Fix wildcard filter matching

Each sub item in the filter tree needs to be directly compared. Using
startsWith causes invalid elements to be marked as matching.

Fixes: QTBUG-92817
Pick-to: 6.4 6.2 5.15
Change-Id: I439d9325b58fd0aa57fb4ead8db48edb4188ebee
Reviewed-by: Karsten Heimrich <karsten.heimrich@qt.io>

src/mqtt/qmqtttopicfilter.cpp

@@ -209,9 +209,24 @@ bool QMqttTopicFilter::match(const QMqttTopicName &name, MatchOptions matchOptio
 
     if (d->filter.endsWith(QLatin1Char('#'))) {
         QStringView root = QStringView{d->filter}.left(d->filter.size() - 1);
+
+        if (root.isEmpty()) // Filter: #
+            return true;
+
         if (root.endsWith(QLatin1Char('/'))) // '#' also represents the parent level!
             root = root.left(root.size() - 1);
-        return topic.startsWith(root);
+
+        const auto filterLevels = root.split(QLatin1Char('/'));
+        const auto topicLevels = QStringView{topic}.split(QLatin1Char('/'));
+
+        if (topicLevels.size() < filterLevels.size())
+            return false;
+
+        for (int i = 0; i < filterLevels.size(); ++i) {
+            if (filterLevels.at(i) != topicLevels.at(i))
+                return false;
+        }
+        return true;
     }
 
     if (d->filter.contains(QLatin1Char('+'))) {

tests/auto/qmqtttopicfilter/tst_qmqtttopicfilter.cpp

@@ -95,6 +95,9 @@ void Tst_QMqttTopicFilter::matches()
 
     QVERIFY(QMqttTopicFilter("$SYS/monitor/+").match(QMqttTopicName("$SYS/monitor/Clients")));
     QVERIFY(QMqttTopicFilter("$SYS/monitor/+").match(QMqttTopicName("$SYS/monitor/Clients"), QMqttTopicFilter::WildcardsDontMatchDollarTopicMatchOption));
+
+    QVERIFY(!QMqttTopicFilter{QLatin1String("x/#")}.match(QLatin1String("xy/foo"))); // QTBUG-92817
+    QVERIFY(!QMqttTopicFilter{QLatin1String("x/y/#")}.match(QLatin1String("x")));
 }
 
 void Tst_QMqttTopicFilter::usableWithQList()