feat(server): compress the cached OpenSky snapshot for clients that accept gzip

The ~860 KB snapshot is held for an adaptive 9-300 s TTL and was served as
identity bytes to every request; no response in server/ set Content-Encoding or
Vary. Gzip it once per refresh beside the cached body, off the event loop, and
return the body and the header that names it together so the two can never
disagree. Live against an anonymous snapshot: 874,002 B to 258,012 B, 3.39x,
decompressing byte-identical to the uncompressed response.

src/tooling/responseCompression.test.mjs

@@ -0,0 +1,129 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { gunzipSync } from 'node:zlib';
+import {
+  acceptsGzip,
+  encodedBody,
+  gzipBody,
+} from '../../server/providers/common/compress.js';
+
+/** A request shaped like the one Node hands a middleware. */
+function request(acceptEncoding) {
+  return {
+    url: '/api/opensky',
+    method: 'GET',
+    headers:
+      acceptEncoding === undefined ? {} : { 'accept-encoding': acceptEncoding },
+  };
+}
+
+/** Repetitive JSON, the shape of an actual snapshot body. */
+function snapshotBody(states = 200) {
+  return JSON.stringify({
+    time: 1700000000,
+    states: Array.from({ length: states }, (_, index) => [
+      `abc${index}`,
+      'CALLSIGN',
+      'United States',
+      1700000000,
+      30.1,
+      -97.7,
+      10000,
+    ]),
+  });
+}
+
+/**
+ * Deterministic high-entropy bytes. Random data would do, but a seeded sequence
+ * keeps the "gzip made it bigger" assertion from depending on luck.
+ */
+function incompressibleBytes(length) {
+  const bytes = Buffer.alloc(length);
+  let state = 123456789;
+  for (let index = 0; index < length; index += 1) {
+    state = (state * 1103515245 + 12345) % 2147483648;
+    bytes[index] = (state >>> 16) & 0xff;
+  }
+  return bytes;
+}
+
+test('accept-encoding is read the way clients actually write it', () => {
+  assert.equal(acceptsGzip(request('gzip')), true);
+  assert.equal(acceptsGzip(request('gzip, deflate, br')), true);
+  assert.equal(acceptsGzip(request('br;q=1.0, gzip;q=0.8')), true);
+  assert.equal(acceptsGzip(request('GZIP')), true);
+  assert.equal(acceptsGzip(request(' gzip ')), true);
+
+  assert.equal(acceptsGzip(request('br, deflate')), false);
+  assert.equal(acceptsGzip(request('identity')), false);
+  assert.equal(acceptsGzip(request('')), false);
+  assert.equal(acceptsGzip(request(undefined)), false);
+  assert.equal(acceptsGzip({}), false);
+  assert.equal(acceptsGzip(undefined), false);
+
+  // A quality of zero names gzip and refuses it (RFC 9110 section 12.4.2),
+  // and an unreadable quality falls back to the encoding every client accepts.
+  assert.equal(acceptsGzip(request('gzip;q=0')), false);
+  assert.equal(acceptsGzip(request('gzip;q=0.000')), false);
+  assert.equal(acceptsGzip(request('gzip;q=bogus')), false);
+  assert.equal(acceptsGzip(request('gzip;q=0.001')), true);
+
+  // `gzip` is not matched as a substring of a neighbouring token.
+  assert.equal(acceptsGzip(request('x-gzip')), false);
+  assert.equal(acceptsGzip(request('gzipped')), false);
+});
+
+test('a body is only compressed when compression pays for itself', async () => {
+  assert.equal(await gzipBody(null), null);
+  assert.equal(await gzipBody(undefined), null);
+  assert.equal(await gzipBody(''), null);
+  assert.equal(
+    await gzipBody('{"states":[]}'),
+    null,
+    'below the frame threshold',
+  );
+  assert.equal(await gzipBody(Buffer.alloc(1023, 'a')), null, 'one byte short');
+
+  const identity = snapshotBody();
+  assert.ok(identity.length > 1024, 'fixture is worth compressing');
+  const compressed = await gzipBody(identity);
+  assert.ok(Buffer.isBuffer(compressed));
+  assert.ok(compressed.byteLength < Buffer.byteLength(identity));
+  assert.equal(gunzipSync(compressed).toString(), identity);
+
+  // Buffers are accepted as-is, and data that gzip cannot shrink is refused
+  // rather than served as a larger response.
+  assert.ok((await gzipBody(Buffer.from(identity))) !== null);
+  assert.equal(await gzipBody(incompressibleBytes(4096)), null);
+});
+
+test('the encoded body and the header that names it are chosen together', async () => {
+  const identity = snapshotBody();
+  const compressed = await gzipBody(identity);
+
+  const negotiated = encodedBody(request('gzip'), identity, compressed);
+  assert.equal(negotiated.body, compressed);
+  assert.equal(negotiated.headers['Content-Encoding'], 'gzip');
+  assert.equal(negotiated.headers.Vary, 'Accept-Encoding');
+  assert.equal(gunzipSync(negotiated.body).toString(), identity);
+
+  // Offered but not wanted: identity bytes, and no Content-Encoding to
+  // contradict them.
+  const refused = encodedBody(request('br'), identity, compressed);
+  assert.equal(refused.body, identity);
+  assert.equal(refused.headers['Content-Encoding'], undefined);
+  assert.equal(refused.headers.Vary, 'Accept-Encoding');
+
+  // Wanted but not available: same answer.
+  const unavailable = encodedBody(request('gzip'), identity, null);
+  assert.equal(unavailable.body, identity);
+  assert.equal(unavailable.headers['Content-Encoding'], undefined);
+
+  // Vary is on both branches: without it a shared cache could hand a stored
+  // response to a client whose Accept-Encoding it was not selected for.
+  assert.equal(unavailable.headers.Vary, 'Accept-Encoding');
+  assert.equal(
+    encodedBody(request(undefined), identity, compressed).headers.Vary,
+    'Accept-Encoding',
+  );
+});