From 15b43b5ded62ad76776d69bc64ce2679ddfa8eb8 Mon Sep 17 00:00:00 2001
From: Mårten Nordheim <marten.nordheim@qt.io>
Date: Tue, 08 Dec 2020 15:47:47 +0100
Subject: [PATCH] QCache: fix updating entries breaking the internal chain

After f08492c6fd9818c7d80b1725355453e179b4d85b was
merged this bug would manifest as an entry appearing twice
in the chain when a updating an existing entry (insert with
an existing key). This could sometimes result in crashes later
as the list filled up and the list was used in trim() to remove
various entries.

Fixes: QTBUG-89176
Pick-to: 6.0
Change-Id: Ide80160fb4317dc0aefe79eec5dce7ec6813e790
---

diff --git a/src/corelib/tools/qcache.h b/src/corelib/tools/qcache.h
index 74784af..41cf9ab 100644
--- a/src/corelib/tools/qcache.h
+++ b/src/corelib/tools/qcache.h
@@ -247,14 +247,15 @@
         if (result.initialized) {
             cost -= n->value.cost;
             result.it.node()->emplace(object, cost);
+            relink(key);
         } else {
             Node::createInPlace(n, key, object, cost);
+            n->prev = &chain;
+            n->next = chain.next;
+            chain.next->prev = n;
+            chain.next = n;
         }
         total += cost;
-        n->prev = &chain;
-        n->next = chain.next;
-        chain.next->prev = n;
-        chain.next = n;
         return true;
     }
     T *object(const Key &key) const noexcept
diff --git a/src/network/kernel/qhostinfo.cpp b/src/network/kernel/qhostinfo.cpp
index b3d6844..8361470 100644
--- a/src/network/kernel/qhostinfo.cpp
+++ b/src/network/kernel/qhostinfo.cpp
@@ -916,7 +916,7 @@
     QObject::connect(QCoreApplication::instance(), &QObject::destroyed,
                      &threadPool, [&](QObject *) { threadPool.waitForDone(); },
                      Qt::DirectConnection);
-    threadPool.setMaxThreadCount(20); // do up to 20 DNS lookups in parallel
+    threadPool.setMaxThreadCount(100); // do up to 20 DNS lookups in parallel
 #endif
 }
 
diff --git a/tests/auto/corelib/tools/qcache/tst_qcache.cpp b/tests/auto/corelib/tools/qcache/tst_qcache.cpp
index f122e45..2d1ca0a 100644
--- a/tests/auto/corelib/tools/qcache/tst_qcache.cpp
+++ b/tests/auto/corelib/tools/qcache/tst_qcache.cpp
@@ -48,6 +48,7 @@
     void take();
     void axioms_on_key_type();
     void largeCache();
+    void internalChainHasFullOrder();
 };
 
 
@@ -414,5 +415,18 @@
     QVERIFY(cache.size() == 0);
 }
 
+// The internal chain could lose track of some objects.
+// Make sure it doesn't happen again.
+void tst_QCache::internalChainHasFullOrder()
+{
+    QCache<QString, int> cache;
+    cache.setMaxCost(20);
+    cache.insert(QString::number(1), new int, 1);
+    cache.insert(QString::number(2), new int, 1);
+    cache.insert(QString::number(1), new int, 1);
+    cache.setMaxCost(0);
+    QCOMPARE(cache.size(), 0);
+}
+
 QTEST_APPLESS_MAIN(tst_QCache)
 #include "tst_qcache.moc"
diff --git a/tests/auto/network/kernel/qhostinfo/tst_qhostinfo.cpp b/tests/auto/network/kernel/qhostinfo/tst_qhostinfo.cpp
index 5390433..e895e94 100644
--- a/tests/auto/network/kernel/qhostinfo/tst_qhostinfo.cpp
+++ b/tests/auto/network/kernel/qhostinfo/tst_qhostinfo.cpp
@@ -106,6 +106,8 @@
     void cache();
 
     void abortHostLookup();
+
+    void manylookup();
 protected slots:
     void resultsReady(const QHostInfo &);
 
@@ -731,5 +733,25 @@
     int id;
 };
 
+#include <QNetworkReply>
+void tst_QHostInfo::manylookup()
+{
+    QNetworkAccessManager manager;
+    auto reply = manager.get(QNetworkRequest(QUrl("https://raw.githubusercontent.com/opendns/public-domain-lists/master/opendns-top-domains.txt")));
+    QTRY_VERIFY(reply->isFinished());
+    auto domains = reply->readAll().split('\n');
+    QTestEventLoop loop;
+    int done = 0;
+    for (int i = 0; i < domains.length(); ++i) {
+        QHostInfo::lookupHost(QString::fromLatin1(domains[i]), &loop,
+                              [&done, &loop](const QHostInfo &h) {
+                                  if (++done == 100)
+                                      loop.exitLoop();
+                              });
+    }
+    loop.enterLoop(5000);
+    QVERIFY(!loop.timeout());
+}
+
 QTEST_MAIN(tst_QHostInfo)
 #include "tst_qhostinfo.moc"