gstack/bin/gstack-redact-prepush

#!/usr/bin/env bun
/**
 * gstack-redact-prepush — git pre-push hook that scans the diff being pushed for
 * HIGH-severity credentials and blocks the push on a hit.
 *
 * THIS IS A GUARDRAIL, NOT ENFORCEMENT. `git push --no-verify` bypasses it, as
 * does `GSTACK_REDACT_PREPUSH=skip`. It catches accidental credential pushes,
 * the most common real-world leak. It does NOT scan history, binary/LFS/submodule
 * files, or non-added lines. History scanning is /cso's job.
 *
 * Git pre-push interface: refs are read from STDIN, one per line:
 *   <local ref> <local sha> <remote ref> <remote sha>
 * We scan the ADDED lines of <remote sha>..<local sha> per ref (what's being
 * pushed). Special cases:
 *   - remote sha all-zeroes  → new branch: diff against merge-base with the
 *     remote's default branch (fallback: scan all commits unique to local ref).
 *   - local sha all-zeroes   → branch delete: nothing to scan, skip.
 *   - force-push             → remote..local still gives the net new content.
 *
 * Behavior:
 *   - HIGH finding in added lines → print + exit 1 (block), for public AND private.
 *   - MEDIUM → warn (non-blocking). LOW/WARN → silent.
 *   - GSTACK_REDACT_PREPUSH=skip → log + exit 0 (escape valve).
 *
 * Installed/uninstalled via `gstack-redact install-prepush-hook` (see the
 * gstack-redact CLI), which chains any pre-existing hook.
 */
import { spawnSync } from "child_process";
import * as fs from "fs";
import * as os from "os";
import * as path from "path";
import { scan, type Finding } from "../lib/redact-engine";
import { mkdirpSync } from "../lib/fs-utils";

const ZERO = /^0+$/;
// The canonical empty-tree object under SHA-1; diffing against it yields all
// content as added. Only a FALLBACK — see emptyTree().
const EMPTY_TREE_SHA1 = "4b825dc642cb6eb9a060e54bf8d69288fbee4904";

/**
 * Permissive git for legitimately-fallible PROBES (symbolic-ref, rev-parse,
 * merge-base) where a non-zero exit is normal control flow. The DIFF call
 * must NOT use this — see gitStrict (#1946 fail-closed).
 */
function git(args: string[]): string {
  const r = spawnSync("git", args, { encoding: "utf8", maxBuffer: 64 * 1024 * 1024 });
  return r.status === 0 ? (r.stdout ?? "") : "";
}

/**
 * Fail-closed git for the diff that decides whether the push is scanned
 * (#1946). status !== 0 covers repo errors; status === null covers a killed
 * process AND maxBuffer overflow — the oversized-diff case is exactly where
 * a large secret-bearing blob is most likely, so "couldn't read the diff"
 * must block, not silently allow.
 */
function gitStrict(args: string[]): string {
  const r = spawnSync("git", args, { encoding: "utf8", maxBuffer: 64 * 1024 * 1024 });
  // status !== 0 covers BOTH a non-zero exit AND null (process killed by a
  // signal or maxBuffer overflow — null !== 0 is true).
  if (r.status !== 0) {
    throw new Error(
      `git ${args[0]} failed (status=${r.status ?? "killed/overflow"}): ${(r.stderr ?? "").slice(0, 300)}`,
    );
  }
  return r.stdout ?? "";
}

/**
 * The empty-tree OID for THIS repository's hash algorithm.
 *
 * The SHA-1 constant does not exist in a SHA-256 repository, so the fallback
 * range `<sha1-empty-tree>..local` makes git error out, gitStrict throws, and
 * every legitimate first push of a new branch is hard-blocked with a diff
 * error. Asking git to hash the empty tree returns the right OID under either
 * algorithm and needs no table of constants.
 */
let _emptyTree: string | undefined;
function emptyTree(): string {
  if (_emptyTree === undefined) {
    // --stdin with empty input rather than /dev/null: portable to Windows.
    const r = spawnSync("git", ["hash-object", "-t", "tree", "--stdin"], {
      encoding: "utf8",
      input: "",
    });
    const oid = r.status === 0 ? (r.stdout ?? "").trim() : "";
    _emptyTree = /^[0-9a-f]{40,64}$/i.test(oid) ? oid : EMPTY_TREE_SHA1;
  }
  return _emptyTree;
}

/** True when the object exists in the local odb (cat-file -e signals via exit code). */
function objectExists(sha: string): boolean {
  const r = spawnSync("git", ["cat-file", "-e", sha], { encoding: "utf8" });
  return r.status === 0;
}

/**
 * The remote this push targets, as git hands it to pre-push in $1 (and its URL
 * as $2); the installed hook wrapper forwards "$@". Null when the name is
 * unavailable (stdin/CLI invocation) or does not name a CONFIGURED remote — a
 * URL push has no remote-tracking namespace at all — and every caller then
 * falls back to the historical origin-shaped behavior.
 */
let _pushRemote: string | null | undefined;
function pushRemote(): string | null {
  if (_pushRemote === undefined) {
    const name = process.argv[2];
    const configured = name
      ? git(["remote"]).split("\n").map((s) => s.trim()).filter(Boolean).includes(name)
      : false;
    _pushRemote = configured ? name! : null;
  }
  // `?? null` only narrows away the not-yet-computed sentinel; by here the
  // cache is always set, and null legitimately means "no tracking namespace".
  return _pushRemote ?? null;
}

/**
 * The remote-tracking exclusion used when narrowing to "commits new to the
 * remote" (#2592 catch-up merges, #2573 rebased force-pushes).
 *
 * Narrowed to the PUSH TARGET's namespace (S1): a bare `--remotes` excludes
 * commits reachable from ANY remote-tracking ref, so a secret that had only
 * ever been fetched from (or pushed to) a private/local-path remote was never
 * scanned when later pushed to a PUBLIC remote — "already left this machine"
 * is not "already reached THIS remote". Falling back to the bare form scans
 * LESS than the narrowed form would, but never less than the hook historically
 * did.
 */
function remotesExclusion(): string {
  const name = pushRemote();
  return name ? `--remotes=${name}/*` : "--remotes";
}

/**
 * The push target's default branch, or null when it has none we can resolve.
 *
 * ⚠ SCOPED TO THE PUSH TARGET, NOT HARDCODED TO origin.
 *
 * remotesExclusion() above already reasons this way for the rev-list exclusion,
 * but this probe kept asking origin regardless of where the push was going, and
 * the two together are what pick the scan range. The consequence was a
 * fail-OPEN, measured: with HEAD == origin/main, `git push publish HEAD:main`
 * to a brand-new second remote resolved a merge-base of HEAD against
 * origin/main, producing the range HEAD..HEAD — an empty diff — so the entire
 * tree, credentials included, shipped to an unknown (possibly public) remote
 * with exit 0 and nothing scanned. That is the exact moment a scan matters most.
 *
 * Returning null when the target remote has no resolvable default branch drops
 * the caller through to the rev-list step, which is already scoped to the push
 * target and correctly reports "all of this is new" for a never-fetched remote.
 * Scanning more is the safe direction here and matches the S1 reasoning above.
 */
function defaultRemoteBranch(): string | null {
  const remote = pushRemote() ?? "origin";
  // <remote>/HEAD → <remote>/main, fall back to main/master.
  const sym = git(["symbolic-ref", `refs/remotes/${remote}/HEAD`]).trim();
  if (sym) return sym.replace("refs/remotes/", "");
  for (const b of [`${remote}/main`, `${remote}/master`]) {
    if (git(["rev-parse", "--verify", b]).trim()) return b;
  }
  return null;
}

/**
 * Base commit for a push whose remote tip we cannot use directly, ordered from
 * most precise to most conservative. Returns null when nothing can anchor the
 * range, i.e. the whole history really is new content.
 */
function unknownRemoteTipBase(localSha: string): string | null {
  // 1. The common case: a merge-base with the remote's default branch. Null
  //    means the PUSH TARGET has no default branch we can see (never fetched),
  //    in which case there is nothing legitimate to anchor on — fall through.
  const def = defaultRemoteBranch();
  const base = def ? git(["merge-base", localSha, def]).trim() : "";
  if (base) return base;

  // 2. No merge-base. defaultRemoteBranch() guessed a ref that does not exist
  //    (default branch named trunk/develop, origin/HEAD unset), or history is
  //    disjoint. Anything reachable from localSha but from NO remote-tracking
  //    branch is what this push actually adds; the parent of its oldest commit
  //    is the real base.
  //
  //    Without this we drop straight to EMPTY_TREE and re-scan content that is
  //    already on the remote. That is not merely wasteful, it is wrong in two
  //    ways: a secret pushed long ago gets re-reported as if THIS push
  //    introduced it (telling the operator to rotate a key over someone else's
  //    old commit), and on any real repository the input overshoots the
  //    engine's byte cap, so `engine.input_too_large` blocks having scanned
  //    NOTHING — "scans more, never less" inverted into "scans nothing".
  //
  //    The exclusion is scoped to the PUSH TARGET's tracking refs (see
  //    remotesExclusion): content on some OTHER remote has left this machine,
  //    but it has not reached the remote being pushed to — a secret that only
  //    ever hit a private remote must still be scanned on its way to a public
  //    one (S1).
  const newCommits = git(["rev-list", "--reverse", localSha, "--not", remotesExclusion()]).trim();
  if (newCommits) {
    const oldest = newCommits.split("\n")[0];
    const parent = git(["rev-parse", "--verify", `${oldest}^`]).trim();
    if (parent) return parent;
    // Oldest new commit is a root commit: there is no parent to anchor on.
  }

  // 3. Nothing to anchor on — a genuinely fresh repository with no remote refs.
  //    Every commit IS new content, so scanning it all is the correct answer.
  return null;
}

/**
 * The commits this push actually adds — reachable from localSha and from NO
 * remote-tracking ref.
 *
 * ⚠ WHY THIS EXISTS RATHER THAN A TWO-DOT RANGE.
 *
 * `remoteSha..localSha` is "everything new on this branch", which is NOT the
 * same as "everything new to the remote". Merge origin/main into a feature
 * branch and every commit main gained since the branch's last push becomes an
 * added line — content that is already published, already scanned, and not
 * this push's doing.
 *
 * Two things follow, and both were observed:
 *
 *   · FALSE HIGH FINDINGS. A placeholder connection string in a test fixture,
 *     already merged to main by someone else, blocked an unrelated push as
 *     `db.url_with_password` — telling the operator to rotate a credential
 *     over a fixture they had never touched. A
 *     guard that cries wolf on catch-up merges is a guard people learn to
 *     bypass reflexively — which is exactly how a real secret gets through.
 *   · OVERSIZED SCANS. The comment on SCAN_CHUNK_BYTES below records a
 *     1,146,782-byte diff from "a feature branch catching up to a busy main"
 *     blowing the engine's 1 MiB cap. Same root cause, treated there as a size
 *     problem and solved by slicing. Narrowing the range fixes the size too.
 *
 * A two-dot range cannot express this: after merging main, neither the remote
 * tip nor the merge-base with main is an ancestor of the other, so no single
 * base excludes both. `rev-list --not --remotes=<push-remote>/*` is the
 * operation that does, and this file already reasons that way in
 * `unknownRemoteTipBase` step 2. The exclusion is scoped to the push target's
 * tracking namespace (see remotesExclusion): the upstream commits a catch-up
 * merge brings in came from the SAME remote being pushed to, so scoping keeps
 * the #2592 fix intact while a secret known only to some OTHER (private)
 * remote is still scanned on its way to this one (S1).
 *
 * Each commit is diffed alone. `--cc` on a merge shows only the conflict
 * RESOLUTION — content that exists in no parent — so a secret introduced while
 * resolving a merge is still caught, while an ordinary merge contributes
 * nothing. Returns null when the notion does not apply, so callers fall back.
 */
function addedLinesFromNewCommits(localSha: string, remoteSha: string): string | null {
  // remoteSha is what git TELLS us the remote has, and it is authoritative in a
  // way `--remotes` is not: remote-tracking refs can be absent (a fresh clone
  // that never fetched, a push to a remote with no tracking ref) or stale. Drop
  // it and a repo with no tracking refs excludes NOTHING — every commit ever
  // made reads as "new", which re-introduces the false positives from the other
  // direction. So it stays the base; `--remotes` only ADDS exclusions on top.
  if (ZERO.test(remoteSha) || !objectExists(remoteSha)) return null;

  const narrowed = git(["rev-list", localSha, "--not", remoteSha, remotesExclusion()]).trim();
  if (!narrowed) return null;

  // If excluding remote-tracking refs changes nothing, this push has no
  // catch-up commits and the plain range already describes it exactly. Defer to
  // it. That is not just an optimization: it keeps every push that ISN'T a
  // catch-up merge on the original gitStrict diff path, so the fail-closed
  // guarantee (#1946) and its regression test keep exercising the code they
  // were written for. A narrowing that silently retired that test would be a
  // worse trade than the false positives it set out to fix.
  const plain = git(["rev-list", `${remoteSha}..${localSha}`]).trim();
  const asSet = (s: string) => s.split("\n").filter(Boolean).sort().join("\n");
  if (asSet(narrowed) === asSet(plain)) return null;

  const shas = narrowed.split("\n").filter(Boolean);
  // A rewrite of long history should fall back rather than shell out per commit.
  if (shas.length > 500) return null;
  const out: string[] = [];
  for (const sha of shas) {
    // gitStrict: a failed diff must never read as "nothing added" (#1946).
    out.push(gitStrict([
      "show", "--unified=0", "--no-color", "--no-ext-diff", "--no-textconv",
      "--cc", "--format=", sha,
    ]));
  }
  return out.join("\n");
}

/** Return the added-line text for a ref update being pushed. */
function addedLinesFor(localSha: string, remoteSha: string): string {
  // Preferred ONLY when this push carries catch-up commits: scanning them again
  // is the bug. Every other shape falls through to the range logic below.
  const fromNew = addedLinesFromNewCommits(localSha, remoteSha);
  if (fromNew !== null) return collectAddedLines(fromNew);

  let range: string;
  // Set when git NAMED a remote tip we do not have. Any base we derive locally
  // is then a guess about content we cannot see — see the empty-result check
  // at the end of this function.
  let guessedFromUnresolvableTip = false;
  if (ZERO.test(remoteSha) || !objectExists(remoteSha)) {
    // Either a new branch (zero remote sha), or the remote tip object is absent
    // locally (shallow clone, force-push without a prior fetch, CI checkout) so
    // remote..local cannot resolve. Both need a base derived locally; scan MORE
    // rather than hard-blocking a legitimate push (adversarial review finding 8).
    guessedFromUnresolvableTip = !ZERO.test(remoteSha);
    const base = unknownRemoteTipBase(localSha);
    range = base ? `${base}..${localSha}` : `${emptyTree()}..${localSha}`;
  } else {
    // Existing branch (incl. force-push): net new content remote..local.
    range = `${remoteSha}..${localSha}`;
  }
  // -U0: only changed lines; we keep lines starting with '+' (added), drop the
  // +++ file header. Unified diff added lines start with a single '+'.
  // Strict (#1946): a failed diff used to return "" and the push sailed
  // through unscanned — fail open on the exact path the guard exists for.
  //
  // --no-ext-diff: a user's `diff.external` driver replaces the entire diff
  // with its own output — with one set, `git diff` emits zero '+' lines, so an
  // unhardened scanner reads an empty diff and exits 0 on a push full of
  // secrets. Reachable from ordinary user config, not hypothetical. (#2498)
  // --no-textconv: a .gitattributes textconv driver can likewise rewrite
  // content before we ever see it. (#2498)
  const diff = gitStrict([
    "diff", "--unified=0", "--no-color", "--no-ext-diff", "--no-textconv",
    range,
  ]);
  const added = collectAddedLines(diff);

  // ⚠ A GUESS THAT SCANNED NOTHING IS NOT EVIDENCE OF NOTHING.
  //
  // git told us the remote is at a sha we do not have, which is itself proof
  // that our remote-tracking refs do not describe this ref. Every base above is
  // then a guess, and when the guess yields an EMPTY diff we have not concluded
  // "this push adds nothing" — we have concluded nothing at all, while git is
  // about to send whatever the real difference is.
  //
  // Measured fail-open: a four-field ref line with a valid local sha and a
  // well-shaped but absent remote sha, on a repo where HEAD == origin/main,
  // resolved to HEAD..HEAD and allowed a push carrying a live credential with
  // exit 0. The shape check in main() rejects junk like "not-a-sha"; it cannot
  // reject 40 plausible hex digits.
  //
  // Narrow by construction: the legitimate cases this path exists for (shallow
  // clone, CI checkout, force-push without a prior fetch) all produce a
  // NON-empty range and are untouched. Only "we guessed and saw nothing" blocks,
  // and it blocks with the fix in the message: fetch the remote.
  if (guessedFromUnresolvableTip && !added.trim()) {
    throw new Error(
      `remote tip ${remoteSha.slice(0, 12)} is not in the local object database and no ` +
        "local base narrowed this push, so nothing was scanned — run `git fetch` for this " +
        "remote and push again",
    );
  }
  return added;
}

/**
 * Added-line text from a unified diff. Shared by both range strategies so the
 * hunk-aware header handling below cannot drift between them.
 */
function collectAddedLines(diff: string): string {
  const added: string[] = [];
  // Hunk-aware header skip (#2498): `+++ ` is only a FILE HEADER outside a
  // hunk. Inside a hunk, an added content line whose text begins with "++"
  // renders as "+++<content>" — the old blanket startsWith("+++") skip
  // silently dropped exactly those lines from the scan.
  let inHunk = false;
  for (const line of diff.split("\n")) {
    // `diff --` rather than `diff --git`: a merge scanned with --cc emits
    // `diff --cc <path>`, so a --git-only reset left inHunk true across file
    // boundaries and read the next file's `+++ b/...` header as content. Only
    // noise (it over-scans, never under-scans), but the boundary is real.
    if (line.startsWith("diff --")) { inHunk = false; continue; }
    if (line.startsWith("@@")) { inHunk = true; continue; }
    if (!inHunk && (line.startsWith("+++") || line.startsWith("---"))) continue;
    if (line.startsWith("+")) added.push(line.slice(1));
  }
  return added.join("\n");
}

/**
 * Byte budget per scan() call. Kept comfortably under redact-engine's
 * DEFAULT_MAX_BYTES (1 MiB) so a slice never trips its oversize guard.
 */
const SCAN_CHUNK_BYTES = 768 * 1024;

/**
 * Bytes replayed from the end of one slice into the start of the next.
 *
 * ⚠ WHY SLICING WITHOUT OVERLAP IS NOT SAFE.
 *
 * The reasoning this replaced said overlap was unnecessary because "every
 * pattern is single-line, so a line boundary cannot bisect a detectable
 * secret". True of the MATCH, false of the PROXIMITY requirement: five
 * patterns in redact-patterns.ts carry a `nearRegex` and only fire when their
 * qualifying label appears within `nearWindow` characters — aws.secret_key
 * (HIGH) needs `aws_secret_access_key` within 100, gcp.service_account (HIGH)
 * needs `"private_key_id"` within 300. Put the label at the end of one slice
 * and the secret at the start of the next and the pattern never fires at all.
 *
 * Measured, not hypothetical: a 40-char AWS secret with its label straddling
 * the cut produced ZERO HIGH findings and exit 0 — the guard allowed a push it
 * would have blocked had the same bytes sat 41 bytes earlier.
 *
 * 16 KiB is ~55x the largest nearWindow, so the qualifying context always
 * survives the seam with room to spare.
 */
const SCAN_OVERLAP_BYTES = 16 * 1024;

/**
 * Per-line budget. Kept a full overlap below the slice budget so that
 * `carried overlap + one unit` can never exceed SCAN_CHUNK_BYTES, which keeps
 * every scan() call provably under the engine's 1 MiB cap.
 */
const LINE_BUDGET_BYTES = SCAN_CHUNK_BYTES - SCAN_OVERLAP_BYTES;

/**
 * Zero-width characters, matching redact-engine's ZERO_WIDTH set exactly.
 *
 * The engine strips these before matching, so they cost bytes here while
 * contributing nothing to the text a pattern actually sees. Budgeting slices in
 * RAW bytes therefore lets invisible padding decide where the seam falls:
 * ~900 KB of U+200B between a label and its secret pushed them into different
 * slices while their NORMALIZED distance stayed a couple of characters, and the
 * push went out with exit 0. Stripping on ingest makes this file's budget and
 * overlap measure the same thing the engine measures.
 *
 * Finding-preserving by construction — the engine would have removed exactly
 * these characters anyway, so no match is created or destroyed.
 */
const ZERO_WIDTH = /[\u200B\u200C\u200D\u2060\uFEFF]/g;

/** Never cut between the halves of a surrogate pair. */
function safeCut(s: string, i: number): number {
  if (i <= 0 || i >= s.length) return i;
  const prev = s.charCodeAt(i - 1);
  return prev >= 0xd800 && prev <= 0xdbff ? i - 1 : i;
}

/**
 * Cut one over-budget line into byte-bounded pieces that overlap.
 *
 * A minified bundle or a single-line JSON blob is one "line" of megabytes. Handed
 * to the engine whole it trips input_too_large: the push is blocked, which is
 * safe, but blocked WITHOUT the content ever being read — so the operator is
 * told a size error where a credential may be sitting, and learns to bypass.
 * Slicing with overlap reads it for real; the 2 MiB one-liner that used to
 * report `engine.input_too_large` now names the aws.access_key inside it.
 */
function sliceLongLine(line: string, budget: number, overlap: number): string[] {
  const pieces: string[] = [];
  let start = 0;
  while (start < line.length) {
    // One char is never fewer than one byte, so `budget` chars is never short
    // of the byte budget; shrink proportionally until it fits.
    let end = Math.min(line.length, start + budget);
    let bytes = Buffer.byteLength(line.slice(start, end), "utf8");
    while (end > start + 1 && bytes > budget) {
      const scaled = start + Math.max(1, Math.floor((end - start) * (budget / bytes)));
      end = scaled < end ? scaled : end - 1;
      bytes = Buffer.byteLength(line.slice(start, end), "utf8");
    }
    end = Math.max(start + 1, safeCut(line, end));
    pieces.push(line.slice(start, end));
    if (end >= line.length) break;
    // Step back by the overlap so a secret sitting on the cut — and the label
    // that qualifies it — land together in the next piece.
    let back = 0;
    let backBytes = 0;
    while (end - back - 1 > start && backBytes < overlap) {
      backBytes += Buffer.byteLength(line[end - back - 1]!, "utf8");
      back++;
    }
    // start strictly increases: the loop guarantees end - back > start.
    start = Math.max(start + 1, safeCut(line, end - back));
  }
  return pieces;
}

/**
 * Scan added lines in line-aligned slices, unioning the findings.
 *
 * Why: the engine refuses input over its byte cap and fails closed, which is
 * right for one scan() call but wrong as a push policy — a feature branch
 * catching up to a busy main legitimately produces more added lines than the
 * cap (1,146,782 bytes against the 1 MiB default in the push that prompted
 * this, and only ~7% of that was the lockfile). The push then blocked on
 * `engine.input_too_large` — a size error naming no credential — which trains
 * people to reach for --no-verify, defeating the guardrail far more thoroughly
 * than a large diff does.
 *
 * Slices overlap by SCAN_OVERLAP_BYTES and are budgeted in zero-width-stripped
 * bytes, so a seam cannot separate a proximity-qualified secret from its label;
 * see those two constants for the fail-open each one closes.
 *
 * Findings' line/col are slice-relative, which is fine here — this hook only
 * reads severity, id and preview. Do not lift this into the engine, where
 * callers rely on absolute line numbers.
 */
function scanAddedLines(added: string, opts: Parameters<typeof scan>[1]): Finding[] {
  const findings: Finding[] = [];
  // Overlap deliberately re-scans the seam, so one occurrence can surface
  // twice. Dedup on id+preview: line/col are slice-relative and cannot identify
  // an occurrence. Worst case two DIFFERENT occurrences share a preview and are
  // counted once — that lowers a count, never a block.
  const seen = new Set<string>();

  let slice: string[] = [];
  let sliceBytes = 0;

  const flush = (carryOverlap: boolean) => {
    if (slice.length === 0) return;
    for (const f of scan(slice.join("\n"), opts).findings) {
      const key = `${f.id}\u0000${f.preview}`;
      if (seen.has(key)) continue;
      seen.add(key);
      findings.push(f);
    }
    if (!carryOverlap) {
      slice = [];
      sliceBytes = 0;
      return;
    }
    // Re-seed the next slice with the trailing lines worth up to one overlap.
    const tail: string[] = [];
    let tailBytes = 0;
    for (let i = slice.length - 1; i >= 0; i--) {
      const b = Buffer.byteLength(slice[i]!, "utf8") + 1;
      if (tailBytes + b > SCAN_OVERLAP_BYTES) break;
      tail.unshift(slice[i]!);
      tailBytes += b;
    }
    slice = tail;
    sliceBytes = tailBytes;
  };

  // unitBytes counts the newline that rejoins the unit.
  const push = (unit: string, unitBytes: number) => {
    // Close the current slice BEFORE overflowing it, carrying the overlap.
    if (sliceBytes > 0 && sliceBytes + unitBytes > SCAN_CHUNK_BYTES) flush(true);
    slice.push(unit);
    sliceBytes += unitBytes;
  };

  for (const raw of added.split("\n")) {
    const line = raw.replace(ZERO_WIDTH, "");
    const lineBytes = Buffer.byteLength(line, "utf8") + 1;
    if (lineBytes > LINE_BUDGET_BYTES) {
      // An over-budget line cannot share a slice with anything else. Close the
      // current slice first (carrying its tail so context leading INTO the long
      // line survives), then feed the pieces one slice at a time.
      flush(true);
      for (const piece of sliceLongLine(line, LINE_BUDGET_BYTES - 1, SCAN_OVERLAP_BYTES)) {
        push(piece, Buffer.byteLength(piece, "utf8") + 1);
        flush(true);
      }
      continue;
    }
    push(line, lineBytes);
  }
  flush(false);

  return findings;
}

function logSkip(reason: string): void {
  try {
    const home = process.env.GSTACK_HOME || path.join(os.homedir(), ".gstack");
    const dir = path.join(home, "security");
    // mkdirpSync, not bare mkdirSync: bun-on-Windows EEXIST (#2635). This site
    // is try-wrapped by the caller, so the old failure was a silent skip-log
    // loss rather than a crash — the fix makes the log survive, not un-crash.
    mkdirpSync(dir);
    fs.appendFileSync(
      path.join(dir, "prepush-skip.jsonl"),
      JSON.stringify({ ts: new Date().toISOString(), reason }) + "\n",
    );
  } catch {
    // best-effort; never block a push because logging failed
  }
}

function main() {
  if ((process.env.GSTACK_REDACT_PREPUSH || "").toLowerCase() === "skip") {
    logSkip(process.env.GSTACK_REDACT_PREPUSH_REASON || "env-skip");
    process.stderr.write("gstack-redact-prepush: skipped via GSTACK_REDACT_PREPUSH=skip\n");
    process.exit(0);
  }

  const stdin = fs.readFileSync(0, "utf8");
  const refs = stdin
    .split("\n")
    .map((l) => l.trim())
    .filter(Boolean)
    .map((l) => l.split(/\s+/));

  const allHigh: Finding[] = [];
  let mediumCount = 0;

  for (const fields of refs) {
    // Fail CLOSED on a ref line we cannot parse (#2498): git hands pre-push
    // exactly "<local ref> <local sha> <remote ref> <remote sha>" — anything
    // else means we cannot tell WHAT is being pushed, and silently skipping
    // it would leave that ref unscanned.
    const [, localSha, , remoteSha] = fields;
    const shaShaped = (s: string | undefined) => !!s && /^[0-9a-f]{40,64}$/i.test(s);
    if (fields.length !== 4 || !shaShaped(localSha) || !shaShaped(remoteSha)) {
      process.stderr.write(
        "\n⛔ gstack-redact-prepush BLOCKED the push — could not parse a pre-push ref line, " +
          "so its content cannot be scanned.\n" +
          `  line: ${JSON.stringify(fields.join(" "))}\n` +
          "Bypass if you're sure: GSTACK_REDACT_PREPUSH=skip git push   (or git push --no-verify)\n",
      );
      process.exit(1);
    }
    if (ZERO.test(localSha!)) continue; // branch delete → nothing pushed
    let added: string;
    try {
      added = addedLinesFor(localSha, remoteSha || "0");
    } catch (err) {
      // Fail CLOSED (#1946): if we can't compute the pushed diff we can't
      // scan it, and unscanned-but-allowed is the failure mode this hook
      // exists to prevent.
      process.stderr.write(
        "\n⛔ gstack-redact-prepush BLOCKED the push — could not compute the pushed diff, " +
          "so it cannot be scanned for credentials.\n" +
          `  (${err instanceof Error ? err.message.split("\n")[0] : String(err)})\n` +
          "Bypass if you're sure: GSTACK_REDACT_PREPUSH=skip git push   (or git push --no-verify)\n",
      );
      process.exit(1);
    }
    if (!added.trim()) continue;
    // Visibility doesn't change HIGH behavior; pass private so nothing is treated
    // as public-strict (HIGH blocks regardless either way).
    // Sliced (see scanAddedLines) so a large-but-legitimate diff is actually
    // scanned rather than blocked unscanned on the engine's size cap.
    for (const f of scanAddedLines(added, { repoVisibility: "private" })) {
      if (f.severity === "HIGH") allHigh.push(f);
      else if (f.severity === "MEDIUM") mediumCount++;
    }
  }

  if (mediumCount > 0) {
    process.stderr.write(
      `gstack-redact-prepush: ${mediumCount} MEDIUM finding(s) in pushed diff (PII/internal). ` +
        "Not blocking. Review before this becomes public.\n",
    );
  }

  if (allHigh.length > 0) {
    // A scan that could not RUN is not a scan that FOUND something. Reporting
    // "credential(s) in the pushed diff — rotate the credential" for an
    // `engine.*` finding tells the operator to rotate a secret that was never
    // detected, on a diff that was never read. Blocking is still right (fail
    // closed), but the reason must be the true one: a guardrail that cries wolf
    // is a guardrail that gets bypassed by reflex, which is worse than none.
    // Seen live 2026-07-30: a diff of a few hundred bytes reported HIGH
    // engine.input_too_large, because an unresolvable base branch made the hook
    // fall back to EMPTY_TREE..local — i.e. the WHOLE repo (~7 MiB) as "added
    // lines". The size the operator sees and the size the hook measures can
    // therefore differ by four orders of magnitude.
    const unscanned = allHigh.filter((f) => f.id.startsWith("engine."));
    const secrets = allHigh.filter((f) => !f.id.startsWith("engine."));

    if (secrets.length > 0) {
      process.stderr.write(
        "\n⛔ gstack-redact-prepush BLOCKED the push — credential(s) in the pushed diff:\n\n",
      );
      for (const f of secrets) {
        process.stderr.write(`  HIGH  ${f.id}  ${f.preview}\n`);
      }
      process.stderr.write(
        "\nRotate the credential (a pushed secret is compromised) and remove it from the diff.\n",
      );
    }

    if (unscanned.length > 0) {
      process.stderr.write(
        "\n⛔ gstack-redact-prepush BLOCKED the push — the diff could NOT be scanned.\n" +
          "   No credential was found; none was looked for. Blocking fail-closed.\n\n",
      );
      for (const f of unscanned) {
        process.stderr.write(`  ${f.id}: ${f.description}\n`);
      }
      process.stderr.write(
        "\nLikely cause: the base branch could not be resolved, so the whole repo was\n" +
          "treated as added lines. Check `git rev-parse --abbrev-ref origin/HEAD` and\n" +
          "`git merge-base HEAD origin/main`, then push again. Scan the diff yourself\n" +
          "before bypassing: `git diff <base>..HEAD | grep -inE \'password|secret|token|api.?key\'`.\n",
      );
    }

    process.stderr.write(
      "This is a guardrail: `git push --no-verify` or `GSTACK_REDACT_PREPUSH=skip git push` bypass it.\n",
    );
    process.exit(1);
  }

  process.exit(0);
}

main();