perf(runner): use 'wait -n' for parallel job slot on Bash 4.3+

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Performance
 - Faster runtime-error detection: single `case` glob instead of 23-iteration loop in `detect_runtime_error` (#668)
 - Hot-path coverage flag now cached in `_BASHUNIT_COVERAGE_ON`, removing a function dispatch per call (#664)
+- Parallel runner blocks on `wait -n` on Bash 4.3+ instead of polling `jobs -r`, removing sleep-induced slot-release latency (#667)
 
 ## [0.36.0](https://github.com/TypedDevs/bashunit/compare/0.35.0...0.36.0) - 2026-05-07
 

src/runner.sh

@@ -141,14 +141,35 @@ function bashunit::runner::print_verbose_test_summary() {
   printf '%*s\n' "$TERMINAL_WIDTH" '' | tr ' ' '-'
 }
 
+# Returns 0 when this Bash supports `wait -n` (Bash 4.3+), 1 otherwise.
+function bashunit::runner::_supports_wait_n() {
+  local major="${BASH_VERSINFO[0]:-0}"
+  local minor="${BASH_VERSINFO[1]:-0}"
+  if [ "$major" -gt 4 ]; then
+    return 0
+  fi
+  if [ "$major" -eq 4 ] && [ "$minor" -ge 3 ]; then
+    return 0
+  fi
+  return 1
+}
+
 function bashunit::runner::wait_for_job_slot() {
   local max_jobs="${BASHUNIT_PARALLEL_JOBS:-0}"
   if [ "$max_jobs" -le 0 ]; then
     return 0
   fi
 
-  # Adaptive backoff: start at 50ms, grow to 200ms to reduce `jobs -r` overhead
-  # on long-running tests while keeping short tests responsive.
+  if bashunit::runner::_supports_wait_n; then
+    # Bash 4.3+: block until any child exits. No polling, no sleep latency.
+    while [ "$(jobs -r | wc -l)" -ge "$max_jobs" ]; do
+      wait -n 2>/dev/null || break
+    done
+    return 0
+  fi
+
+  # Bash 3.x fallback: adaptive poll starting at 50ms, growing to 200ms to
+  # reduce `jobs -r` overhead on long-running tests while staying responsive.
   local delay="0.05"
   local iterations=0
   while true; do

tests/unit/parallel_test.sh

@@ -55,6 +55,29 @@ function test_wait_for_job_slot_returns_immediately_when_under_limit() {
   assert_successful_code "$?"
 }
 
+function test_supports_wait_n_matches_running_bash_version() {
+  local major="${BASH_VERSINFO[0]:-0}"
+  local minor="${BASH_VERSINFO[1]:-0}"
+  local expected_rc=1
+  if [ "$major" -gt 4 ] || { [ "$major" -eq 4 ] && [ "$minor" -ge 3 ]; }; then
+    expected_rc=0
+  fi
+
+  bashunit::runner::_supports_wait_n
+  assert_same "$expected_rc" "$?"
+}
+
+function test_wait_for_job_slot_releases_when_background_job_finishes() {
+  export BASHUNIT_PARALLEL_JOBS=1
+
+  # Launch a short-lived job, occupy the only slot, then call wait_for_job_slot.
+  # On Bash 4.3+ this exercises the `wait -n` path; on Bash 3.x the poll path.
+  (sleep 0.1) &
+  bashunit::runner::wait_for_job_slot
+
+  assert_successful_code "$?"
+}
+
 # === is_enabled tests ===
 
 function test_parallel_enabled_on_windows() {