Weird debugger behavior, not firing on some cases

[jpmonettas]

Instrumenting code like this makes the debugger behave weirdly, sometimes not firing :

(add-lib 'prismatic/schema {:mvn/version "1.4.1"})
(require '[schema.core])

(schema.core/defn fibo-iter-with-schema
  ([n :- schema.core/Int] (fibo-iter-with-schema 0 1 n))
  ([curr :- schema.core/Int
    nxt :- schema.core/Int
    n :- schema.core/Int]
   (cond
     (zero? n) curr
     :else (recur nxt (+ curr nxt) (dec n)))))

I'm attaching a small video that I think describes it better.

issue.mp4

I'm also not sure why the debugger is popping up on instrumentation, looks like schema.core/defn macroexpansion is calling some already instrumented code.

Environment & Version information

CIDER version information

;; CIDER 1.15.0-snapshot (package: 20240603.825), nREPL 1.3.0-beta2
;; Clojure 1.12.0-beta1, Java 17.0.12-ea

Emacs version

29.4

Operating system

Debian Linux

[github-actions]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed soon if no further activity occurs. Thank you for your contribution and understanding!

[github-actions]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed soon if no further activity occurs. Thank you for your contribution and understanding!

[github-actions]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed soon if no further activity occurs. Thank you for your contribution and understanding!

[bbatsov]

This is an instrumentation/breakpoint-placement problem in cider-nrepl (deciding which forms get breakpoints, and handling macroexpanded code like schema.core/defn and recur). The CIDER client only reacts to the debug-input requests the middleware emits. Moving to cider-nrepl.

[bbatsov]

Dug into this. The concrete misbehavior is that a breakpoint fires at definition time (when you evaluate the schema.core/defn), before you ever call the function - which is what makes the debugger "pop up on instrumentation" and leaves the debug session in a confused state.

Root cause: schema.core/defn runs bookkeeping at definition time (schema.utils/declare-class-schema! / fn-schema-bearer) that references the function's own name. cider-nrepl instruments user-written forms, so that name reference carries a breakpoint and fires when the setup runs. A plain defn has zero definition-time breaks; this is specific to schema evaluating setup code eagerly. Execution-time debugging (breaks inside the body, across recur) actually works correctly.

Why it's not a simple fix: the schema type symbols are already skipped, but the fn-name reference can't be - the same form runs both at definition (bookkeeping, shouldn't break) and at call time (recursive calls, should break). A form-based instrumenter can't tell those apart; it'd need runtime "am I in the definition or in a call?" awareness. So for now this is a known limitation of debugging schema.core/defn (and similar macros that run setup code referencing user forms at definition time). Workaround: wrap only the specific sub-expressions you care about with #break rather than #dbg-ing the whole typed defn.