Add getter and setter for qt_qhash_seed

In some cases it's not possible to use QT_HASH_SEED, specially when
we need to set the environment variable from inside the application,
as dynamically loaded libraries or plugins may create static QHash
instances. That would set qt_qhash_seed to a value different from
-1 and skip the env var value.

For those cases, and when we still want to set qt_qhash_seed, we
provide a way to enforce its value.

Auto-tests accessing qt_qhash_seed directly have been updated
accordingly. Usage in qdoc, uic and rcc has been left as is
for the time being.

Change-Id: I3b35b4fa0223c83b1348a6508641905a2a63266f
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/corelib/tools/qhash.cpp

@@ -290,6 +290,53 @@ static void qt_initialize_qhash_seed()
     }
 }
 
+/*! \relates QHash
+    \since 5.6
+
+    Returns the current global QHash seed.
+
+    The seed is set in any newly created QHash. See \l{qHash} about how this seed
+    is being used by QHash.
+
+    \sa qSetGlobalQHashSeed
+ */
+int qGlobalQHashSeed()
+{
+    return qt_qhash_seed.load();
+}
+
+/*! \relates QHash
+    \since 5.6
+
+    Sets the global QHash seed.
+
+    Manually setting the global QHash seed value should be done only for testing
+    and debugging purposes, when deterministic and reproducible behavior on a QHash
+    is needed. We discourage to do it in production code as it can make your
+    application susceptible to \l{algorithmic complexity attacks}.
+
+    The seed is set in any newly created QHash. See \l{qHash} about how this seed
+    is being used by QHash.
+
+    If the environment variable \c QT_HASH_SEED is set, calling this function will
+    result in a no-op.
+
+    Passing the value -1 will reinitialize the global QHash seed to a random value.
+
+    \sa qGlobalQHashSeed
+ */
+void qSetGlobalQHashSeed(int newSeed)
+{
+    if (qEnvironmentVariableIsSet("QT_HASH_SEED"))
+        return;
+    if (newSeed == -1) {
+        int x(qt_create_qhash_seed() & INT_MAX);
+        qt_qhash_seed.store(x);
+    } else {
+        qt_qhash_seed.store(newSeed & INT_MAX);
+    }
+}
+
 /*!
     \internal
 
@@ -1132,7 +1179,8 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     where you temporarily need deterministic behavior, for example for debugging or
     regression testing. To disable the randomization, define the environment
     variable \c QT_HASH_SEED. The contents of that variable, interpreted as a
-    decimal value, will be used as the seed for qHash().
+    decimal value, will be used as the seed for qHash(). Alternatively, you can
+    call the qSetGlobalQHashSeed() function.
 
     \sa QHashIterator, QMutableHashIterator, QMap, QSet
 */

src/corelib/tools/qhash.h

@@ -1037,6 +1037,9 @@ Q_INLINE_TEMPLATE int QMultiHash<Key, T>::count(const Key &key, const T &value)
     return n;
 }
 
+Q_CORE_EXPORT int qGlobalQHashSeed();
+Q_CORE_EXPORT void qSetGlobalQHashSeed(int newSeed);
+
 Q_DECLARE_ASSOCIATIVE_ITERATOR(Hash)
 Q_DECLARE_MUTABLE_ASSOCIATIVE_ITERATOR(Hash)
 

tests/auto/corelib/tools/qhashfunctions/tst_qhashfunctions.cpp

@@ -50,6 +50,8 @@ private Q_SLOTS:
     void qthash();
     void range();
     void rangeCommutative();
+
+    void setGlobalQHashSeed();
 };
 
 void tst_QHashFunctions::qhash()
@@ -207,5 +209,21 @@ void tst_QHashFunctions::rangeCommutative()
     (void)qHashRangeCommutative(hashables, hashables + numHashables);
 }
 
+void tst_QHashFunctions::setGlobalQHashSeed()
+{
+    // Setter works as advertised
+    qSetGlobalQHashSeed(0x10101010);
+    QCOMPARE(qGlobalQHashSeed(), 0x10101010);
+
+    // Creating a new QHash doesn't reset the seed
+    QHash<QString, int> someHash;
+    someHash.insert("foo", 42);
+    QCOMPARE(qGlobalQHashSeed(), 0x10101010);
+
+    // Reset works as advertised
+    qSetGlobalQHashSeed(-1);
+    QVERIFY(qGlobalQHashSeed() != -1);
+}
+
 QTEST_APPLESS_MAIN(tst_QHashFunctions)
 #include "tst_qhashfunctions.moc"

tests/auto/corelib/tools/qset/tst_qset.cpp

@@ -970,24 +970,6 @@ void tst_QSet::initializerList()
 #endif
 }
 
-QT_BEGIN_NAMESPACE
-extern Q_CORE_EXPORT QBasicAtomicInt qt_qhash_seed; // from qhash.cpp
-QT_END_NAMESPACE
-
-class QtQHashSeedSaver {
-    int oldSeed, newSeed;
-public:
-    explicit QtQHashSeedSaver(int seed)
-        : oldSeed(qt_qhash_seed.fetchAndStoreRelaxed(seed)),
-          newSeed(seed)
-    {}
-    ~QtQHashSeedSaver()
-    {
-        // only restore when no-one else changed the seed in the meantime:
-        qt_qhash_seed.testAndSetRelaxed(newSeed, oldSeed);
-    }
-};
-
 void tst_QSet::qhash()
 {
     //
@@ -995,14 +977,14 @@ void tst_QSet::qhash()
     //
     {
         // create some deterministic initial state:
-        const QtQHashSeedSaver seed1(0);
+        qSetGlobalQHashSeed(0);
 
         QSet<int> s1;
         s1.reserve(4);
         s1 << 400 << 300 << 200 << 100;
 
         // also change the seed:
-        const QtQHashSeedSaver seed2(0x10101010);
+        qSetGlobalQHashSeed(0x10101010);
 
         QSet<int> s2;
         s2.reserve(100); // provoke different bucket counts
@@ -1049,7 +1031,7 @@ void tst_QSet::intersects()
     s1 << 200;
     QVERIFY(s1.intersects(s2));
 
-    const QtQHashSeedSaver seedSaver(0x10101010);
+    qSetGlobalQHashSeed(0x10101010);
     QSet<int> s3;
     s3 << 500;
     QVERIFY(!s1.intersects(s3));

tests/auto/dbus/qdbusxmlparser/tst_qdbusxmlparser.cpp

@@ -64,15 +64,10 @@ private slots:
     void properties();
 };
 
-QT_BEGIN_NAMESPACE
-// Avoid QHash randomization so that the order of the XML attributes is stable
-extern Q_CORE_EXPORT QBasicAtomicInt qt_qhash_seed; // from qhash.cpp
-QT_END_NAMESPACE
-
 void tst_QDBusXmlParser::initTestCase()
 {
     // Always initialize the hash seed to 0 to get reliable test results
-    qt_qhash_seed.store(0);
+    qSetGlobalQHashSeed(0);
 }
 
 void tst_QDBusXmlParser::parsing_data()

tests/auto/other/lancelot/tst_lancelot.cpp

@@ -259,13 +259,9 @@ void tst_Lancelot::paint(QPaintDevice *device, GraphicsEngine engine, const QStr
 QTEST_MAIN(tst_Lancelot)
 #undef main
 
-QT_BEGIN_NAMESPACE
-extern Q_DECL_IMPORT QBasicAtomicInt qt_qhash_seed; // from qhash.cpp
-QT_END_NAMESPACE
-
 int main(int argc, char *argv[])
 {
-    qt_qhash_seed.store(0);   // Avoid rendering variations caused by QHash randomization
+    qSetGlobalQHashSeed(0);   // Avoid rendering variations caused by QHash randomization
 
     QBaselineTest::handleCmdLineArgs(&argc, &argv);
     return _realmain(argc, argv);