Studio: bootstrap install-script allowlist on PR that introduces it

The previous commit tightened the install-script gate so a PR cannot
add new allowlist entries on its own head branch -- they must already
exist on the base ref. That created a chicken-and-egg problem for the
very PR that introduces studio/frontend/.install-script-allowlist:
base does not have the file yet, so every head entry is "new" and the
gate refuses to land.

Resolve by making the head-only rejection conditional on the base
ref actually having the allowlist file. The workflow signals
"missing on base" by removing the temp file after a failed git show
(previously it was always truncated to empty, which the checker
could not distinguish from an empty-by-intent base). The checker
treats a missing base allowlist path as bootstrap mode and accepts
the head allowlist as-is for that single run. Once the file exists
on base, every subsequent PR is back under the strict head-only
rejection rule.

.github/workflows/security-audit.yml

@@ -1105,10 +1105,15 @@ jobs:
             > /tmp/base-package-lock.json
           # Pull the TRUSTED allowlist from the base ref so a PR cannot
           # allowlist its own new postinstall dependency in the same diff
-          # the checker scans. Missing file is OK (empty allowlist).
-          git show "$BASE_SHA:studio/frontend/.install-script-allowlist" \
-            > /tmp/base-install-script-allowlist 2>/dev/null \
-            || : > /tmp/base-install-script-allowlist
+          # the checker scans. If the file does NOT exist on base, REMOVE
+          # the temp file -- the checker treats a missing base allowlist
+          # as bootstrap mode (the PR that introduces the file is allowed
+          # to populate it; once it lands, subsequent PRs must respect
+          # the head-only rejection rule).
+          if ! git show "$BASE_SHA:studio/frontend/.install-script-allowlist" \
+              > /tmp/base-install-script-allowlist 2>/dev/null; then
+            rm -f /tmp/base-install-script-allowlist
+          fi
 
       - name: Diff for newly-added install-script deps
         if: github.event_name == 'pull_request'

scripts/check_new_install_scripts.py

@@ -328,28 +328,52 @@ def main(argv: list[str] | None = None) -> int:
 
     try:
         head_allowlist = _load_allowlist(head_allowlist_path)
-        base_allowlist = _load_allowlist(base_allowlist_path)
     except ValueError as exc:
         print(f"[install-script-diff] ERROR: {exc}", file = sys.stderr)
         return 2
 
-    # Refuse a PR that adds new allowlist entries on its own head branch.
-    # Allowlist deltas must land in a separate, trusted commit on base
-    # first; otherwise the same PR could approve its own postinstall.
-    added_head_only = sorted(head_allowlist - base_allowlist)
-    if added_head_only:
+    # Bootstrap: if the BASE ref has no allowlist file at all, this is
+    # the PR that creates it. There is no prior allowlist to diff
+    # against, and refusing every head entry here would make the gate
+    # unlandable. The workflow signals "missing on base" by NOT writing
+    # the temp file (rm -f after a failed ``git show``), which is what
+    # we detect here. Once the file exists on base, future PRs must
+    # land allowlist deltas there first.
+    if not base_allowlist_path.exists():
         print(
-            "[install-script-diff] FAIL: install-script allowlist entries "
-            "must already exist on the base branch; do not let a PR "
-            "allowlist its own new postinstall dependency.",
-            file = sys.stderr,
+            f"[install-script-diff] bootstrap: {base_allowlist_path} "
+            "missing on base; accepting head allowlist as-is for this run.",
+            flush = True,
         )
-        for entry in added_head_only:
-            print(f"  head-only allowlist entry: {entry}", file = sys.stderr)
-        return 1
+        allowlist = head_allowlist
+    else:
+        try:
+            base_allowlist = _load_allowlist(base_allowlist_path)
+        except ValueError as exc:
+            print(f"[install-script-diff] ERROR: {exc}", file = sys.stderr)
+            return 2
 
-    # Only the trusted base allowlist participates in the skip set.
-    allowlist = base_allowlist
+        # Refuse a PR that adds new allowlist entries on its own head
+        # branch. Allowlist deltas must land in a separate trusted
+        # commit on base first; otherwise the same PR could approve
+        # its own postinstall dependency.
+        added_head_only = sorted(head_allowlist - base_allowlist)
+        if added_head_only:
+            print(
+                "[install-script-diff] FAIL: install-script allowlist "
+                "entries must already exist on the base branch; do not "
+                "let a PR allowlist its own new postinstall dependency.",
+                file = sys.stderr,
+            )
+            for entry in added_head_only:
+                print(
+                    f"  head-only allowlist entry: {entry}",
+                    file = sys.stderr,
+                )
+            return 1
+
+        # Only the trusted base allowlist participates in the skip set.
+        allowlist = base_allowlist
 
     findings = diff_new_install_scripts(base_lock, head_lock)
     if allowlist: