_once_with()
 
+    def test_send_heartbeat(self):
+        """The send_heartbeat method writes the expected output."""
+        conn = Connection()
+        with patch.object(conn, 'Transport') as transport_mock:
+            handshake(conn, transport_mock)
+            transport_mock().write.reset_mock()
+            conn.send_heartbeat()
+            transport_mock().write.assert_called_once_with(
+                memoryview(bytearray(b'\x08\x00\x00\x00\x00\x00\x00\xce'))
+            )
+
 
 class test_channel:
     # Integration tests. Tests verify the correctness of communication between
diff --git a/t/unit/test_method_framing.py b/t/unit/test_method_framing.py
index bd9c465..96ecf03 100644
--- a/t/unit/test_method_framing.py
+++ b/t/unit/test_method_framing.py
@@ -138,11 +138,24 @@ class test_frame_writer:
         assert 'body'.encode('utf-16') in memory.tobytes()
         assert msg.properties['content_encoding'] == 'utf-16'
 
-    def test_frame_max_update(self):
-        msg = Message(body='t' * (self.connection.frame_max + 10))
-        frame = 2, 1, spec.Basic.Publish, b'x' * 10, msg
+    def test_write_frame__fast__buffer_store_resize(self):
+        """The buffer_store is resized when the connection's frame_max is increased."""
+        small_msg = Message(body='t')
+        small_frame = 2, 1, spec.Basic.Publish, b'x' * 10, small_msg
+        self.g(*small_frame)
+        self.write.assert_called_once()
+        write_arg = self.write.call_args[0][0]
+        assert isinstance(write_arg, memoryview)
+        assert len(write_arg) < self.connection.frame_max
+        self.connection.reset_mock()
+
+        # write a larger message to the same frame_writer after increasing frame_max
+        large_msg = Message(body='t' * (self.connection.frame_max + 10))
+        large_frame = 2, 1, spec.Basic.Publish, b'x' * 10, large_msg
+        original_frame_max = self.connection.frame_max
         self.connection.frame_max += 100
-        self.g(*frame)
-        self.write.assert_called()
-        memory = self.write.call_args[0][0]
-        assert isinstance(memory, memoryview)
+        self.g(*large_frame)
+        self.write.assert_called_once()
+        write_arg = self.write.call_args[0][0]
+        assert isinstance(write_arg, memoryview)
+        assert len(write_arg) > original_frame_max