rewrite FastAPIInstrumentor:build_middleware_stack to become failsafe

instrumentation/opentelemetry-instrumentation-fastapi/src/opentelemetry/instrumentation/fastapi/__init__.py

@@ -289,22 +289,33 @@ def instrument_app(
  schema_url=_get_schema_url(sem_conv_opt_in_mode),
  )
 
- # Instead of using `app.add_middleware` we monkey patch `build_middleware_stack` to insert our middleware
- # as the outermost middleware.
- # Otherwise `OpenTelemetryMiddleware` would have unhandled exceptions tearing through it and would not be able
- # to faithfully record what is returned to the client since it technically cannot know what `ServerErrorMiddleware` is going to do.
-
+ # In order to make traces available at any stage of the request
+ # processing - including exception handling - we wrap ourselves as
+ # the new, outermost middleware. However in order to prevent
+ # exceptions from user-provided hooks of tearing through, we wrap
+ # them to return without failure unconditionally.
  def build_middleware_stack(self: Starlette) -> ASGIApp:
+ def failsafe(func):
+ @functools.wraps(func)
+ def wrapper(*args, **kwargs):
+ try:
+ return func(*args, **kwargs)
+ except Exception:
+ pass
+
+ return wrapper
+
  inner_server_error_middleware: ASGIApp = ( # type: ignore
  self._original_build_middleware_stack() # type: ignore
  )
- otel_middleware = OpenTelemetryMiddleware(
+
+ return OpenTelemetryMiddleware(
  inner_server_error_middleware,
  excluded_urls=excluded_urls,
  default_span_details=_get_default_span_details,
- server_request_hook=server_request_hook,
- client_request_hook=client_request_hook,
- client_response_hook=client_response_hook,
+ server_request_hook=failsafe(server_request_hook),
+ client_request_hook=failsafe(client_request_hook),
+ client_response_hook=failsafe(client_response_hook),
  # Pass in tracer/meter to get __name__and __version__ of fastapi instrumentation
  tracer=tracer,
  meter=meter,
@@ -313,23 +324,6 @@ def build_middleware_stack(self: Starlette) -> ASGIApp:
  http_capture_headers_sanitize_fields=http_capture_headers_sanitize_fields,
  exclude_spans=exclude_spans,
  )
- # Wrap in an outer layer of ServerErrorMiddleware so that any exceptions raised in OpenTelemetryMiddleware
- # are handled.
- # This should not happen unless there is a bug in OpenTelemetryMiddleware, but if there is we don't want that
- # to impact the user's application just because we wrapped the middlewares in this order.
- if isinstance(
- inner_server_error_middleware, ServerErrorMiddleware
- ): # usually true
- outer_server_error_middleware = ServerErrorMiddleware(
- app=otel_middleware,
- )
- else:
- # Something else seems to have patched things, or maybe Starlette changed.
- # Just create a default ServerErrorMiddleware.
- outer_server_error_middleware = ServerErrorMiddleware(
- app=otel_middleware
- )
- return outer_server_error_middleware
 
  app._original_build_middleware_stack = app.build_middleware_stack
  app.build_middleware_stack = types.MethodType(