Q[Multi]Hash::reserve(): do nothing if desired size is less than current

Calling Q[Multi]Hash::reserve(n) when n is much smaller than the
current amount of elements in the hash, could result in an infinite
loop, because at some point the algorithm could not find a free bucket
for the element.

Fixing it by returning early if the new desired capacity is less than
current.

Fixes: QTBUG-102067
Change-Id: I38ef0b2168c4e2a317eedf91b2155b1fdffb1c27
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit d11941db41f00525f907417bbcd6ac5ee30d8485)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 37 insertions, 0 deletions

diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 3190a2e34e..9b46fc5c0f 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -857,6 +857,9 @@ public:
     inline qsizetype capacity() const noexcept { return d ? qsizetype(d->numBuckets >> 1) : 0; }
     void reserve(qsizetype size)
     {
+        // reserve(0) is used in squeeze()
+        if (size && (this->capacity() >= size))
+            return;
         if (isDetached())
             d->rehash(size);
         else
@@ -1374,6 +1377,9 @@ public:
     inline qsizetype capacity() const noexcept { return d ? qsizetype(d->numBuckets >> 1) : 0; }
     void reserve(qsizetype size)
     {
+        // reserve(0) is used in squeeze()
+        if (size && (this->capacity() >= size))
+            return;
         if (isDetached())
             d->rehash(size);
         else
diff --git a/tests/auto/corelib/tools/qhash/tst_qhash.cpp b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
index 50c2b0d84e..9982c08392 100644
--- a/tests/auto/corelib/tools/qhash/tst_qhash.cpp
+++ b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
@@ -97,6 +97,7 @@ private slots:
     void fineTuningInEmptyHash();
 
     void reserveShared();
+    void reserveLessThanCurrentAmount();
 
     void QTBUG98265();
 
@@ -2640,6 +2641,36 @@ void tst_QHash::reserveShared()
     QCOMPARE(hash.capacity(), oldCap);
 }
 
+void tst_QHash::reserveLessThanCurrentAmount()
+{
+    {
+        QHash<int, int> hash;
+        for (int i = 0; i < 1000; ++i)
+            hash.insert(i, i * 10);
+
+        // This used to hang in an infinite loop: QTBUG-102067
+        hash.reserve(1);
+
+        // Make sure that hash still has all elements
+        for (int i = 0; i < 1000; ++i)
+            QCOMPARE(hash.value(i), i * 10);
+    }
+    {
+        QMultiHash<int, int> hash;
+        for (int i = 0; i < 1000; ++i) {
+            hash.insert(i, i * 10);
+            hash.insert(i, i * 10 + 1);
+        }
+
+        // This used to hang in infinite loop: QTBUG-102067
+        hash.reserve(1);
+
+        // Make sure that hash still has all elements
+        for (int i = 0; i < 1000; ++i)
+            QCOMPARE(hash.values(i), QList<int>() << i * 10 + 1 << i * 10);
+    }
+}
+
 void tst_QHash::QTBUG98265()
 {
     QMultiHash<QUuid, QByteArray> a;