fix(scripts): fail loud when absorbed-heading fork-point read errors

scripts/check-changelog-parity.sh

@@ -445,7 +445,28 @@ for manifest in "${manifests[@]}"; do
     # Named apart from $head_version / $base_version above: those are MANIFEST
     # versions the rest of this loop body compares and interpolates, and a
     # one-character alias between the two would be a silent bug.
-    mapfile -t fork_heading_versions < <(git show "$merge_base:$changelog" 2>/dev/null | changelog_versions -)
+    #
+    # Read the fork-point changelog via COMMAND substitution with status
+    # checked — never process substitution, which swallows git's exit status and
+    # turns a failed read into an empty heading list (fail-open, #2324). Probe
+    # with git ls-tree first: a path absent from the tree is exit 0 with empty
+    # output; an unusable rev is non-zero. (git cat-file -e cannot distinguish
+    # the two — it exits 128 for both — so a change set ADDING a changelog would
+    # fail the gate.)
+    fork_heading_versions=()
+    if base_listing="$(git ls-tree -r --name-only "$merge_base" -- "$changelog")"; then
+      if [[ -n "$base_listing" ]]; then
+        base_body=""
+        if ! base_body="$(git show "$merge_base:$changelog")"; then
+          echo "check-changelog-parity: 'git show $merge_base:$changelog' failed; refusing to pass without checking." >&2
+          exit 2
+        fi
+        mapfile -t fork_heading_versions < <(printf '%s\n' "$base_body" | changelog_versions -)
+      fi
+    else
+      echo "check-changelog-parity: 'git ls-tree -r --name-only $merge_base -- $changelog' failed; refusing to pass without checking." >&2
+      exit 2
+    fi
     mapfile -t head_heading_versions < <(changelog_versions "$changelog")
     head_documents=()
     if ((${#head_heading_versions[@]} > 0)); then

scripts/check-changelog-parity.test.sh

@@ -237,6 +237,32 @@ rc=$?
 if [[ $rc -ne 0 && "$out" == *"ABSORBED CHANGELOG HEADING"*"alpha"* && "$out" == *"## [0.51.8]"* ]]; then ok "absorbed predecessor release heading fails --check-bump"; else fail "absorbed heading not caught: rc=$rc out='$out'"; fi
 rm -rf "$repo"
 
+# ABSORBED HEADING + GIT SHOW FAILURE: a fork-point read that genuinely fails
+# must fail loud, never read as "nothing to preserve" and pass (#2324).
+repo="$(mk_repo)"
+git_init "$repo"
+mk_plugin "$repo" alpha 0.51.8 yes
+printf '# Changelog\n\n## [0.51.8]\n\n- eight\n\n## [0.51.7]\n' >"$repo/plugins/alpha/CHANGELOG.md"
+git -C "$repo" add -A >/dev/null && git -C "$repo" commit -qm base
+base="$(git -C "$repo" rev-parse HEAD)"
+printf '{ "name": "alpha", "version": "0.51.9" }\n' >"$repo/plugins/alpha/.claude-plugin/plugin.json"
+printf '# Changelog\n\n## [0.51.9]\n\n- eight\n\n## [0.51.7]\n' >"$repo/plugins/alpha/CHANGELOG.md"
+git -C "$repo" add -A >/dev/null && git -C "$repo" commit -qm bump
+shim_dir="$(mktemp -d)"
+cat >"$shim_dir/git" <<'SHIM'
+#!/usr/bin/env bash
+if [[ "${1:-}" == show && "${2:-}" == *CHANGELOG.md ]]; then
+  echo "simulated object-store failure" >&2
+  exit 128
+fi
+exec /usr/bin/git "$@"
+SHIM
+chmod +x "$shim_dir/git"
+out="$(cd "$repo" && PATH="$shim_dir:$PATH" bash scripts/check-changelog-parity.sh --check-bump "$base" 2>&1)"
+rc=$?
+rm -rf "$shim_dir" "$repo"
+if [[ $rc -eq 2 && "$out" == *"git show"* && "$out" == *"failed"* ]]; then ok "absorbed-heading check fails loud when git show CHANGELOG fails"; else fail "absorbed-heading git failure not loud: rc=$rc out='$out'"; fi
+
 # RELABELLED, NOT DELETED: the same bad resolution renames the predecessor's
 # heading to the new version instead of adding one. That IS a deletion of the
 # predecessor version, and must read as one however the line is written.