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

tests/buildWatch.test.js

@@ -0,0 +1,104 @@
+// `make dev` runs `node build.js --watch`
+// (https://git.eeqj.de/sneak/AutistMask/issues/332). These drive build.js's
+// watch() over a temp directory with a stand-in for build(), so nothing here
+// builds or writes dist/.
+
+const fs = require("fs");
+const os = require("os");
+const path = require("path");
+
+const { watch } = require("../build");
+
+let dir;
+let watchers = [];
+
+beforeEach(() => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), "autistmask-watch-"));
+    fs.mkdirSync(path.join(dir, "nested"));
+    jest.spyOn(console, "log").mockImplementation(() => {});
+    jest.spyOn(console, "error").mockImplementation(() => {});
+});
+
+afterEach(() => {
+    for (const watcher of watchers) watcher.close();
+    watchers = [];
+    fs.rmSync(dir, { recursive: true, force: true });
+    jest.restoreAllMocks();
+});
+
+const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
+
+// Wait until `condition()` holds; fail the test if it has not within 3s.
+async function until(condition) {
+    const deadline = Date.now() + 3000;
+    while (!condition()) {
+        if (Date.now() > deadline) throw new Error("timed out waiting");
+        await sleep(10);
+    }
+}
+
+// Save the way many editors do: write a new copy, then rename it over the
+// original, so the file at that path is a different one afterwards.
+function saveByRename(file, contents) {
+    fs.writeFileSync(`${file}.tmp`, contents);
+    fs.renameSync(`${file}.tmp`, file);
+}
+
+test("builds at start, then once per change to a file in a subdirectory", async () => {
+    const file = path.join(dir, "nested", "a.js");
+    fs.writeFileSync(file, "1");
+    let builds = 0;
+    watchers = watch([dir], async () => {
+        builds++;
+    });
+    await until(() => builds === 1);
+
+    fs.writeFileSync(file, "2");
+    await until(() => builds === 2);
+    await sleep(300);
+    expect(builds).toBe(2);
+});
+
+test("keeps seeing a file that is saved by renaming a new copy over it", async () => {
+    const file = path.join(dir, "nested", "a.js");
+    fs.writeFileSync(file, "1");
+    let builds = 0;
+    watchers = watch([dir], async () => {
+        builds++;
+    });
+    await until(() => builds === 1);
+
+    saveByRename(file, "2");
+    await until(() => builds === 2);
+    saveByRename(file, "3");
+    await until(() => builds === 3);
+});
+
+test("a failed build is reported and watching carries on", async () => {
+    let builds = 0;
+    watchers = watch([dir], async () => {
+        builds++;
+        if (builds === 1) throw new Error("unexpected token");
+    });
+    await until(() => console.error.mock.calls.length === 1);
+    expect(console.error).toHaveBeenCalledWith(
+        "Build failed: unexpected token",
+    );
+
+    fs.writeFileSync(path.join(dir, "a.js"), "1");
+    await until(() => builds === 2);
+});
+
+test("a change made while a build runs causes one more build after it", async () => {
+    let builds = 0;
+    watchers = watch([dir], async () => {
+        builds++;
+        if (builds === 1) {
+            fs.writeFileSync(path.join(dir, "a.js"), "1");
+            await sleep(200);
+        }
+    });
+    await until(() => builds === 2);
+    await sleep(300);
+    expect(builds).toBe(2);
+});