Fix double compress when compression enabled and compressed file exists

If the handler has `.enable_compression` set and we use the pre-compressed
.gz we would compress twice. The caller may not know if the `.gz` file
exists or not or have a mixed use case where some files are
pre-compressed and some are compressed on the fly based if they have
limited storage

fixes #8011

aiohttp/web_fileresponse.py

@@ -251,6 +251,10 @@ async def prepare(self, request: "BaseRequest") -> Optional[AbstractStreamWriter
             self.headers[hdrs.CONTENT_ENCODING] = encoding
         if gzip:
             self.headers[hdrs.VARY] = hdrs.ACCEPT_ENCODING
+            # Disable compression if we are already sending
+            # a compressed file since we don't want to double
+            # compress.
+            self._compression = False
 
         self.etag = etag_value  # type: ignore[assignment]
         self.last_modified = st.st_mtime  # type: ignore[assignment]

tests/test_web_sendfile_functional.py

@@ -249,6 +249,32 @@ async def handler(request):
     await client.close()
 
 
+async def test_static_file_with_gziped_counter_part_enable_compression(
+    aiohttp_client: Any, sender: Any
+):
+    """Test that enable_compression does not double compress when a .gz file is also present."""
+    filepath = pathlib.Path(__file__).parent / "hello.txt"
+
+    async def handler(request):
+        resp = sender(filepath)
+        resp.enable_compression()
+        return resp
+
+    app = web.Application()
+    app.router.add_get("/", handler)
+    client = await aiohttp_client(app)
+
+    resp = await client.get("/")
+    assert resp.status == 200
+    body = await resp.read()
+    assert body == b"hello aiohttp\n"
+    assert resp.headers["Content-Type"] == "text/plain"
+    assert resp.headers.get("Content-Encoding") == "gzip"
+    resp.close()
+    await resp.release()
+    await client.close()
+
+
 async def test_static_file_with_content_encoding(
     aiohttp_client: Any, sender: Any
 ) -> None: