Cache svg paths

Rebuilding them for each access from the settings is costly

python/core/auto_generated/qgsapplication.sip.in

@@ -281,6 +281,13 @@ Returns the path to the icons image directory.
     static QString srsDatabaseFilePath();
 %Docstring
 Returns the path to the srs.db file.
+%End
+
+    static void setSvgPaths( const QStringList &svgPaths );
+%Docstring
+Sets the paths to svg directories and invalidates the svg path list cache.
+
+.. versionadded:: 3.18
 %End
 
     static QStringList svgPaths();

src/app/options/qgsoptions.cpp

@@ -1464,7 +1464,7 @@ void QgsOptions::saveOptions()
   {
     pathsList << mListSVGPaths->item( i )->text();
   }
-  mSettings->setValue( QStringLiteral( "svg/searchPathsForSVG" ), pathsList );
+  QgsApplication::setSvgPaths( pathsList );
 
   pathsList.clear();
   for ( int i = 0; i < mListComposerTemplatePaths->count(); ++i )

src/core/qgsapplication.cpp

@@ -70,6 +70,7 @@
 #include "qgsmeshlayer.h"
 #include "qgsfeaturestore.h"
 #include "qgslocator.h"
+#include "qgsreadwritelocker.h"
 
 #include "gps/qgsgpsconnectionregistry.h"
 #include "processing/qgsprocessingregistry.h"
@@ -1050,27 +1051,46 @@ QString QgsApplication::srsDatabaseFilePath()
   }
 }
 
+void QgsApplication::setSvgPaths( const QStringList &svgPaths )
+{
+  QgsSettings().setValue( QStringLiteral( "svg/searchPathsForSVG" ), svgPaths );
+  members()->mSvgPathCacheValid = false;
+}
+
 QStringList QgsApplication::svgPaths()
 {
-  //local directories to search when looking for an SVG with a given basename
-  //defined by user in options dialog
-  QgsSettings settings;
-  const QStringList pathList = settings.value( QStringLiteral( "svg/searchPathsForSVG" ) ).toStringList();
+  static QReadWriteLock lock;
+
+  QgsReadWriteLocker locker( lock, QgsReadWriteLocker::Read );
 
-  // maintain user set order while stripping duplicates
-  QStringList paths;
-  for ( const QString &path : pathList )
+  if ( members()->mSvgPathCacheValid )
   {
-    if ( !paths.contains( path ) )
-      paths.append( path );
+    return members()->mSvgPathCache;
   }
-  for ( const QString &path : qgis::as_const( *sDefaultSvgPaths() ) )
+  else
   {
-    if ( !paths.contains( path ) )
-      paths.append( path );
-  }
+    locker.changeMode( QgsReadWriteLocker::Write );
+    //local directories to search when looking for an SVG with a given basename
+    //defined by user in options dialog
+    QgsSettings settings;
+    const QStringList pathList = settings.value( QStringLiteral( "svg/searchPathsForSVG" ) ).toStringList();
 
-  return paths;
+    // maintain user set order while stripping duplicates
+    QStringList paths;
+    for ( const QString &path : pathList )
+    {
+      if ( !paths.contains( path ) )
+        paths.append( path );
+    }
+    for ( const QString &path : qgis::as_const( *sDefaultSvgPaths() ) )
+    {
+      if ( !paths.contains( path ) )
+        paths.append( path );
+    }
+    members()->mSvgPathCache = paths;
+
+    return paths;
+  }
 }
 
 QStringList QgsApplication::layoutTemplatePaths()

src/core/qgsapplication.h

@@ -316,6 +316,13 @@ class CORE_EXPORT QgsApplication : public QApplication
     //! Returns the path to the srs.db file.
     static QString srsDatabaseFilePath();
 
+    /**
+     * Sets the paths to svg directories and invalidates the svg path list cache.
+     *
+     * \since QGIS 3.18
+     */
+    static void setSvgPaths( const QStringList &svgPaths );
+
     //! Returns the paths to svg directories.
     static QStringList svgPaths();
 
@@ -1005,6 +1012,8 @@ class CORE_EXPORT QgsApplication : public QApplication
       QgsBookmarkManager *mBookmarkManager = nullptr;
       QgsStyleModel *mStyleModel = nullptr;
       QString mNullRepresentation;
+      QStringList mSvgPathCache;
+      bool mSvgPathCacheValid = false;
 
       ApplicationMembers();
       ~ApplicationMembers();