Support Starlette/FastAPI app.host (#4157)

In Starlette/FastAPI you're able to create subapps. When using
`transaction_style="url"` in our integration, this would throw an
exception because we try to access `route.path` to determine the
transaction name, but `Host` routes have no `path` attribute.

Closes #2631

Author: sentrivana

sentry_sdk/integrations/starlette.py

@@ -693,7 +693,11 @@ def _transaction_name_from_router(scope):
     for route in router.routes:
         match = route.matches(scope)
         if match[0] == Match.FULL:
-            return route.path
+            try:
+                return route.path
+            except AttributeError:
+                # routes added via app.host() won't have a path attribute
+                return scope.get("path")
 
     return None
 

tests/integrations/fastapi/test_fastapi.py

@@ -682,3 +682,38 @@ async def _error():
     client.get("/error")
 
     assert len(events) == int(expected_error)
+
+
+@pytest.mark.parametrize("transaction_style", ["endpoint", "url"])
+def test_app_host(sentry_init, capture_events, transaction_style):
+    sentry_init(
+        traces_sample_rate=1.0,
+        integrations=[
+            StarletteIntegration(transaction_style=transaction_style),
+            FastApiIntegration(transaction_style=transaction_style),
+        ],
+    )
+
+    app = FastAPI()
+    subapp = FastAPI()
+
+    @subapp.get("/subapp")
+    async def subapp_route():
+        return {"message": "Hello world!"}
+
+    app.host("subapp", subapp)
+
+    events = capture_events()
+
+    client = TestClient(app)
+    client.get("/subapp", headers={"Host": "subapp"})
+
+    assert len(events) == 1
+
+    (event,) = events
+    assert "transaction" in event
+
+    if transaction_style == "url":
+        assert event["transaction"] == "/subapp"
+    else:
+        assert event["transaction"].endswith("subapp_route")