KVStoreEntry for retrieving the body doesn't appear to be an async operation

[vcarvajal-sigsci]

Calling arrayBuffer(), json(), or text() on KVStoreEntry eventually reaches the function read_from_handle_all inside of runtime/fastly/builtins/fetch/request-response.cpp. This function will read the entire body of the object into memory before returning, and there is no suspension point while it waits for chunks of the body (in other words it doesn’t return control to the event loop until after all the chunks have been read).

[kablesanhk]

#1179