ersatztv/scripts/pr-changed-files.sh

#!/usr/bin/env bash
# Exhaustively enumerate a PR's changed file paths, or fail closed.
#
# ersatztv#649. This is the ONE implementation of the security-critical half of the merge gate.
# It exists because the same logic was written twice — once in `.claude/hooks/pretooluse-merge-consent.sh`
# (advisory: a failure produces a human prompt) and once in `.gitea/workflows/review-verdict.yml`
# (ENFORCED: it writes the branch-protection-required `review-verdict/h10` status). The advisory copy
# accumulated four rounds of hardening (ersatztv#643) that the enforced copy never received, leaving the
# copy with real authority strictly weaker than the copy without. Two copies of a security predicate
# drift; one cannot.
#
# SCOPE — mechanism, not policy. This script answers exactly one question: "what is the complete set of
# paths this PR touches, at one head, or can we not tell?" It deliberately does NOT classify the PR.
# The two callers' allow-lists differ ON PURPOSE and must stay separate:
#   * the hook's docs-only pattern also lets .claude/ .gitea/ .husky/ through, which is safe there only
#     because it falls through to a HUMAN PROMPT;
#   * the workflow's is narrower, because there a match posts a green status with nobody in the loop.
# Sharing the enumeration fixes the drift; sharing the classification would erase an intended difference.
#
# CONTRACT
#   Usage: pr-changed-files.sh <owner> <repo> <pr> <expected-head-sha> <expected-base-ref>
#   stdout: newline-delimited paths, BOTH sides of every rename, no blank lines. May be empty.
#   exit 0  the enumeration is COMPLETE and bound to <expected-head-sha> AND <expected-base-ref>.
#           stdout is authoritative.
#   exit 1  the enumeration could NOT be completed or verified. stdout is meaningless — the caller
#           MUST fail closed (withhold any exemption). A diagnostic goes to stderr.
#   exit 2  usage error.
# Callers must treat any non-zero exit as "no exemption". Never read stdout without checking the status.
#
# WHY THE BASE REF IS AN ARGUMENT, AND WHY IT IS NOT OPTIONAL (ersatztv#698 route 1).
# `/pulls/{n}/files` computes the diff against the PR's **live** base, which is mutable. Retargeting a
# PR changes the enumerated file set without moving the head sha, so head-binding alone does not bind
# the ANSWER — only the commit it is nominally about. Reproduced live on this instance: a PR opened
# into `main` and retargeted mid-run to a scratch base enumerated as docs-only and was granted
# `review-verdict/h10=success`, while its diff against `main` carried a C# file (probe PR #703).
#
# REQUIRED rather than optional on purpose. An optional binding on a shared security primitive is an
# opt-out, and the caller that forgets it is precisely the caller that needed it — silently. Five
# arguments or exit 2.
#
# This NARROWS the window, it does not erase it. The base is re-read after the paging round trips
# alongside the head, so a retarget that is still in effect at that point fails closed; a retarget
# that opens and closes strictly between the files call and the re-read is not observable from here.
# Pinning the diff to two shas would close it, and Gitea 1.25.4 cannot serve that: `compare/{base}...
# {head}` returns `total_commits`/`commits` and NO `files`, and a `--depth=1` fetch of the two shas
# has no merge base, so a three-dot diff is impossible while a two-dot one over-reports every commit
# `main` gained since the branch point (both measured, #698). The remainder is covered one level up
# instead, by the workflow reclassifying on `edited` rather than trusting a machine-written success.
#
# AUTH/TRANSPORT is caller-supplied via env, because the two callers authenticate differently:
#   ETV_GITEA_TOKEN | GITEA_TOKEN   -> `Authorization: token`
#   ETV_GITEA_BASICAUTH             -> curl -u user:pass
#   ETV_GITEA_URL | GITEA_BASE_URL  -> API base; defaults to the homelab Gitea. A value ending in
#                                     /api/v1 is used as-is, otherwise /api/v1 is appended.
#
# jq COMPATIBILITY (ersatztv#648). This runs on the CI runner, which ships **jq 1.6**, while it is
# authored on Macs shipping 1.8.x. It is therefore written to the 1.6-compatible subset:
#   * never rely on `jq -e`'s exit status over EMPTY input — 1.6 exits 0 where >=1.7 exits 4, which is
#     precisely the fail-open that ersatztv#647 found live in the enforced gate. Emptiness is always
#     checked explicitly in shell FIRST.
#   * never use `contains()` for substring tests — on 1.6 `contains("<NUL>")` is true for every string.
#   * never distinguish a parse error from "no output" by exit code — 1.6 returns 4 for both.
# See docs/ci-cd.md -> "The jq contract".

set -euo pipefail

if [ "$#" -ne 5 ]; then
  echo "usage: pr-changed-files.sh <owner> <repo> <pr> <expected-head-sha> <expected-base-ref>" >&2
  exit 2
fi

owner=$1
repo=$2
pr=$3
expected_sha=$4
expected_base=$5

if [ -z "$owner" ] || [ -z "$repo" ] || [ -z "$pr" ] || [ -z "$expected_sha" ] || [ -z "$expected_base" ]; then
  echo "pr-changed-files: empty owner/repo/pr/sha/base argument" >&2
  exit 2
fi

base_url="${ETV_GITEA_URL:-${GITEA_BASE_URL:-http://192.168.1.95:3000}}"
case "$base_url" in
  */api/v1) : ;;
  */)       base_url="${base_url}api/v1" ;;
  *)        base_url="${base_url}/api/v1" ;;
esac

# Empty output on ANY failure, so every caller path treats a transport error the same way. The
# emptiness is then rejected explicitly below — never inferred from a jq exit code.
gq() {
  local path="$1"
  if [ -n "${ETV_GITEA_TOKEN:-}" ]; then
    curl -sf -H "Authorization: token $ETV_GITEA_TOKEN" "$base_url/$path" 2>/dev/null || true
  elif [ -n "${GITEA_TOKEN:-}" ]; then
    curl -sf -H "Authorization: token $GITEA_TOKEN" "$base_url/$path" 2>/dev/null || true
  elif [ -n "${ETV_GITEA_BASICAUTH:-}" ]; then
    curl -sf -u "$ETV_GITEA_BASICAUTH" "$base_url/$path" 2>/dev/null || true
  else
    printf ''
  fi
}

if [ -z "${ETV_GITEA_TOKEN:-}" ] && [ -z "${GITEA_TOKEN:-}" ] && [ -z "${ETV_GITEA_BASICAUTH:-}" ]; then
  echo "pr-changed-files: no Gitea credentials in env — cannot enumerate, failing closed" >&2
  exit 1
fi

# Bind the BASE before the first page is requested (ersatztv#698 route 1). Checking only afterwards
# would leave the common case — a PR retargeted before the enumeration even starts — indistinguishable
# from an honest one, because every page would agree with every other page while all of them described
# a diff against the wrong base. Both ends are checked; neither alone is sufficient.
prjson_before=$(gq "repos/$owner/$repo/pulls/$pr")
if [ -z "${prjson_before//[[:space:]]/}" ]; then
  echo "pr-changed-files: could not read PR #$pr to bind the base ref before enumerating — failing closed" >&2
  exit 1
fi
base_before=$(printf '%s' "$prjson_before" | jq -r '.base.ref // ""' 2>/dev/null || true)
if [ -z "$base_before" ] || [ "$base_before" != "$expected_base" ]; then
  echo "pr-changed-files: PR #$pr targets '${base_before:-<unreadable>}', not the expected '$expected_base' — the diff would be computed against a different base, failing closed" >&2
  exit 1
fi

PAGE_SIZE=50
MAX_PAGES=40          # 2000 files; beyond this we refuse rather than guess

files=""
page=1
complete=no

while [ "$page" -le "$MAX_PAGES" ]; do
  raw=$(gq "repos/$owner/$repo/pulls/$pr/files?limit=${PAGE_SIZE}&page=${page}")

  # An EMPTY body is rejected in SHELL, before jq sees it. `jq -e` over empty input exits 4 on
  # jq >= 1.7 but 0 on jq 1.6, and the runner ships 1.6 — leaving this to jq's exit status is the
  # exact fail-open ersatztv#647 found in the enforced copy. A transport failure must never
  # masquerade as a legitimate short final page.
  if [ -z "${raw//[[:space:]]/}" ]; then
    echo "pr-changed-files: empty/unreadable response for page ${page}" >&2
    complete=no; break
  fi

  # VALIDATE EVERY FIELD THE EXTRACTION BELOW CONSUMES, on EVERY row.
  #
  # * Top-level type alone is not enough: `[{}]` is a well-formed array whose rows carry no
  #   `filename`, so it contributes no paths, looks like a short page, and would complete the
  #   enumeration from a PARTIAL list — the same failure one level down. It also rejects arrays of
  #   scalars, which would otherwise make the extraction fail under `set -e`.
  # * CR/LF in a path is rejected outright. `chunk` flattens paths into newline-delimited text, so a
  #   filename containing a newline splits into TWO lines matched against the allow-list separately:
  #   "safe.md\ndocs/Program.cs" yields `safe.md` and `docs/Program.cs`, both of which pass, while the
  #   real single path ends in `.cs`. Git permits newlines in filenames, so this is reachable and was
  #   reproduced against the hook.
  # * `previous_filename` is validated on EVERY row, not only `renamed` ones, because `chunk` emits it
  #   for every row regardless of `.status`. Validating it only where it is semantically "supposed to"
  #   appear left a hole one predicate wide: a `status: "modified"` row carrying a newline in
  #   `previous_filename` was reproducibly exempted. The validation domain must match the CONSUMPTION
  #   domain.
  # * `..` is rejected because the callers' allow-lists anchor `^docs/`, so `docs/../ErsatzTV/Program.cs`
  #   matches one. Git will not produce such a path; this guard's job is to fail closed on unexpected
  #   2xx shapes rather than assume a well-behaved peer.
  # * `.status` is checked against a CLOSED set. Be precise about what this does and does not do:
  #   the extraction below emits `(.previous_filename // empty)` UNCONDITIONALLY, so a present
  #   `previous_filename` is never dropped on account of `.status`. What the closed set actually buys
  #   is rejecting rows whose vocabulary we do not recognise — where a source path may be absent, or
  #   carried in some other field we are not reading. Without it, `"Renamed"` with a capital R, or an
  #   absent status, silently takes the `else true` branch of the clause below and skips the
  #   "renamed rows MUST carry previous_filename" requirement entirely. (An earlier version of this
  #   comment claimed the source path would be "dropped", which is not the mechanism; a maintainer
  #   who tested that claim would find it false and might conclude the check is redundant.)
  #   `modified` is accepted alongside `changed` deliberately: live Gitea 1.25.4 emits `changed`, but a
  #   closed allow-list built from the wrong vocabulary is a worse failure than the hole it closes — it
  #   would gate every genuine docs-only PR on any version that spells it differently. The property is
  #   "reject values we do not recognise", not "enumerate one version exactly".
  if ! printf '%s' "$raw" \
       | jq -e 'def ok: type == "string" and length > 0
                        and (test("[\\r\\n]") | not)
                        and (split("/") | index("..") | not);
                type == "array" and all(.[];
                  (.filename | ok)
                  and (.previous_filename == null or (.previous_filename | ok))
                  and ((.status // "") as $s | ($s | type) == "string"
                       and (["added","deleted","changed","modified","renamed","copied"] | index($s)) != null)
                  and (if .status == "renamed"
                       then (.previous_filename | type == "string" and length > 0)
                       else true end))' \
         >/dev/null 2>&1; then
    echo "pr-changed-files: page ${page} failed row validation" >&2
    complete=no; break
  fi

  # BOTH sides of a rename: Gitea reports a `git mv` as ONE row whose `filename` is the DESTINATION,
  # with the source only in `previous_filename`. Reading `filename` alone lets a PR move a protected
  # file INTO docs/ and pass as docs-only (verified live: `.gitea/workflows/renovate.yml` ->
  # `docs/innocuous-note.md` showed no protected path). One renamed row is therefore ONE row but TWO
  # paths, which is why the two counts below are computed differently.
  n=$(printf '%s' "$raw" | jq -r 'length')
  chunk=$(printf '%s' "$raw" | jq -r '.[] | (.filename // empty), (.previous_filename // empty)')
  [ -n "$chunk" ] && files=$(printf '%s\n%s' "$files" "$chunk")

  # Terminate ONLY on an explicitly validated EMPTY page — never on a merely SHORT one.
  # "Fewer than 50 rows means last page" assumes the server's page size is the 50 we asked for, but
  # Gitea caps `limit` at the server-wide MAX_RESPONSE_ITEMS (default 50, configurable) and is free to
  # return fewer. A 30-row page followed by a page of code would complete the enumeration over a
  # PARTIAL list — the same fail-open, reached without any transport error. Costs one extra request;
  # the MAX_PAGES cap still fails closed.
  if [ "$n" -eq 0 ]; then complete=yes; break; fi
  page=$((page + 1))
done

if [ "$complete" != yes ]; then
  echo "pr-changed-files: enumeration incomplete (stopped at page ${page}) — failing closed" >&2
  exit 1
fi

# Bind the enumeration to ONE head. Paging is several round-trips; a force-push between them means
# page 1 came from head A and page 2 from head B, so the assembled list belongs to no single commit —
# B's code page can be skipped entirely while B's docs page reads as a clean short tail. Re-read the
# head and refuse if it moved.
prjson=$(gq "repos/$owner/$repo/pulls/$pr")
if [ -z "${prjson//[[:space:]]/}" ]; then
  echo "pr-changed-files: could not re-read PR head to bind the enumeration — failing closed" >&2
  exit 1
fi
sha_after=$(printf '%s' "$prjson" | jq -r '.head.sha // ""' 2>/dev/null || true)
if [ -z "$sha_after" ] || [ "$sha_after" != "$expected_sha" ]; then
  echo "pr-changed-files: head moved during enumeration (${expected_sha:0:7} -> ${sha_after:0:7}) — failing closed" >&2
  exit 1
fi

# The same round-trip window applies to the BASE, and the head check cannot see it: retargeting moves
# the diff without moving the head sha (ersatztv#698 route 1). Comparing `.base.ref` — the branch NAME,
# never its tip — is deliberate and matches `post-review-verdict.sh` (ersatztv#632): a base that merely
# ADVANCES is ordinary churn, while comparing tips would fail every enumeration on every unrelated
# merge to `main`.
base_after=$(printf '%s' "$prjson" | jq -r '.base.ref // ""' 2>/dev/null || true)
if [ -z "$base_after" ] || [ "$base_after" != "$expected_base" ]; then
  echo "pr-changed-files: base moved during enumeration ('$expected_base' -> '${base_after:-<unreadable>}') — the enumerated diff is against a base this PR no longer targets, failing closed" >&2
  exit 1
fi

printf '%s\n' "$files" | grep -v '^$' || true
exit 0