Add friendly, descriptive errors when opening a project which has

preset non-default coordinate operations set between a crs pair,
which use grids that are not available on the local system

python/core/auto_generated/qgscoordinatetransformcontext.sip.in

@@ -212,6 +212,7 @@ be used.
 .. versionadded:: 3.8
 %End
 
+
     bool readXml( const QDomElement &element, const QgsReadWriteContext &context, QStringList &missingTransforms /Out/ );
 %Docstring
 Reads the context's state from a DOM ``element``.

src/app/qgsappcoordinateoperationhandlers.cpp

@@ -18,6 +18,7 @@
 #include "qgsmessagebar.h"
 #include "qgsmessagebaritem.h"
 #include "qgsmessageoutput.h"
+#include "qgsproject.h"
 
 //
 // QgsAppMissingRequiredGridHandler
@@ -47,9 +48,19 @@ QgsAppMissingGridHandler::QgsAppMissingGridHandler( QObject *parent )
     emit coordinateOperationCreationError( sourceCrs, destinationCrs, error );
   } );
 
+  QgsCoordinateTransform::setCustomMissingGridUsedByContextHandler( [ = ]( const QgsCoordinateReferenceSystem & sourceCrs,
+      const QgsCoordinateReferenceSystem & destinationCrs,
+      const QgsDatumTransform::TransformDetails & desired )
+  {
+    emit missingGridUsedByContextHandler( sourceCrs, destinationCrs, desired );
+  } );
+
   connect( this, &QgsAppMissingGridHandler::missingRequiredGrid, this, &QgsAppMissingGridHandler::onMissingRequiredGrid, Qt::QueuedConnection );
   connect( this, &QgsAppMissingGridHandler::missingPreferredGrid, this, &QgsAppMissingGridHandler::onMissingPreferredGrid, Qt::QueuedConnection );
   connect( this, &QgsAppMissingGridHandler::coordinateOperationCreationError, this, &QgsAppMissingGridHandler::onCoordinateOperationCreationError, Qt::QueuedConnection );
+  connect( this, &QgsAppMissingGridHandler::missingGridUsedByContextHandler, this, &QgsAppMissingGridHandler::onMissingGridUsedByContextHandler, Qt::QueuedConnection );
+
+  connect( QgsProject::instance(), &QgsProject::cleared, this, [ = ] { mAlreadyWarnedPairsForProject.clear(); } );
 
 }
 
@@ -197,6 +208,61 @@ void QgsAppMissingGridHandler::onCoordinateOperationCreationError( const QgsCoor
   bar->pushWidget( widget, Qgis::Critical, 0 );
 }
 
+void QgsAppMissingGridHandler::onMissingGridUsedByContextHandler( const QgsCoordinateReferenceSystem &sourceCrs, const QgsCoordinateReferenceSystem &destinationCrs, const QgsDatumTransform::TransformDetails &desired )
+{
+  if ( !shouldWarnAboutPairForCurrentProject( sourceCrs, destinationCrs ) )
+    return;
+
+  const QString shortMessage = tr( "Cannot use project transform between %1 and %2" ).arg( displayIdentifierForCrs( sourceCrs, true ),
+                               displayIdentifierForCrs( destinationCrs, true ) );
+
+  QString gridMessage;
+  for ( const QgsDatumTransform::GridDetails &grid : desired.grids )
+  {
+    if ( !grid.isAvailable )
+    {
+      QString m = tr( "This transformation requires the grid file “%1”, which is not available for use on the system." ).arg( grid.shortName );
+      if ( !grid.url.isEmpty() )
+      {
+        if ( !grid.packageName.isEmpty() )
+        {
+          m += ' ' +  tr( "This grid is part of the <i>%1</i> package, available for download from <a href=\"%2\">%2</a>." ).arg( grid.packageName, grid.url );
+        }
+        else
+        {
+          m += ' ' + tr( "This grid is available for download from <a href=\"%1\">%1</a>." ).arg( grid.url );
+        }
+      }
+      gridMessage += QStringLiteral( "<li>%1</li>" ).arg( m );
+    }
+  }
+  if ( !gridMessage.isEmpty() )
+  {
+    gridMessage = "<ul>" + gridMessage + "</ul>";
+  }
+
+  const QString longMessage = tr( "<p>This project specifies a preset transform between <i>%1</i> and <i>%2</i>, which is not available for use on the system.</p>" ).arg( displayIdentifierForCrs( sourceCrs ),
+                              displayIdentifierForCrs( destinationCrs ) )
+                              + gridMessage
+                              + tr( "<p>The operation specified for use in the project is:</p><p><code>%1</code></p>" ).arg( desired.proj ) ;
+
+
+  QgsMessageBar *bar = QgisApp::instance()->messageBar();
+  QgsMessageBarItem *widget = bar->createMessage( QString(), shortMessage );
+  QPushButton *detailsButton = new QPushButton( tr( "Details" ) );
+  connect( detailsButton, &QPushButton::clicked, this, [longMessage]
+  {
+    // dlg has deleted on close
+    QgsMessageOutput * dlg( QgsMessageOutput::createMessageOutput() );
+    dlg->setTitle( tr( "Project Transformation Not Available" ) );
+    dlg->setMessage( longMessage, QgsMessageOutput::MessageHtml );
+    dlg->showMessage();
+  } );
+
+  widget->layout()->addWidget( detailsButton );
+  bar->pushWidget( widget, Qgis::Critical, 0 );
+}
+
 bool QgsAppMissingGridHandler::shouldWarnAboutPair( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &dest )
 {
   if ( mAlreadyWarnedPairs.contains( qMakePair( source, dest ) ) || mAlreadyWarnedPairs.contains( qMakePair( dest, source ) ) )
@@ -207,3 +273,14 @@ bool QgsAppMissingGridHandler::shouldWarnAboutPair( const QgsCoordinateReference
   mAlreadyWarnedPairs.append( qMakePair( source, dest ) );
   return true;
 }
+
+bool QgsAppMissingGridHandler::shouldWarnAboutPairForCurrentProject( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &dest )
+{
+  if ( mAlreadyWarnedPairsForProject.contains( qMakePair( source, dest ) ) || mAlreadyWarnedPairsForProject.contains( qMakePair( dest, source ) ) )
+  {
+    return false;
+  }
+
+  mAlreadyWarnedPairsForProject.append( qMakePair( source, dest ) );
+  return true;
+}

src/app/qgsappcoordinateoperationhandlers.h

@@ -45,6 +45,10 @@ class QgsAppMissingGridHandler : public QObject
                                            const QgsCoordinateReferenceSystem &destinationCrs,
                                            const QString &error );
 
+    void missingGridUsedByContextHandler( const QgsCoordinateReferenceSystem &sourceCrs,
+                                          const QgsCoordinateReferenceSystem &destinationCrs,
+                                          const QgsDatumTransform::TransformDetails &desired );
+
   private slots:
 
     void onMissingRequiredGrid( const QgsCoordinateReferenceSystem &sourceCrs,
@@ -59,11 +63,17 @@ class QgsAppMissingGridHandler : public QObject
     void onCoordinateOperationCreationError( const QgsCoordinateReferenceSystem &sourceCrs,
         const QgsCoordinateReferenceSystem &destinationCrs,
         const QString &error );
+
+    void onMissingGridUsedByContextHandler( const QgsCoordinateReferenceSystem &sourceCrs,
+                                            const QgsCoordinateReferenceSystem &destinationCrs,
+                                            const QgsDatumTransform::TransformDetails &desired );
   private:
 
     bool shouldWarnAboutPair( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &dest );
+    bool shouldWarnAboutPairForCurrentProject( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &dest );
 
     QList< QPair< QgsCoordinateReferenceSystem, QgsCoordinateReferenceSystem > > mAlreadyWarnedPairs;
+    QList< QPair< QgsCoordinateReferenceSystem, QgsCoordinateReferenceSystem > > mAlreadyWarnedPairsForProject;
 
 };
 

src/core/qgscoordinatetransform.cpp

@@ -969,3 +969,8 @@ void QgsCoordinateTransform::setCustomCoordinateOperationCreationErrorHandler( c
 {
   QgsCoordinateTransformPrivate::setCustomCoordinateOperationCreationErrorHandler( handler );
 }
+
+void QgsCoordinateTransform::setCustomMissingGridUsedByContextHandler( const std::function<void ( const QgsCoordinateReferenceSystem &, const QgsCoordinateReferenceSystem &, const QgsDatumTransform::TransformDetails & )> &handler )
+{
+  QgsCoordinateTransformPrivate::setCustomMissingGridUsedByContextHandler( handler );
+}

src/core/qgscoordinatetransform.h

@@ -447,6 +447,7 @@ class CORE_EXPORT QgsCoordinateTransform
      *
      * \see setCustomMissingPreferredGridHandler()
      * \see setCustomCoordinateOperationCreationErrorHandler()
+     * \see setCustomMissingGridUsedByContextHandler()
      *
      * \note Not available in Python bindings
      * \since QGIS 3.8
@@ -466,6 +467,7 @@ class CORE_EXPORT QgsCoordinateTransform
      *
      * \see setCustomMissingRequiredGridHandler()
      * \see setCustomCoordinateOperationCreationErrorHandler()
+     * \see setCustomMissingGridUsedByContextHandler()
      *
      * \note Not available in Python bindings
      * \since QGIS 3.8
@@ -482,6 +484,7 @@ class CORE_EXPORT QgsCoordinateTransform
      *
      * \see setCustomMissingRequiredGridHandler()
      * \see setCustomMissingPreferredGridHandler()
+     * \see setCustomMissingGridUsedByContextHandler()
      *
      * \note Not available in Python bindings
      * \since QGIS 3.8
@@ -490,6 +493,21 @@ class CORE_EXPORT QgsCoordinateTransform
         const QgsCoordinateReferenceSystem &destinationCrs,
         const QString &error )> &handler );
 
+    /**
+     * Sets a custom handler to use when a coordinate operation was specified for use between \a sourceCrs and
+     * \a destinationCrs by the transform context, yet the coordinate operation could not be created. The \a desiredOperation argument
+     * specifies the desired transform details as specified by the context.
+     *
+     * \see setCustomMissingRequiredGridHandler()
+     * \see setCustomMissingPreferredGridHandler()
+     * \see setCustomCoordinateOperationCreationErrorHandler()
+     *
+     * \note Not available in Python bindings
+     * \since QGIS 3.8
+     */
+    static void setCustomMissingGridUsedByContextHandler( const std::function< void( const QgsCoordinateReferenceSystem &sourceCrs,
+        const QgsCoordinateReferenceSystem &destinationCrs,
+        const QgsDatumTransform::TransformDetails &desiredOperation )> &handler );
 #endif
 
   private:

src/core/qgscoordinatetransform_p.cpp

@@ -47,6 +47,10 @@ std::function< void( const QgsCoordinateReferenceSystem &sourceCrs,
                      const QgsCoordinateReferenceSystem &destinationCrs,
                      const QString &error )> QgsCoordinateTransformPrivate::sCoordinateOperationCreationErrorHandler = nullptr;
 
+std::function< void( const QgsCoordinateReferenceSystem &sourceCrs,
+                     const QgsCoordinateReferenceSystem &destinationCrs,
+                     const QgsDatumTransform::TransformDetails &desiredOperation )> QgsCoordinateTransformPrivate::sMissingGridUsedByContextHandler = nullptr;
+
 #if PROJ_VERSION_MAJOR<6
 #ifdef USE_THREAD_LOCAL
 thread_local QgsProjContextStore QgsCoordinateTransformPrivate::mProjContext;
@@ -298,7 +302,29 @@ ProjData QgsCoordinateTransformPrivate::threadLocalProjData()
 #if PROJ_VERSION_MAJOR>=6
   QgsProjUtils::proj_pj_unique_ptr transform;
   if ( !mProjCoordinateOperation.isEmpty() )
+  {
     transform.reset( proj_create( context, mProjCoordinateOperation.toUtf8().constData() ) );
+    if ( !transform || !proj_coordoperation_is_instantiable( context, transform.get() ) )
+    {
+      if ( sMissingGridUsedByContextHandler )
+      {
+        QgsDatumTransform::TransformDetails desired;
+        desired.proj = mProjCoordinateOperation;
+        desired.accuracy = -1; //unknown, can't retrieve from proj as we can't instantiate the op
+        desired.grids = QgsProjUtils::gridsUsed( mProjCoordinateOperation );
+        sMissingGridUsedByContextHandler( mSourceCRS, mDestCRS, desired );
+      }
+      else
+      {
+        const QString err = QObject::tr( "Could not use operation specified in project between %1 and %2. (Wanted to use: %3)." ).arg( mSourceCRS.authid() )
+                            .arg( mDestCRS.authid() )
+                            .arg( mProjCoordinateOperation );
+        QgsMessageLog::logMessage( err, QString(), Qgis::Critical );
+      }
+
+      transform.reset();
+    }
+  }
 
   QString nonAvailableError;
   if ( !transform ) // fallback on default proj pathway
@@ -508,6 +534,11 @@ void QgsCoordinateTransformPrivate::setCustomCoordinateOperationCreationErrorHan
   sCoordinateOperationCreationErrorHandler = handler;
 }
 
+void QgsCoordinateTransformPrivate::setCustomMissingGridUsedByContextHandler( const std::function<void ( const QgsCoordinateReferenceSystem &, const QgsCoordinateReferenceSystem &, const QgsDatumTransform::TransformDetails & )> &handler )
+{
+  sMissingGridUsedByContextHandler = handler;
+}
+
 #if PROJ_VERSION_MAJOR<6
 QString QgsCoordinateTransformPrivate::stripDatumTransform( const QString &proj4 ) const
 {

src/core/qgscoordinatetransform_p.h

@@ -182,6 +182,17 @@ class QgsCoordinateTransformPrivate : public QSharedData
         const QgsCoordinateReferenceSystem &destinationCrs,
         const QString &error )> &handler );
 
+    /**
+     * Sets a custom handler to use when a coordinate operation was specified for use between \a sourceCrs and
+     * \a destinationCrs by the transform context, yet the coordinate operation could not be created. The \a desiredOperation argument
+     * specifies the desired transform details as specified by the context.
+     *
+     * \since QGIS 3.8
+     */
+    static void setCustomMissingGridUsedByContextHandler( const std::function< void( const QgsCoordinateReferenceSystem &sourceCrs,
+        const QgsCoordinateReferenceSystem &destinationCrs,
+        const QgsDatumTransform::TransformDetails &desiredOperation )> &handler );
+
   private:
 
 #if PROJ_VERSION_MAJOR<6
@@ -206,6 +217,10 @@ class QgsCoordinateTransformPrivate : public QSharedData
     static std::function< void( const QgsCoordinateReferenceSystem &sourceCrs,
                                 const QgsCoordinateReferenceSystem &destinationCrs,
                                 const QString &error )> sCoordinateOperationCreationErrorHandler;
+
+    static std::function< void( const QgsCoordinateReferenceSystem &sourceCrs,
+                                const QgsCoordinateReferenceSystem &destinationCrs,
+                                const QgsDatumTransform::TransformDetails &desiredOperation )> sMissingGridUsedByContextHandler;
 };
 
 /// @endcond

src/core/qgscoordinatetransformcontext.cpp

@@ -223,8 +223,7 @@ bool QgsCoordinateTransformContext::readXml( const QDomElement &element, const Q
     if ( !QgsProjUtils::coordinateOperationIsAvailable( coordinateOp ) )
     {
       // not possible in current Proj 6 api!
-      // missingTransforms.append( QgsProjUtils::nonAvailableGrids( coordinateOp ) );
-      missingTransforms.append( coordinateOp ); // yuck, we don't want to expose this string to users!
+      // QgsCoordinateTransform will alert users to this, we don't need to use missingTransforms here
       result = false;
     }
 

src/core/qgscoordinatetransformcontext.h

@@ -205,6 +205,8 @@ class CORE_EXPORT QgsCoordinateTransformContext
      */
     QString calculateCoordinateOperation( const QgsCoordinateReferenceSystem &source, const QgsCoordinateReferenceSystem &destination ) const;
 
+    // TODO QGIS 4.0 - remove missingTransforms, not used for Proj >= 6.0 builds
+
     /**
      * Reads the context's state from a DOM \a element.
      *

src/core/qgsproject.cpp

@@ -1232,7 +1232,7 @@ bool QgsProject::readProjectFile( const QString &filename )
   mCrs = projectCrs;
 
   QStringList datumErrors;
-  if ( !mTransformContext.readXml( doc->documentElement(), context, datumErrors ) )
+  if ( !mTransformContext.readXml( doc->documentElement(), context, datumErrors ) && !datumErrors.empty() )
   {
     emit missingDatumTransforms( datumErrors );
   }