refs #659 Method to get timestamp from revision file without loading value.

src/blackmisc/datacache.cpp

@@ -135,6 +135,11 @@ namespace BlackMisc
         QTimer::singleShot(0, &m_serializer, [this, key, timestamp] { m_revision.overrideTimestamp(key, timestamp); });
     }
 
+    qint64 CDataCache::getTimestampOnDisk(const QString &key)
+    {
+        return m_revision.getTimestampOnDisk(key);
+    }
+
     void CDataCache::pinValue(const QString &key)
     {
         QTimer::singleShot(0, &m_serializer, [this, key] { m_revision.pinValue(key); });
@@ -558,6 +563,37 @@ namespace BlackMisc
         m_lockFile.unlock();
     }
 
+    qint64 CDataCacheRevision::getTimestampOnDisk(const QString &key)
+    {
+        QMutexLocker lock(&m_mutex);
+
+        if (m_lockFile.isLocked()) { return m_timestamps.value(key); }
+
+        if (! m_lockFile.lock())
+        {
+            CLogMessage(this).error("Failed to lock %1: %2") << m_basename << lockFileError(m_lockFile);
+            m_lockFile.unlock();
+            return 0;
+        }
+
+        qint64 result = 0;
+        QFile revisionFile(m_basename + "/.rev");
+        if (revisionFile.exists())
+        {
+            if (revisionFile.open(QFile::ReadOnly | QFile::Text))
+            {
+                auto json = QJsonDocument::fromJson(revisionFile.readAll()).object();
+                result = static_cast<qint64>(json.value("timestamps").toObject().value(key).toDouble());
+            }
+            else
+            {
+                CLogMessage(this).error("Failed to open %1: %2") << revisionFile.fileName() << revisionFile.errorString();
+            }
+        }
+        m_lockFile.unlock();
+        return result;
+    }
+
     void CDataCacheRevision::pinValue(const QString &key)
     {
         Q_ASSERT(! m_updateInProgress);

src/blackmisc/datacache.h

@@ -141,6 +141,9 @@ namespace BlackMisc
         //! Causes the new timestamp to be written to the revision file.
         void overrideTimestamp(const QString &key, qint64 timestamp);
 
+        //! Read the revision file to get a timestamp.
+        qint64 getTimestampOnDisk(const QString &key);
+
         //! Set the flag which will cause the value to be pre-loaded.
         void pinValue(const QString &key);
 
@@ -241,6 +244,9 @@ namespace BlackMisc
         //! Method used for implementing timestamp renewal.
         void renewTimestamp(const QString &key, qint64 timestamp);
 
+        //! Method used for implementing loading timestamp without value.
+        qint64 getTimestampOnDisk(const QString &key);
+
         //! Method used for implementing pinning values.
         void pinValue(const QString &key);
 
@@ -311,6 +317,13 @@ namespace BlackMisc
         //! 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); }
 
+        //! Get the timestamp of the value, or of the deferred value that is available to be loaded.
+        QDateTime getAvailableTimestamp() const
+        {
+            if (Trait::isDeferred()) { return QDateTime::fromMSecsSinceEpoch(CDataCache::instance()->getTimestampOnDisk(this->getKey())); }
+            return this->getTimestamp();
+        }
+
         //! If the value is load-deferred, trigger the deferred load (async).
         void admit() { if (Trait::isDeferred()) { CDataCache::instance()->admitValue(Trait::key(), true); } }