fix: make dev rebuilds dist/ on source changes (closes #332)

make dev passed --watch to a build.js that read no arguments, so it
built once and exited. build.js --watch now builds, then builds again
after every change to a file under src/, manifest/ or icons/, until
interrupted; any other argument fails. Each directory gets its own
watcher, because Node's recursive watch on Linux loses a file that an
editor saves by renaming a new copy over it. A watch build writes no
build receipt and cannot be verified; README.md and the Makefile say
so and point to make build.

Model: opus-5-5

build.js

@@ -15,6 +15,15 @@ const DIST_CHROME = path.join(DIST, "chrome");
 const DIST_FIREFOX = path.join(DIST, "firefox");
 const SRC = path.join(__dirname, "src");
 
+// What `make dev` watches: the directories whose files build() bundles,
+// compiles or copies. A change anywhere else, package.json and build.js
+// included, starts no build; restart make dev after one.
+const WATCHED_DIRS = [
+    SRC,
+    path.join(__dirname, "manifest"),
+    path.join(__dirname, "icons"),
+];
+
 // 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
@@ -441,6 +450,10 @@ function getBuildInfo() {
 async function build() {
     console.log("Building AutistMask extension...");
 
+    // Under make dev this runs once per rebuild in the same process, and each
+    // build accounts only for what it emits itself.
+    emittedFiles.length = 0;
+
     const receiptPath = receiptTarget();
     if (!receiptPath) {
         console.warn(
@@ -597,27 +610,94 @@ async function build() {
     console.log("Build complete: dist/chrome/ and dist/firefox/");
 }
 
-// Run only as a program. Required as a module — which is how
-// tests/buildForbiddenInputs.test.js reaches the checks below — this file
-// builds nothing and writes nothing.
-if (require.main === module) {
-    build().catch((err) => {
-        console.error(
-            `Build failed: ${err && err.message ? err.message : err}`,
-        );
-        process.exit(1);
-    });
+// make dev: build, then build again after every change under `dirs`, until
+// interrupted. A failed build is reported and watching carries on, so a
+// half-finished edit does not end it. A change that arrives while a build is
+// running is not lost: it causes one more build as soon as that one finishes.
+// A directory created after this starts is not watched until a restart.
+//
+// make dev sets no AUTISTMASK_BUILD_RECEIPT, so these builds write no receipt
+// and nothing can verify what they leave in dist/.
+//
+// `rebuild` is build() everywhere but tests/buildWatch.test.js, which also
+// closes the returned watchers.
+function watch(dirs, rebuild) {
+    let building = false;
+    let changedAgain = false;
+
+    async function run() {
+        if (building) {
+            changedAgain = true;
+            return;
+        }
+        building = true;
+        do {
+            // Let the rest of one save's events arrive first, so that one
+            // save is one build.
+            await new Promise((resolve) => setTimeout(resolve, 100));
+            changedAgain = false;
+            try {
+                await rebuild();
+            } catch (err) {
+                console.error(
+                    `Build failed: ${err && err.message ? err.message : err}`,
+                );
+            }
+        } while (changedAgain);
+        building = false;
+        console.log("Watching for changes (Ctrl-C to stop)...");
+    }
+
+    // One watcher per directory rather than fs.watch's recursive option: on
+    // Linux, Node's recursive watch watches each file, and stops seeing one
+    // that an editor saves by renaming a new copy over it. A directory's
+    // watcher reports every change to the files in it, however they are saved.
+    const subdirectories = dirs.flatMap((dir) =>
+        fs
+            .readdirSync(dir, { recursive: true, withFileTypes: true })
+            .filter((entry) => entry.isDirectory())
+            .map((entry) => path.join(entry.parentPath, entry.name)),
+    );
+    const watchers = [...dirs, ...subdirectories].map((dir) =>
+        fs.watch(dir, run),
+    );
+    run();
+    return watchers;
 }
 
-// Exported for tests/buildForbiddenInputs.test.js only. The prohibition these
-// three functions enforce is the guarantee behind
-// https://git.eeqj.de/sneak/AutistMask/issues/324, and `make check` does not
-// run `make build` — so they are unit tested against synthetic metafiles
-// rather than being exercised only by CI, where "it ran" is not "it works".
+// Run only as a program. Required as a module — which is how
+// tests/buildForbiddenInputs.test.js and tests/buildWatch.test.js reach the
+// functions below — this file builds nothing and writes nothing.
+if (require.main === module) {
+    const args = process.argv.slice(2);
+    // An argument this file does not know fails rather than being ignored.
+    if (args.length > 1 || (args.length === 1 && args[0] !== "--watch")) {
+        console.error("usage: node build.js [--watch]");
+        process.exit(2);
+    }
+    if (args[0] === "--watch") {
+        watch(WATCHED_DIRS, build);
+    } else {
+        build().catch((err) => {
+            console.error(
+                `Build failed: ${err && err.message ? err.message : err}`,
+            );
+            process.exit(1);
+        });
+    }
+}
+
+// Exported for tests only: watch() for tests/buildWatch.test.js, the rest for
+// tests/buildForbiddenInputs.test.js. The prohibition those enforce is the
+// guarantee behind https://git.eeqj.de/sneak/AutistMask/issues/324, and
+// `make check` does not run `make build` — so they are unit tested against
+// synthetic metafiles rather than being exercised only by CI, where "it ran"
+// is not "it works".
 module.exports = {
     importChain,
     newForbiddenRecord,
     recordBundledInputs,
     assertNoForbiddenInputs,
     assertForbiddenTableCovered,
+    watch,
 };