In slapd just about all of the operation handlers send their results from the backend directly to the client. The notable exception is for the Bind operation, which sends failures directly to the client, but has to return successes to the frontend to allow final processing to occur there. This difference causes a fair bit of complexity when writing Bind handlers in backends and overlays.
In 2.3 we ran into other sequencing issues that could only be solved by moving some of the Bind cleanup into a callback (connection_bind_cb). We have an opportunity now to move all of the postprocessing into that callback, allowing us to remove all special case handling of Binds in the backends and overlays. Of course, making this change will touch every backend that handles Bind operations. It may impact overlays that intercept Binds as well.
In particular, overlays that insert response callbacks on Bind operations can potentially break in the current code. I.e., if an overlay inserts a callback at the head of the stack (instead of at the end) then currently it will simply not get called in the case of a successful Bind, because the overlay framework tears down the callback stack before it returns to the frontend. If the callback was inserted using malloc'd memory then it will cause a memory leak as well.
I plan to file an ITS on this for RE24, not sure how soon I'll get to the actual changes though.