[processing] Add a flag for algorithms to indicate that they should

not be available from the standalone qgis_process tool (e.g. "select by "
algorithms, which have no meaning outside of a GUI application)

These algorithms are hidden from the algorithm list for qgis_process
and cannot be run by the tool

python/core/auto_generated/processing/qgsprocessingalgorithm.sip.in

@@ -49,6 +49,7 @@ Abstract base class for processing algorithms.
       FlagCustomException,
       FlagPruneModelBranchesBasedOnAlgorithmResults,
       FlagSkipGenericModelLogging,
+      FlagNotAvailableInStandaloneTool,
       FlagDeprecated,
     };
     typedef QFlags<QgsProcessingAlgorithm::Flag> Flags;

src/core/processing/qgsprocessingalgorithm.h

@@ -79,6 +79,7 @@ class CORE_EXPORT QgsProcessingAlgorithm
       FlagCustomException = 1 << 10, //!< Algorithm raises custom exception notices, don't use the standard ones
       FlagPruneModelBranchesBasedOnAlgorithmResults = 1 << 11, //!< Algorithm results will cause remaining model branches to be pruned based on the results of running the algorithm
       FlagSkipGenericModelLogging = 1 << 12, //!< When running as part of a model, the generic algorithm setup and results logging should be skipped
+      FlagNotAvailableInStandaloneTool = 1 << 13, //!< Algorithm should not be available from the standalone "qgis_process" tool. Used to flag algorithms which make no sense outside of the QGIS application, such as "select by..." style algorithms.
       FlagDeprecated = FlagHideFromToolbox | FlagHideFromModeler, //!< Algorithm is deprecated
     };
     Q_DECLARE_FLAGS( Flags, Flag )

src/process/qgsprocess.cpp

@@ -314,6 +314,9 @@ void QgsProcessingExec::listAlgorithms()
     const QList<const QgsProcessingAlgorithm *> algorithms = provider->algorithms();
     for ( const QgsProcessingAlgorithm *algorithm : algorithms )
     {
+      if ( algorithm->flags() & QgsProcessingAlgorithm::FlagDeprecated || algorithm->flags() & QgsProcessingAlgorithm::FlagNotAvailableInStandaloneTool )
+        continue;
+
       std::cout << "\t" << algorithm->id().toLocal8Bit().constData() << "\t" << algorithm->displayName().toLocal8Bit().constData() << "\n";
     }
 
@@ -450,6 +453,12 @@ int QgsProcessingExec::execute( const QString &id, const QVariantMap &params, co
       std::cerr << QStringLiteral( "Algorithm %1 not found!\n" ).arg( id ).toLocal8Bit().constData();
       return 1;
     }
+
+    if ( alg->flags() & QgsProcessingAlgorithm::FlagNotAvailableInStandaloneTool )
+    {
+      std::cerr << QStringLiteral( "The \"%1\" algorithm is not available for use outside of the QGIS desktop application\n" ).arg( id ).toLocal8Bit().constData();
+      return 1;
+    }
   }
 
   std::cout << "\n----------------\n";