fix: SMT session deadlocks on unread solver stderr; set-logic lost after (reset) - #228
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…(reset) Two related defects in the typechecking SMT session (Act.SMT): 1. spawnSolver creates the solver's stderr as a pipe that nothing ever reads. Solvers emit warnings there; once ~64KB accumulate, the solver blocks in write() mid-session and act blocks in poll() waiting for stdout - a permanent deadlock at zero CPU on both sides. Large specs hit this reliably: `act type` appears to hang on an SMT blow-up but is actually deadlocked on a full pipe. Fixed by draining stderr with a background thread. 2. checkSat prepends (reset) to every query, but (reset) clears the (set-logic ALL) sent by the spawn-time preamble, so every query after the first runs without a logic. This is also what makes cvc5 emit its set-logic warning triplet per query - the stderr volume that fills the pipe in (1). Fixed by re-sending the preamble after each (reset). With both fixes a ~600-line spec that previously deadlocked cvc5 in under a minute typechecks to completion.
320 trivial behaviours = 320+ queries, each preceded by (reset); before the fix that meant one solver warning block per query (~270 bytes with cvc5) written to a stderr pipe nothing read. This file generates ~86KB of pre-fix warning volume - past any kernel's pipe capacity (observed blocking as low as 8KB under pipe-buffer pressure) - so unfixed act hangs on it indefinitely (cvc5 blocked in write, act in poll, both at zero CPU); fixed act typechecks it in ~5s. Runs via the existing tests/typing/pass wildcard. Note the failure mode on regression is a hang, not a failing exit - CI job timeouts are the backstop.
lefterislazar
approved these changes
Jul 31, 2026
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