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>

2 files changed, 52 insertions, 1 deletions

diff --git a/src/corelib/tools/qhash.cpp b/src/corelib/tools/qhash.cpp
index 1f3ea36121..b334a697a9 100644
--- a/src/corelib/tools/qhash.cpp
+++ b/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
 */
diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index e367cc0068..8d65a018ae 100644
--- a/src/corelib/tools/qhash.h
+++ b/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)