Fix handling of chunked+gzipped response when first chunk does not gi…

…ve uncompressed data (#3477)

aiohttp/streams.py

@@ -254,15 +254,23 @@ def end_http_chunk_receiving(self) -> None:
         if self._http_chunk_splits is None:
             raise RuntimeError("Called end_chunk_receiving without calling "
                                "begin_chunk_receiving first")
-        if not self._http_chunk_splits or \
-                self._http_chunk_splits[-1] != self.total_bytes:
-            self._http_chunk_splits.append(self.total_bytes)
 
-            # wake up readchunk when end of http chunk received
-            waiter = self._waiter
-            if waiter is not None:
-                self._waiter = None
-                set_result(waiter, False)
+        pos = self._http_chunk_splits[-1] if self._http_chunk_splits else 0
+
+        if self.total_bytes == pos:
+            # Note, when chunked + gzip is used, we can receive some chunk
+            # of compressed data, but that data may not be enough for gzip FSM
+            # to yield any uncompressed data. That's why current position may
+            # not change after receiving a chunk.
+            return
+
+        self._http_chunk_splits.append(self.total_bytes)
+
+        # wake up readchunk when end of http chunk received
+        waiter = self._waiter
+        if waiter is not None:
+            self._waiter = None
+            set_result(waiter, False)
 
     async def _wait(self, func_name: str) -> None:
         # StreamReader uses a future to link the protocol feed_data() method

tests/test_streams.py

@@ -719,6 +719,35 @@ async def test_readchunk_with_other_read_calls(self) -> None:
         assert b'' == data
         assert not end_of_chunk
 
+    async def test_read_empty_chunks(self) -> None:
+        loop = asyncio.get_event_loop()
+        stream = self._make_one()
+
+        # Simulate empty first chunk. This is significant special case
+        stream.begin_http_chunk_receiving()
+        stream.end_http_chunk_receiving()
+
+        stream.begin_http_chunk_receiving()
+        stream.feed_data(b'ungzipped')
+        stream.end_http_chunk_receiving()
+
+        # Possible when compression is enabled.
+        stream.begin_http_chunk_receiving()
+        stream.end_http_chunk_receiving()
+
+        # is also possible
+        stream.begin_http_chunk_receiving()
+        stream.end_http_chunk_receiving()
+
+        stream.begin_http_chunk_receiving()
+        stream.feed_data(b' data')
+        stream.end_http_chunk_receiving()
+
+        stream.feed_eof()
+
+        data = await stream.read()
+        assert data == b'ungzipped data'
+
     async def test_readchunk_separate_http_chunk_tail(self) -> None:
         """Test that stream.readchunk returns (b'', True) when end of
         http chunk received after body