refs #581, #592 Allow to renew a stale value by updating the timestamp on disk without altering the value.

src/blackmisc/datacache.cpp

@@ -105,6 +105,11 @@ namespace BlackMisc
         QTimer::singleShot(0, &m_serializer, [this, key, ttl] { m_revision.setTimeToLive(key, ttl); });
     }
 
+    void CDataCache::renewTimestamp(const QString &key, qint64 timestamp)
+    {
+        QTimer::singleShot(0, &m_serializer, [this, key, timestamp] { m_revision.overrideTimestamp(key, timestamp); });
+    }
+
     QString lockFileError(const QLockFile &lock)
     {
         switch (lock.error())
@@ -385,6 +390,41 @@ namespace BlackMisc
         m_timesToLive.insert(key, ttl);
     }
 
+    void CDataCacheRevision::overrideTimestamp(const QString &key, qint64 timestamp)
+    {
+        Q_ASSERT(! m_updateInProgress);
+        Q_ASSERT(! m_lockFile.isLocked());
+
+        if (! m_lockFile.lock())
+        {
+            CLogMessage(this).error("Failed to lock %1: %2") << m_basename << lockFileError(m_lockFile);
+            m_lockFile.unlock();
+            return;
+        }
+
+        CAtomicFile revisionFile(m_basename + "/.rev");
+        if (revisionFile.exists())
+        {
+            if (! revisionFile.open(QFile::ReadWrite | QFile::Text))
+            {
+                CLogMessage(this).error("Failed to open %1: %2") << revisionFile.fileName() << revisionFile.errorString();
+                m_lockFile.unlock();
+                return;
+            }
+
+            auto json = QJsonDocument::fromJson(revisionFile.readAll()).object();
+            auto timestamps = json.value("timestamps").toObject();
+            timestamps.insert(key, timestamp);
+            json.insert("timestamps", timestamps);
+
+            if (! (revisionFile.seek(0) && revisionFile.resize(0) && revisionFile.write(QJsonDocument(json).toJson()) && revisionFile.checkedClose()))
+            {
+                CLogMessage(this).error("Failed to write to %1: %2") << revisionFile.fileName() << revisionFile.errorString();
+            }
+        }
+        m_lockFile.unlock();
+    }
+
     QJsonObject CDataCacheRevision::toJson(const QMap<QString, qint64> &timestamps)
     {
         QJsonObject result;

src/blackmisc/datacache.h

@@ -76,6 +76,9 @@ namespace BlackMisc
         //! Set TTL value that will be written to the revision file.
         void setTimeToLive(const QString &key, int ttl);
 
+        //! Causes the new timestamp to be written to the revision file.
+        void overrideTimestamp(const QString &key, qint64 timestamp);
+
     private:
         mutable QMutex m_mutex { QMutex::Recursive };
         bool m_updateInProgress = false;
@@ -157,6 +160,9 @@ namespace BlackMisc
         //! Method used for implementing TTL.
         void setTimeToLive(const QString &key, int ttl);
 
+        //! Method used for implementing timestamp renewal.
+        void renewTimestamp(const QString &key, qint64 timestamp);
+
     private:
         CDataCache();
 
@@ -204,6 +210,9 @@ namespace BlackMisc
         //! True if the current timestamp is older than the TTL (time to live).
         bool isStale() const { return this->getTimestamp() + Trait::timeToLive() > QDateTime::currentMSecsSinceEpoch(); }
 
+        //! Don't change the value, but write a new timestamp, to extend the life of the value.
+        void renewTimestamp(qint64 timestamp) { return CDataCache::instance()->renewTimestamp(this->getKey(), timestamp); }
+
         //! Return a future providing the value. If the value is still loading, the future will wait for it.
         //! If the value is not present, the variant is null. Bypasses async get and inhibits notification slot.
         std::future<CVariant> syncLoad() { return CDataCache::instance()->syncLoad(m_owner, this->getKey()); }