Cache generated WKT if it's in the preferred format

Some frequently called code paths call this, and it's rather costly
to calculate on the proj side.

(cherry picked from commit 65296e9)

src/core/qgscoordinatereferencesystem.cpp

@@ -597,6 +597,7 @@ bool QgsCoordinateReferenceSystem::loadFromDatabase( const QString &db, const QS
 
   QgsDebugMsgLevel( "load CRS from " + db + " where " + expression + " is " + value, 3 );
   d->mIsValid = false;
+  d->mWktPreferred.clear();
 
   QFileInfo myInfo( db );
   if ( !myInfo.exists() )
@@ -644,6 +645,7 @@ bool QgsCoordinateReferenceSystem::loadFromDatabase( const QString &db, const QS
     d->mEllipsoidAcronym = statement.columnAsText( 3 );
 #endif
     d->mProj4 = statement.columnAsText( 4 );
+    d->mWktPreferred.clear();
     d->mSRID = statement.columnAsText( 5 ).toLong();
     d->mAuthId = statement.columnAsText( 6 );
     d->mIsGeographic = statement.columnAsText( 7 ).toInt() != 0;
@@ -841,6 +843,7 @@ bool QgsCoordinateReferenceSystem::createFromWkt( const QString &wkt )
 
   d->mIsValid = false;
   d->mProj4.clear();
+  d->mWktPreferred.clear();
   if ( wkt.isEmpty() )
   {
     QgsDebugMsgLevel( QStringLiteral( "theWkt is uninitialized, operation failed" ), 4 );
@@ -899,6 +902,7 @@ bool QgsCoordinateReferenceSystem::createFromProj( const QString &projString )
   {
     d->mIsValid = false;
     d->mProj4.clear();
+    d->mWktPreferred.clear();
     return false;
   }
 
@@ -928,6 +932,7 @@ bool QgsCoordinateReferenceSystem::createFromProj( const QString &projString )
   myProj4String = myProj4String.trimmed();
 
   d->mIsValid = false;
+  d->mWktPreferred.clear();
 #if PROJ_VERSION_MAJOR>=6
   // first, try to use proj to do this for us...
   const QString projCrsString = myProj4String + ( myProj4String.contains( QStringLiteral( "+type=crs" ) ) ? QString() : QStringLiteral( " +type=crs" ) );
@@ -1466,6 +1471,7 @@ void QgsCoordinateReferenceSystem::setProjString( const QString &proj4String )
 {
   d.detach();
   d->mProj4 = proj4String;
+  d->mWktPreferred.clear();
 
   QgsLocaleNumC l;
   QString trimmed = proj4String.trimmed();
@@ -1521,6 +1527,7 @@ bool QgsCoordinateReferenceSystem::setWktString( const QString &wkt, bool allowP
 {
   bool res = false;
   d->mIsValid = false;
+  d->mWktPreferred.clear();
 
 #if PROJ_VERSION_MAJOR>=6
   // TODO - remove allowProjFallback when we require proj 6+ to build
@@ -1896,6 +1903,13 @@ QString QgsCoordinateReferenceSystem::toWkt( WktVariant variant, bool multiline,
 #if PROJ_VERSION_MAJOR>=6
   if ( PJ *obj = d->threadLocalProjObject() )
   {
+    const bool isDefaultPreferredFormat = variant == WKT_PREFERRED && !multiline;
+    if ( isDefaultPreferredFormat && !d->mWktPreferred.isEmpty() )
+    {
+      // can use cached value
+      return d->mWktPreferred;
+    }
+
     PJ_WKT_TYPE type = PJ_WKT1_GDAL;
     switch ( variant )
     {
@@ -1922,7 +1936,15 @@ QString QgsCoordinateReferenceSystem::toWkt( WktVariant variant, bool multiline,
     const QByteArray multiLineOption = QStringLiteral( "MULTILINE=%1" ).arg( multiline ? QStringLiteral( "YES" ) : QStringLiteral( "NO" ) ).toLocal8Bit();
     const QByteArray indentatationWidthOption = QStringLiteral( "INDENTATION_WIDTH=%1" ).arg( multiline ? QString::number( indentationWidth ) : QStringLiteral( "0" ) ).toLocal8Bit();
     const char *const options[] = {multiLineOption.constData(), indentatationWidthOption.constData(), nullptr};
-    return QString( proj_as_wkt( QgsProjContext::get(), obj, type, options ) );
+    QString res = proj_as_wkt( QgsProjContext::get(), obj, type, options );
+
+    if ( isDefaultPreferredFormat )
+    {
+      // cache result for later use
+      d->mWktPreferred = res;
+    }
+
+    return res;
   }
   return QString();
 #else
@@ -2024,6 +2046,8 @@ bool QgsCoordinateReferenceSystem::readXml( const QDomNode &node )
       node = srsNode.namedItem( QStringLiteral( "geographicflag" ) );
       d->mIsGeographic = node.toElement().text().compare( QLatin1String( "true" ) );
 
+      d->mWktPreferred.clear();
+
       //make sure the map units have been set
       setMapUnits();
     }
@@ -2384,6 +2408,7 @@ bool QgsCoordinateReferenceSystem::loadFromAuthCode( const QString &auth, const
 {
   d.detach();
   d->mIsValid = false;
+  d->mWktPreferred.clear();
 
   PJ_CONTEXT *pjContext = QgsProjContext::get();
   QgsProjUtils::proj_pj_unique_ptr crs( proj_create_from_database( pjContext, auth.toUtf8().constData(), code.toUtf8().constData(), PJ_CATEGORY_CRS, false, nullptr ) );
@@ -2409,6 +2434,7 @@ bool QgsCoordinateReferenceSystem::loadFromAuthCode( const QString &auth, const
 
   d->mIsValid = true;
   d->mProj4 = proj4;
+  d->mWktPreferred.clear();
   d->mDescription = QString( proj_get_name( crs.get() ) );
   d->mAuthId = QStringLiteral( "%1:%2" ).arg( auth, code );
   d->mIsGeographic = testIsGeographic( crs.get() );

src/core/qgscoordinatereferencesystem_p.h

@@ -71,6 +71,7 @@ class QgsCoordinateReferenceSystemPrivate : public QSharedData
       , mCRS( nullptr )
 #endif
       , mProj4( other.mProj4 )
+      , mWktPreferred( other.mWktPreferred )
       , mAxisInvertedDirty( other.mAxisInvertedDirty )
       , mAxisInverted( other.mAxisInverted )
     {
@@ -178,7 +179,7 @@ class QgsCoordinateReferenceSystemPrivate : public QSharedData
 
     mutable QString mProj4;
 
-    mutable QString mWkt
+    mutable QString mWktPreferred;
 
     //! True if presence of an inverted axis needs to be recalculated
     mutable bool mAxisInvertedDirty = false;