Unwrap view errors occurring under a BaseHTTPMiddleware

starlette/middleware/base.py

@@ -34,7 +34,7 @@ async def coro() -> None:
             try:
                 message = await recv_stream.receive()
             except anyio.EndOfStream:
-                raise RuntimeError("No response returned.")
+                raise RuntimeError("No response returned.") from None
 
             assert message["type"] == "http.response.start"
 
@@ -50,11 +50,28 @@ async def body_stream() -> typing.AsyncGenerator[bytes, None]:
             response.raw_headers = message["headers"]
             return response
 
-        async with anyio.create_task_group() as task_group:
-            request = Request(scope, receive=receive)
-            response = await self.dispatch_func(request, call_next)
-            await response(scope, receive, send)
-            task_group.cancel_scope.cancel()
+        try:
+            async with anyio.create_task_group() as task_group:
+                request = Request(scope, receive=receive)
+                response = await self.dispatch_func(request, call_next)
+                await response(scope, receive, send)
+                task_group.cancel_scope.cancel()
+        except anyio.ExceptionGroup as exc_group:
+            first, *rest = exc_group.exceptions
+            if str(first) == "No response returned.":
+                # "No response returned", and we got an exception group, meaning
+                # the app raised an exception.
+                # Raise the latter in priority, so that we:
+                # a/ match the behavior before 0.16.0 (see regression #1255).
+                # b/ keep `anyio` as an implementation detail internally.
+                assert len(rest) == 1
+                (exc,) = rest
+                raise exc from None
+            else:
+                # User may have used `anyio` directly and triggered an
+                # `ExceptionGroup` which they may want to manage themselves.
+                # Let it be.
+                raise
 
     async def dispatch(
         self, request: Request, call_next: RequestResponseEndpoint