test: verify hash seed uniqueness

This tests that the hash seed used by V8 for hashing is random.

PR-URL: nodejs-private/node-private#84
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

Makefile

@@ -237,6 +237,9 @@ test-internet: all
 test-debugger: all
 	$(PYTHON) tools/test.py debugger
 
+test-hash-seed: all
+	$(NODE) test/pummel/test-hash-seed.js
+
 test-known-issues: all
 	$(PYTHON) tools/test.py known_issues --expect-fail
 
@@ -256,7 +259,7 @@ test-timers:
 test-timers-clean:
 	$(MAKE) --directory=tools clean
 
-test-v8: v8
+test-v8: v8 test-hash-seed
 #	note: performs full test unless QUICKCHECK is specified
 	deps/v8/tools/run-tests.py --arch=$(V8_ARCH) \
         --mode=$(BUILDTYPE_LOWER) $(V8_TEST_NO_I18N) $(QUICKCHECK_ARG) \
@@ -734,4 +737,5 @@ lint-ci: lint
 	bench-all bench bench-misc bench-array bench-buffer bench-net \
 	bench-http bench-fs bench-tls cctest run-ci lint-ci bench-ci \
 	test-v8 test-v8-intl test-v8-benchmarks test-v8-all v8 \
-	$(TARBALL)-headers test-ci test-ci-native test-ci-js build-ci doc-only
+	$(TARBALL)-headers test-ci test-ci-native test-ci-js build-ci doc-only \
+	test-hash-seed

test/fixtures/guess-hash-seed.js

@@ -0,0 +1,175 @@
+/* eslint-disable required-modules */
+'use strict';
+function min(arr) {
+  let res = arr[0];
+  for (let i = 1; i < arr.length; i++) {
+    const val = arr[i];
+    if (val < res)
+      res = val;
+  }
+  return res;
+}
+function run_repeated(n, fn) {
+  const res = [];
+  for (let i = 0; i < n; i++) res.push(fn());
+  return res;
+}
+
+const INT_MAX = 0x7fffffff;
+
+// from src/js/collection.js
+// key must be a signed 32-bit number!
+function ComputeIntegerHash(key/*, seed*/) {
+  let hash = key;
+  hash = hash ^ 0/*seed*/;
+  hash = ~hash + (hash << 15);  // hash = (hash << 15) - hash - 1;
+  hash = hash ^ (hash >>> 12);
+  hash = hash + (hash << 2);
+  hash = hash ^ (hash >>> 4);
+  hash = (hash * 2057) | 0;  // hash = (hash + (hash << 3)) + (hash << 11);
+  hash = hash ^ (hash >>> 16);
+  return hash & 0x3fffffff;
+}
+
+const kNofHashBitFields = 2;
+const kHashShift = kNofHashBitFields;
+const kHashBitMask = 0xffffffff >>> kHashShift;
+const kZeroHash = 27;
+
+function string_to_array(str) {
+  const res = new Array(str.length);
+  for (let i = 0; i < str.length; i++) {
+    res[i] = str.charCodeAt(i);
+  }
+  return res;
+}
+
+function gen_specialized_hasher(str) {
+  const str_arr = string_to_array(str);
+  return Function('seed', `
+    var running_hash = seed;
+    ${str_arr.map((c) => `
+      running_hash += ${c};
+      running_hash &= 0xffffffff;
+      running_hash += (running_hash << 10);
+      running_hash &= 0xffffffff;
+      running_hash ^= (running_hash >>> 6);
+      running_hash &= 0xffffffff;
+    `).join('')}
+    running_hash += (running_hash << 3);
+    running_hash &= 0xffffffff;
+    running_hash ^= (running_hash >>> 11);
+    running_hash &= 0xffffffff;
+    running_hash += (running_hash << 15);
+    running_hash &= 0xffffffff;
+    if ((running_hash & ${kHashBitMask}) == 0) {
+      return ${kZeroHash};
+    }
+    return running_hash;
+  `);
+}
+
+// adapted from HashToEntry
+function hash_to_bucket(hash, numBuckets) {
+  return (hash & ((numBuckets) - 1));
+}
+
+function time_set_lookup(set, value) {
+  const t1 = process.hrtime();
+  for (let i = 0; i < 100; i++) {
+    // annoyingly, SetHas() is JS code and therefore potentially optimizable.
+    // However, SetHas() looks up the table using native code, and it seems like
+    // that's sufficient to prevent the optimizer from doing anything?
+    set.has(value);
+  }
+  const t = process.hrtime(t1);
+  const secs = t[0];
+  const nanos = t[1];
+  return secs * 1e9 + nanos;
+}
+
+// Set with 256 buckets; bucket 0 full, others empty
+const tester_set_buckets = 256;
+const tester_set = new Set();
+let tester_set_treshold;
+(function() {
+  // fill bucket 0 and find extra numbers mapping to bucket 0 and a different
+  // bucket `capacity == numBuckets * 2`
+  let needed = Math.floor(tester_set_buckets * 1.5) + 1;
+  let positive_test_value;
+  let negative_test_value;
+  for (let i = 0; true; i++) {
+    if (i > INT_MAX) throw new Error('i too high');
+    if (hash_to_bucket(ComputeIntegerHash(i), tester_set_buckets) !== 0) {
+      negative_test_value = i;
+      break;
+    }
+  }
+  for (let i = 0; needed > 0; i++) {
+    if (i > INT_MAX) throw new Error('i too high');
+    if (hash_to_bucket(ComputeIntegerHash(i), tester_set_buckets) === 0) {
+      needed--;
+      if (needed == 0) {
+        positive_test_value = i;
+      } else {
+        tester_set.add(i);
+      }
+    }
+  }
+
+  // calibrate Set access times for accessing the full bucket / an empty bucket
+  const pos_time =
+    min(run_repeated(10000, time_set_lookup.bind(null, tester_set,
+                                                 positive_test_value)));
+  const neg_time =
+    min(run_repeated(10000, time_set_lookup.bind(null, tester_set,
+                                                 negative_test_value)));
+  tester_set_treshold = (pos_time + neg_time) / 2;
+  // console.log(`pos_time: ${pos_time}, neg_time: ${neg_time},`,
+  //             `threshold: ${tester_set_treshold}`);
+})();
+
+// determine hash seed
+const slow_str_gen = (function*() {
+  let strgen_i = 0;
+  outer:
+  while (1) {
+    const str = '#' + (strgen_i++);
+    for (let i = 0; i < 1000; i++) {
+      if (time_set_lookup(tester_set, str) < tester_set_treshold)
+        continue outer;
+    }
+    yield str;
+  }
+})();
+
+const first_slow_str = slow_str_gen.next().value;
+// console.log('first slow string:', first_slow_str);
+const first_slow_str_special_hasher = gen_specialized_hasher(first_slow_str);
+let seed_candidates = [];
+//var t_before_first_seed_brute = performance.now();
+for (let seed_candidate = 0; seed_candidate < 0x100000000; seed_candidate++) {
+  if (hash_to_bucket(first_slow_str_special_hasher(seed_candidate),
+                     tester_set_buckets) == 0) {
+    seed_candidates.push(seed_candidate);
+  }
+}
+// console.log(`got ${seed_candidates.length} candidates`);
+//  after ${performance.now()-t_before_first_seed_brute}
+while (seed_candidates.length > 1) {
+  const slow_str = slow_str_gen.next().value;
+  const special_hasher = gen_specialized_hasher(slow_str);
+  const new_seed_candidates = [];
+  for (const seed_candidate of seed_candidates) {
+    if (hash_to_bucket(special_hasher(seed_candidate), tester_set_buckets) ==
+        0) {
+      new_seed_candidates.push(seed_candidate);
+    }
+  }
+  seed_candidates = new_seed_candidates;
+  // console.log(`reduced to ${seed_candidates.length} candidates`);
+}
+if (seed_candidates.length != 1)
+  throw new Error('no candidates remaining');
+const seed = seed_candidates[0];
+console.log(seed);

test/pummel/test-hash-seed.js

@@ -0,0 +1,20 @@
+'use strict';
+
+const REPETITIONS = 2;
+
+const assert = require('assert');
+const common = require('../common');
+const cp = require('child_process');
+const path = require('path');
+const targetScript = path.resolve(common.fixturesDir, 'guess-hash-seed.js');
+const seeds = [];
+
+for (let i = 0; i < REPETITIONS; ++i) {
+  const seed = cp.spawnSync(process.execPath, [targetScript],
+    { encoding: 'utf8' }).stdout.trim();
+  seeds.push(seed);
+}
+
+console.log(`Seeds: ${seeds}`);
+const hasDuplicates = (new Set(seeds)).size !== seeds.length;
+assert.strictEqual(hasDuplicates, false);