Use std::atomic_flag

Going through QSettings doesn't work quite well on Windows; setting it
in Quotest and immediately testing through another QSettings instance
inside NetworkAccessManager (in another thread) returns the previous
value. In fact, there's no general guarantee for that, as almost all
QSettings member functions are reentrant but not thread-safe. The flag
is only initialised from QSettings once at the first NAM creation, and its
changes are only propagated to QSettings but never read again.
Implication: changes in QSettings files when the client is running will
only be reflected at the next run (or may be overwritten altogether),
so just don't change settings files while the client is running, ok?

Quotient/networkaccessmanager.cpp

@@ -21,6 +21,10 @@
 using namespace Quotient;
 
 namespace {
+
+static constexpr auto DirectMediaRequestsSetting =
+    "Network/allow_direct_media_requests"_ls;
+
 class {
 public:
     void addBaseUrl(const QString& accountId, const QUrl& baseUrl)
@@ -47,15 +51,42 @@ class {
     {
         return QReadLocker{ &namLock }, ignoredSslErrors;
     }
+    void allowDirectMediaRequests(bool allow)
+    {
+        if (allow)
+            directMediaRequestsAreAllowed.test_and_set();
+        else
+            directMediaRequestsAreAllowed.clear();
+    }
+    bool directMediaRequestsAllowed() const
+    {
+        return directMediaRequestsAreAllowed.test();
+    }
 
 private:
     mutable QReadWriteLock namLock{};
     QHash<QString, QUrl> baseUrls{};
     QList<QSslError> ignoredSslErrors{};
+    // This one is small enough to be atomic and not need a read-write lock
+    std::atomic_flag directMediaRequestsAreAllowed = false;
 } d;
 
+std::once_flag directMediaRequestsInitFlag;
+
 } // anonymous namespace
 
+void NetworkAccessManager::allowDirectMediaRequests(bool allow, bool permanent)
+{
+    d.allowDirectMediaRequests(allow);
+    if (permanent)
+        QSettings().setValue(DirectMediaRequestsSetting, allow);
+}
+
+bool NetworkAccessManager::directMediaRequestsAllowed()
+{
+    return d.directMediaRequestsAllowed();
+}
+
 void NetworkAccessManager::addBaseUrl(const QString& accountId,
                                       const QUrl& homeserver)
 {
@@ -85,6 +116,11 @@ void NetworkAccessManager::clearIgnoredSslErrors()
 
 NetworkAccessManager* NetworkAccessManager::instance()
 {
+    // Initialise direct media requests allowance at the very first NAM creation
+    std::call_once(directMediaRequestsInitFlag, [] {
+        NetworkAccessManager::allowDirectMediaRequests(
+            QSettings().value(DirectMediaRequestsSetting).toBool());
+    });
     thread_local auto* nam = [] {
         auto* namInit = new NetworkAccessManager();
         connect(QThread::currentThread(), &QThread::finished, namInit,
@@ -122,9 +158,7 @@ QNetworkReply* NetworkAccessManager::createRequest(
     if (accountId.isEmpty()) {
         // Using QSettings here because Quotient::NetworkSettings
         // doesn't provide multi-threading guarantees
-        if (static thread_local const QSettings s;
-            s.value("Network/allow_direct_media_requests"_ls).toBool()) //
-        {
+        if (directMediaRequestsAllowed()) {
             // Best effort with an unauthenticated request directly to the media
             // homeserver (rather than via own homeserver)
             auto* mxcReply = new MxcReply(MxcReply::Deferred);

Quotient/networkaccessmanager.h

@@ -12,7 +12,9 @@ namespace Quotient {
 class QUOTIENT_API NetworkAccessManager : public QNetworkAccessManager {
     Q_OBJECT
 public:
-    using QNetworkAccessManager::QNetworkAccessManager;
+    static void allowDirectMediaRequests(bool allow = true,
+                                         bool permanent = true);
+    static bool directMediaRequestsAllowed();
 
     static void addBaseUrl(const QString& accountId, const QUrl& homeserver);
     static void dropBaseUrl(const QString& accountId);

quotest/quotest.cpp

@@ -492,18 +492,15 @@ struct DownloadRunner {
 bool TestSuite::testDownload(const TestToken& thisTest, const QUrl& mxcUrl)
 {
     // Testing direct media requests needs explicit allowance
-    QSettings s;
-    static constexpr auto DirectMediaRequestsSetting =
-        "Network/allow_direct_media_requests"_ls;
-    s.setValue(DirectMediaRequestsSetting, true);
+    NetworkAccessManager::allowDirectMediaRequests(true, false);
     if (const auto result = DownloadRunner::run(mxcUrl, 1);
         result.front() != QNetworkReply::NoError) {
         clog << "Direct media request to "
              << mxcUrl.toDisplayString().toStdString()
              << " was allowed but failed" << endl;
         FAIL_TEST();
     }
-    s.setValue(DirectMediaRequestsSetting, false);
+    NetworkAccessManager::allowDirectMediaRequests(false, false);
     if (const auto result = DownloadRunner::run(mxcUrl, 1);
         result.front() == QNetworkReply::NoError) {
         clog << "Direct media request to "