fix: address coordinate review findings

Issue: #224
User-Visible: no

test/serialized-write-queue.test.mjs

@@ -0,0 +1,53 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { enqueueSerializedWrite } from '../test-build/serialized-write-queue.js';
+
+function deferred() {
+  let resolve;
+  const promise = new Promise((done) => { resolve = done; });
+  return { promise, resolve };
+}
+
+test('a queued config write reads edits made while the previous write is in flight (#224)', async () => {
+  const firstStarted = deferred();
+  const releaseFirst = deferred();
+  let localConfig = 'first';
+  const observed = [];
+  let chain = Promise.resolve();
+
+  const enqueue = () => {
+    chain = enqueueSerializedWrite(chain, async () => {
+      observed.push(localConfig);
+      if (observed.length === 1) {
+        firstStarted.resolve();
+        await releaseFirst.promise;
+      }
+    });
+    return chain;
+  };
+
+  const first = enqueue();
+  await firstStarted.promise;
+  localConfig = 'second';
+  const second = enqueue();
+  localConfig = 'latest edit during await';
+
+  assert.deepEqual(observed, ['first'], 'the second write waits its turn');
+  releaseFirst.resolve();
+  await Promise.all([first, second]);
+  assert.deepEqual(observed, ['first', 'latest edit during await']);
+});
+
+test('a failed write does not poison the next queued edit (#224)', async () => {
+  const seen = [];
+  const failed = enqueueSerializedWrite(Promise.resolve(), async () => {
+    seen.push('failed');
+    throw new Error('offline');
+  });
+  const recovered = enqueueSerializedWrite(failed, async () => { seen.push('recovered'); });
+
+  await assert.rejects(failed, /offline/);
+  await recovered;
+  assert.deepEqual(seen, ['failed', 'recovered']);
+});