checkers: the bash floor is 5.1, where wait -n -p arrived (fd0j)

find_modern_bash accepted any 5.x, so a bash 5.0 was pinned and
pr-reviews-test.sh then failed on an invalid option. The probe now
reads major and minor; a 5.0 case and a 5.1 control guard it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/code/tooling.md

@@ -645,10 +645,12 @@ one place, with no per-invocation nix cost:
 calls it and the local gate calls it, so there is no second invocation to drift.
 It does three things a bare `./scripts/X-test.sh` does not:
 
-1. **Pins a bash ≥ 5.** The ambient one is tried first, so a runner that already
-   has bash 5.2 pays nothing; nix supplies one only when the ambient bash is
-   older, which on macOS outside the dev shell it is. No bash ≥ 5 and no nix is
-   **exit 3** — a refusal, never a quiet run under 3.2.
+1. **Pins a bash ≥ 5.1.** The ambient one is tried first, so a runner that
+   already has bash 5.2 pays nothing; nix supplies one only when the ambient
+   bash is older, which on macOS outside the dev shell it is. No bash ≥ 5.1 and
+   no nix is **exit 3** — a refusal, never a quiet run under 3.2. The floor is
+   5.1 and not 5 because `pr-reviews-test.sh` uses `wait -n -p`, and `-p`
+   arrived in 5.1. Under 5.0 it is an invalid option (improvise-fd0j).
 2. **Scrubs the environment to an allowlist** (`env -i` plus `PATH`, `HOME`,
    `TMPDIR`, the `NIX_*` and `GITHUB_*` names, extensible through
    `CHECKERS_KEEP`). This is the part that closes the direnv hole above, and it

context/test-evidence/tooling.md

@@ -843,3 +843,27 @@ recycled pid.
 passed and 4 failed. The four timeout cases fail with exit 124; the fake-clock
 one has made about 400 polls by then. Before the guard the same rival never
 finished, and an outer `timeout 30` killed it at exit 124.
+
+### The bash floor is 5.1 — `scripts/checkers-test.sh` (improvise-fd0j)
+
+**What broke.** `pr-reviews-test.sh` calls `wait -n -p`, and `-p` arrived in
+bash 5.1. `checkers.sh`'s `find_modern_bash` accepted any major version 5. So a
+bash 5.0 on `PATH` was pinned, and that suite then failed on an invalid option
+rather than on anything it tests.
+
+**Fix.** The probe reads major and minor in one call and requires
+`major*100 + minor >= 501`. The exit-3 message and the header say 5.1.
+`context/code/tooling.md` states why the floor is 5.1.
+
+**Cases** (section 5b). A fake bash answering `5 0` with no nix → exit 3. The
+control, a fake answering `5 1` → exit 0, proves that refusal comes from the
+version and not from the fake.
+
+**Rivals**, run against the real suite in a copied sandbox:
+- **Floor 5.0** (`-ge 500`) fails "a bash 5.0 with no nix is exit 3, not 5.x"
+  and nothing else.
+- **The pre-fix runner** fails only the 5.1 control. Its probe asked for the
+  major version alone, so the fake's `5 1` was not a number and was refused.
+  The 5.0 case passes against it for the same reason, so that case alone would
+  not have distinguished the old code. The 5.0-floor rival is the one that
+  does.

scripts/checkers-test.sh

@@ -141,6 +141,19 @@ got=$?
 [ "$got" = 3 ] && ok "an old bash with no nix is exit 3" \
                || bad "an old bash with no nix is exit 3" "exit $got, wanted 3"
 
+# 5b. The floor is 5.1, not 5: `wait -n -p` is 5.1 (improvise-fd0j). The 5.1
+# control proves the 5.0 refusal is the version and not the fake itself.
+for v in "5 0:3:a bash 5.0 with no nix is exit 3, not 5.x" \
+         "5 1:0:a bash 5.1 is accepted"; do
+  printf '#!/bin/sh\ncase "$*" in *BASH_VERSINFO*) echo %s; exit 0 ;; esac\nexit 0\n' \
+    "${v%%:*}" > "$work/fakebin/bash"
+  want="${v#*:}"; name="${want#*:}"; want="${want%%:*}"
+  env PATH="$work/fakebin:/usr/bin:/bin" HOME="$HOME" \
+    "${BASH:-bash}" "$d/checkers.sh" any > "$work/out" 2>&1
+  got=$?
+  [ "$got" = "$want" ] && ok "$name" || bad "$name" "exit $got, wanted $want"
+done
+
 # ---------------------------------------------------------------------------
 # 6. A named suite that does not exist is a usage error, not a silent no-op.
 d="$(sandbox missing)"

scripts/checkers.sh

@@ -5,7 +5,7 @@
 #
 # It does three things a bare `./scripts/X-test.sh` does not:
 #
-#   1. pins a bash >= 5, taken from nix when the ambient one is older, so the
+#   1. pins a bash >= 5.1, taken from nix when the ambient one is older, so the
 #      macOS system bash 3.2 can never be the shell a suite passes under;
 #   2. scrubs the environment down to an allowlist, because `nix develop`
 #      exports `name`, `out`, `system`, `shell`, `builder` and two dozen more
@@ -30,7 +30,7 @@ usage: checkers.sh [--list] [--self-test] [name ...]
 
 exit 0  every suite passed
 exit 1  a suite failed, or a shebang is not `#!/usr/bin/env bash`
-exit 3  no bash >= 5 could be found and nix could not supply one
+exit 3  no bash >= 5.1 could be found and nix could not supply one
 exit 4  usage error
 
 THIS SCRIPT'S `$?` MUST BE THE ONE YOU READ. Nothing may follow it in the same
@@ -46,14 +46,17 @@ ALLOW="HOME TMPDIR TMP TEMP TERM LANG LC_ALL USER LOGNAME
        NIX_PATH NIX_REMOTE NIX_SSL_CERT_FILE SSL_CERT_FILE
        XDG_CACHE_HOME XDG_CONFIG_HOME XDG_DATA_HOME"
 
-# Prints the absolute path of a bash >= 5, or nothing. The ambient one is tried
+# Prints the absolute path of a bash >= 5.1, or nothing. The ambient one is tried
 # first so a machine that already has a modern bash pays no nix startup at all.
 find_modern_bash() {
-  local candidate major
+  local candidate major minor
   candidate="$(command -v bash || true)"
   if [ -n "$candidate" ]; then
-    major="$("$candidate" -c 'echo "${BASH_VERSINFO[0]}"' 2>/dev/null || echo 0)"
-    if [ "$major" -ge 5 ] 2>/dev/null; then echo "$candidate"; return 0; fi
+    read -r major minor < <("$candidate" -c 'echo "${BASH_VERSINFO[0]} ${BASH_VERSINFO[1]}"' \
+      2>/dev/null || echo 0 0)
+    if [ "$(( ${major:-0} * 100 + ${minor:-0} ))" -ge 501 ] 2>/dev/null; then
+      echo "$candidate"; return 0
+    fi
   fi
   command -v nix >/dev/null 2>&1 || return 0
   nix shell nixpkgs#bash --command bash -c 'command -v bash' 2>/dev/null || true
@@ -122,7 +125,7 @@ main() {
 
   local pinned; pinned="$(find_modern_bash)"
   if [ -z "$pinned" ]; then
-    echo 'checkers: no bash >= 5 on PATH and nix cannot supply one.' >&2
+    echo 'checkers: no bash >= 5.1 on PATH and nix cannot supply one.' >&2
     echo '  Enter the dev shell (direnv allow / nix develop) and retry.' >&2
     return 3
   fi