build: make verify-build take an explicit expectation and a build receipt (closes #309)

verify-build read its expectation from AUTISTMASK_DEBUG in its own environment
and the Makefile invoked it bare, so an operator with that variable exported
who ran the release target got an INSECURE debug build — every wallet it
creates uses the publicly committed test phrase — verified green, exit 0. It
also had no provenance: a 26-byte file containing the right marker string
passed, the content script and manifest.json were never inspected, and an
entire hand-written dist/ passed.

--expect release|debug and --receipt PATH are now both required, with no
defaults and nothing read from the environment. build.js records every file it
emits with its sha256 and writes the receipt; the Makefile mktemps it outside
the repo per invocation with a trap, and build.js refuses a receipt path inside
dist/. Verification runs three passes in a load-bearing order — receipt shape,
full dist/ walk, then per-file bytes — so an unwalkable subtree cannot make
files look absent. dist/constants-bundles.txt, which was an unsigned trust root
living inside the tree it vouched for, is gone.

What this proves is bounded and stated as such: dist/ is byte-for-byte the
output of the build.js run that just finished, within one make build
invocation. It proves nothing about the honesty of the source tree or build.js,
and nothing to anyone handed a dist/ from elsewhere — that is signing, #310.
The standalone make verify-build target is removed because its only input would
be dist/ itself, i.e. the artifact vouching for itself.

Verified: make check green, test-verify-build 39 cases (was 18), test-e2e 55/55
and test-e2e-firefox 8/8 with make build running uncached inside both images.
All four original bypasses now exit 1. Mutations: digests disabled fails
exactly 4 cases, dropping the dist/ walk fails exactly 8, restoring the ambient
fallback fails exactly 1.

build.js

@@ -1,5 +1,6 @@
 const fs = require("fs");
 const path = require("path");
+const crypto = require("crypto");
 const { execSync } = require("child_process");
 const esbuild = require("esbuild");
 
@@ -8,12 +9,29 @@ const DIST_CHROME = path.join(DIST, "chrome");
 const DIST_FIREFOX = path.join(DIST, "firefox");
 const SRC = path.join(__dirname, "src");
 
-// The module whose compiled DEBUG state script/verify-build asserts, and the
-// manifest naming every emitted bundle that ends up containing it. The
-// manifest is derived from esbuild's own dependency graph rather than from a
-// hardcoded list, so it tracks the bundle layout instead of rotting with it.
+// The module whose compiled DEBUG state script/verify-build asserts. Which
+// bundles contain it is derived from esbuild's own dependency graph rather
+// than from a hardcoded list, so it tracks the bundle layout instead of
+// rotting with it.
 const AUDITED_MODULE = "src/shared/constants.js";
-const BUNDLE_MANIFEST = path.join(DIST, "constants-bundles.txt");
+
+// The build receipt: every file this build emits, with its sha256 and whether
+// it is one of the audited bundles. script/verify-build is handed this and
+// checks dist/ against it, so the file list comes from the build that just ran
+// rather than being read back out of the tree it is supposed to vouch for.
+//
+// The path is supplied by the caller, not chosen here, and the Makefile makes
+// a fresh one per invocation outside the repo: that is what ties a receipt to
+// one build rather than leaving a standing file anyone can write.
+const RECEIPT_HEADER = "autistmask-build-receipt v1";
+const RECEIPT_ENV = "AUTISTMASK_BUILD_RECEIPT";
+
+// Every emitted path must be plainly nameable, because the receipt is a
+// line-oriented text file consumed by a POSIX shell script and a path with a
+// space or a newline in it could not be read back unambiguously. Nothing this
+// build emits looks like that; if that ever changes, the build fails here
+// rather than writing a receipt that cannot be checked.
+const SAFE_EMITTED_PATH = /^dist\/[A-Za-z0-9._][A-Za-z0-9._/-]*$/;
 
 function ensureDir(dir) {
     fs.mkdirSync(dir, { recursive: true });
@@ -31,10 +49,10 @@ function repoRelative(p) {
 // searching the minified text, it does not depend on what survived minification.
 //
 // The ".js" filter below is the only place that assumption lives:
-// script/verify-build searches every file and symlink under dist/ for a
-// marker, without filtering by extension, and hard-fails if it cannot walk the
-// whole tree, so a bundle emitted under some other extension fails there as
-// unlisted rather than escaping both checks at once.
+// script/verify-build reads every file the receipt names, whatever its
+// extension, and fails on any that carries a debug marker without being
+// recorded as an audited bundle — so a bundle emitted under some other
+// extension fails there rather than escaping both checks at once.
 function outputsContainingAuditedModule(metafile) {
     return Object.entries(metafile.outputs)
         .filter(([outFile, info]) => {
@@ -46,6 +64,94 @@ function outputsContainingAuditedModule(metafile) {
         .map(([outFile]) => repoRelative(outFile));
 }
 
+// Every file this build writes under dist/, recorded as it is written. This is
+// the build's own account of what it emitted; it is never recovered by
+// listing dist/, because a file that is in dist/ without this build having put
+// it there is exactly what the receipt exists to expose.
+const emittedFiles = [];
+
+function recordEmitted(absPath) {
+    emittedFiles.push(absPath);
+}
+
+// Copying is the only other way a file reaches dist/; esbuild and the Tailwind
+// CLI record their outputs where they are invoked.
+function copyEmitted(src, dest) {
+    fs.copyFileSync(src, dest);
+    recordEmitted(dest);
+}
+
+function sha256File(absPath) {
+    return crypto
+        .createHash("sha256")
+        .update(fs.readFileSync(absPath))
+        .digest("hex");
+}
+
+// Write the receipt for the files this build emitted. Deliberately records no
+// build mode: which mode was asked for is script/verify-build's argument, so
+// build.js cannot vouch for build.js. All the receipt says is "these bytes,
+// under these names, are what I wrote, and these ones bundle constants.js".
+function writeReceipt(receiptPath, auditedBundles) {
+    const audited = new Set(auditedBundles);
+    const paths = [...new Set(emittedFiles.map(repoRelative))].sort();
+
+    for (const p of paths) {
+        if (!SAFE_EMITTED_PATH.test(p)) {
+            throw new Error(
+                `emitted path cannot be written to a build receipt: ${JSON.stringify(p)}`,
+            );
+        }
+    }
+
+    // A bundle esbuild reported but that nothing recorded as emitted means the
+    // two halves have drifted apart, and the receipt would then leave an
+    // audited bundle out. Fail rather than emit a short receipt.
+    for (const bundle of audited) {
+        if (!paths.includes(bundle)) {
+            throw new Error(
+                `${bundle} contains ${AUDITED_MODULE} but was not recorded as emitted`,
+            );
+        }
+    }
+    if (audited.size === 0) {
+        throw new Error(
+            `no emitted bundle contains ${AUDITED_MODULE}, which is never correct`,
+        );
+    }
+
+    const lines = [RECEIPT_HEADER, `root ${fs.realpathSync(__dirname)}`];
+    for (const p of paths) {
+        const flag = audited.has(p) ? "A" : "P";
+        lines.push(`file ${sha256File(path.join(__dirname, p))} ${flag} ${p}`);
+    }
+    fs.writeFileSync(receiptPath, lines.map((l) => `${l}\n`).join(""));
+
+    console.log(
+        `Build receipt: ${paths.length} emitted file(s), ${audited.size} ` +
+            `containing ${AUDITED_MODULE} (${receiptPath})`,
+    );
+}
+
+// Where the receipt goes, decided before anything is emitted so a build that
+// cannot produce a checkable receipt fails before it writes any artifacts.
+// Inside dist/ is refused: a receipt that lives in the tree it describes can
+// be rewritten by whoever rewrites the tree, which is the hole this replaces.
+function receiptTarget() {
+    const requested = process.env[RECEIPT_ENV];
+    if (!requested) {
+        return null;
+    }
+    const resolved = path.resolve(requested);
+    if (resolved === DIST || resolved.startsWith(DIST + path.sep)) {
+        throw new Error(
+            `${RECEIPT_ENV} points inside dist/ (${resolved}). The receipt ` +
+                `describes dist/ and must not live in it.`,
+        );
+    }
+    return resolved;
+}
+
 // DEBUG is a build-time flag, off unless explicitly requested. It is the only
 // thing that makes the hardcoded test mnemonic reachable, so the opt-in must be
 // exact: anything other than the literal "1" (unset, empty, "true", a typo)
@@ -87,6 +193,15 @@ function getBuildInfo() {
 async function build() {
     console.log("Building AutistMask extension...");
 
+    const receiptPath = receiptTarget();
+    if (!receiptPath) {
+        console.warn(
+            `WARNING: ${RECEIPT_ENV} is unset, so this build writes no ` +
+                `receipt and script/verify-build cannot verify what it ` +
+                `emitted. Build through make build / make build-debug.`,
+        );
+    }
+
     const buildInfo = getBuildInfo();
     console.log("Build info:", buildInfo);
 
@@ -108,19 +223,21 @@ async function build() {
     };
 
     // Emitted bundles that contain constants.js, accumulated across every
-    // esbuild run below and written out for script/verify-build.
+    // esbuild run below and recorded in the receipt for script/verify-build.
     const auditedBundles = [];
 
     // compile tailwind CSS
     console.log("Compiling Tailwind CSS...");
     const tailwindInput = path.join(SRC, "popup", "styles", "main.css");
     const tailwindOutput = path.join(DIST, "styles.css");
+
+    // Start from an empty dist/, so what is there afterwards is what this
+    // build put there and nothing else. Leftovers from an earlier build are
+    // not covered by this build's receipt, and script/verify-build rejects
+    // any file it did not emit rather than ignoring it.
+    fs.rmSync(DIST, { recursive: true, force: true });
     ensureDir(DIST);
 
-    // Drop any manifest from a previous build before emitting anything, so a
-    // build that never gets around to writing one cannot be verified against
-    // a stale list.
-    fs.rmSync(BUNDLE_MANIFEST, { force: true });
     // The locally installed binary, not `npx` — npx silently fetches from the
     // registry when the binary is absent, which is an unpinned network fetch
     // in the middle of a build.
@@ -134,6 +251,7 @@ async function build() {
         `"${tailwindBin}" -i "${tailwindInput}" -o "${tailwindOutput}" --minify`,
         { stdio: "inherit" },
     );
+    recordEmitted(tailwindOutput);
 
     // Every bundle goes through here, so metafile collection cannot be
     // forgotten when a new entry point is added.
@@ -149,6 +267,7 @@ async function build() {
             metafile: true,
             define,
         });
+        recordEmitted(outfile);
         auditedBundles.push(...outputsContainingAuditedModule(result.metafile));
     }
 
@@ -182,39 +301,39 @@ async function build() {
         );
 
         // copy popup HTML
-        fs.copyFileSync(
+        copyEmitted(
             path.join(SRC, "popup", "index.html"),
             path.join(distDir, "src", "popup", "index.html"),
         );
 
         // place compiled CSS next to popup HTML
-        fs.copyFileSync(
+        copyEmitted(
             tailwindOutput,
             path.join(distDir, "src", "popup", "styles.css"),
         );
     }
 
     // copy manifests
-    fs.copyFileSync(
+    copyEmitted(
         path.join(__dirname, "manifest", "chrome.json"),
         path.join(DIST_CHROME, "manifest.json"),
     );
-    fs.copyFileSync(
+    copyEmitted(
         path.join(__dirname, "manifest", "firefox.json"),
         path.join(DIST_FIREFOX, "manifest.json"),
     );
 
-    // Written last so a build that died partway through leaves no manifest
-    // at all, which script/verify-build treats as a hard failure rather than
-    // as "nothing to check".
-    const manifest = [...new Set(auditedBundles)].sort();
-    fs.writeFileSync(BUNDLE_MANIFEST, manifest.map((p) => `${p}\n`).join(""));
-    console.log(
-        `Bundles containing ${AUDITED_MODULE}: ${manifest.length} ` +
-            `(listed in ${repoRelative(BUNDLE_MANIFEST)})`,
-    );
+    // Written last so a build that died partway through leaves no receipt at
+    // all, which script/verify-build treats as a hard failure rather than as
+    // "nothing to check".
+    if (receiptPath) {
+        writeReceipt(receiptPath, auditedBundles);
+    }
 
     console.log("Build complete: dist/chrome/ and dist/firefox/");
 }
 
-build();
+build().catch((err) => {
+    console.error(`Build failed: ${err && err.message ? err.message : err}`);
+    process.exit(1);
+});