When (read-line) is preceded by (read), it already has a newline in the stream, and returns "" immediately rather than prompting for input.

(read) then (read-line) works fine in the stock Clojure repl because the REPL shares STDIN with the in used by the normal (read) and (read-line) calls. So when the REPL's loop does clojure.main/repl-read (to get my evaluation input "(read-line)"), it skips whitespace before the read for the thing to eval, reads that thing, and then skips a newline, if that's the next thing: https://github.com/clojure/clojure/blob/master/src/clj/clojure/main.clj#L145-161

That allows the .read calls generated by the (read-line) execution to start after a trailing newline left in the stream.

We can do the same thing in nrepl.middleware.session/add-stdin on an eval message, but we need to check .ready first, to avoid sending unnecessary :need-input requests back to the client. I think this does belong in add-stdin, since it's essentially a stdin concern, even though it acts on the eval message.

I've got one test exposing the problem, and one proving we only eat newlines, not any old trailing character. I had to stick :stdin-reader on the session to enable consuming the newline from the middleware - not sure whether there's a way to do without it that I'm not seeing.