perf(runner): fold clock perl probe into the first read

CHANGELOG.md

@@ -13,6 +13,7 @@
 - `--jobs auto` / `-j auto` caps parallel concurrency at the CPU core count (portable across Linux/macOS/BSD); the default stays unlimited (#766)
 
 ### Changed
+- Faster cold start: selecting the `perl` clock reads the time once instead of an empty `Time::HiRes` probe followed by a separate first read (2 -> 1 `perl` fork per cold start on bash 3.2 / macOS). No behaviour change
 - Faster cold start: the deferred-output scratch files (failures/skipped/incomplete/risky/profile/rerun) now share one run-unique directory instead of forking `mktemp` once each (6 -> 0 `mktemp` forks per cold start; ~62ms -> ~50ms per nested run on bash 3.2). No behaviour change (#798)
 - Faster cold start: `check_os` detects the OS with a single `uname` fork at load instead of two (matters across the acceptance suite's nested `bashunit` runs). No behaviour change (#798)
 - Faster test execution: removed the remaining round-2 fork leftovers — `wait_for_job_slot` counts running background jobs without forking `wc` on every poll (145 -> 0 `wc` forks across a small parallel run), and `check_duplicate_functions` folds its per-file grep+awk+sort+uniq scan into a single awk pass. No behaviour change (#761)

src/clock.sh

@@ -33,12 +33,23 @@ function bashunit::clock::_choose_impl() {
     esac
   fi
 
-  # 3. Try Perl with Time::HiRes
+  # 3. Try Perl with Time::HiRes. Probe by reading the actual time (not an empty
+  # `-e ""`) so the pending first read reuses this fork instead of paying a
+  # second one; a non-digit/empty result means perl or Time::HiRes is missing, so
+  # fall through. The value is seeded into the return slot for now_to_slot.
   attempts[attempts_count]="Perl"
   attempts_count=$((attempts_count + 1))
-  if bashunit::dependencies::has_perl && perl -MTime::HiRes -e "" &>/dev/null; then
-    _BASHUNIT_CLOCK_NOW_IMPL="perl"
-    return 0
+  if bashunit::dependencies::has_perl; then
+    local perl_now
+    perl_now="$(perl -MTime::HiRes -e 'printf("%.0f\n", Time::HiRes::time() * 1000000000)' 2>/dev/null)"
+    case "$perl_now" in
+    '' | *[!0-9]*) ;;
+    *)
+      _BASHUNIT_CLOCK_NOW_IMPL="perl"
+      _BASHUNIT_CLOCK_NOW_OUT="$perl_now"
+      return 0
+      ;;
+    esac
   fi
 
   # 4. Try Python 3 with time module
@@ -99,7 +110,14 @@ _BASHUNIT_CLOCK_NOW_OUT=""
 # Returns: 0 on success, 1 when no clock implementation is available.
 function bashunit::clock::now_to_slot() {
   if [ -z "$_BASHUNIT_CLOCK_NOW_IMPL" ]; then
+    _BASHUNIT_CLOCK_NOW_OUT=""
     bashunit::clock::_choose_impl || return 1
+    # _choose_impl may have already read the current time while selecting an
+    # interpreter impl (e.g. perl); reuse that value for this first read instead
+    # of forking a second interpreter.
+    if [ -n "$_BASHUNIT_CLOCK_NOW_OUT" ]; then
+      return 0
+    fi
   fi
 
   case "$_BASHUNIT_CLOCK_NOW_IMPL" in

tests/acceptance/bashunit_coldstart_forks_test.sh

@@ -23,3 +23,25 @@ function test_coldstart_does_not_fork_mktemp_per_scratch_file() {
 
   assert_less_than 1 "$mktemp_forks"
 }
+
+# Regression guard for the clock cold start. Selecting the `perl` clock impl used
+# to fork perl twice: once for an empty `Time::HiRes` probe and once for the
+# first real time read. Choosing an interpreter impl must cost at most one fork
+# now (the probe reads the actual time and seeds it for the first read). On
+# platforms that pick a fork-free/`date` clock (e.g. Linux) this is zero.
+function test_coldstart_selects_clock_impl_without_a_probe_fork() {
+  if bashunit::check_os::is_windows; then
+    bashunit::skip "process tracing is unreliable under Git Bash" && return
+  fi
+
+  local trace
+  trace="$(PS4='+ ' bash -x ./bashunit --version 2>&1 >/dev/null)"
+
+  # Count real interpreter executions used by the clock (perl/python/node) at
+  # cold start. `command -v <interp>` is a shell builtin and never appears as an
+  # executed command line, so it is not counted.
+  local interp_forks
+  interp_forks="$(printf '%s\n' "$trace" | grep -cE '^\++ +(perl|python|node) ' || true)"
+
+  assert_less_or_equal_than 1 "$interp_forks"
+}