Use a custom proj error handler to collect descriptive coordinate

operation creation errors, and report these back to users Because these errors contain useful information for users to debug, e.g. "Source and target ellipsoid do not belong to the same celestial body"
qgis · Jun 11, 2019 · 46f364a · 46f364a
1 parent d35197f
commit 46f364a
Showing 1 changed file with 41 additions and 1 deletion.
diff --git a/src/core/qgscoordinatetransform_p.cpp b/src/core/qgscoordinatetransform_p.cpp
@@ -268,6 +268,26 @@ void QgsCoordinateTransformPrivate::calculateTransforms( const QgsCoordinateTran
 #endif
 }
 
+#if PROJ_VERSION_MAJOR>=6
+static void proj_collecting_logger( void *user_data, int /*level*/, const char *message )
+{
+  QStringList *dest = reinterpret_cast< QStringList * >( user_data );
+  dest->append( QString( message ) );
+}
+
+static void proj_logger( void *, int level, const char *message )
+{
+  if ( level == PJ_LOG_ERROR )
+  {
+    QgsDebugMsg( QString( message ) );
+  }
+  else if ( level == PJ_LOG_DEBUG )
+  {
+    QgsDebugMsgLevel( QString( message ), 3 );
+  }
+}
+#endif
+
 ProjData QgsCoordinateTransformPrivate::threadLocalProjData()
 {
   QgsReadWriteLocker locker( mProjLock, QgsReadWriteLocker::Read );
@@ -303,6 +323,10 @@ ProjData QgsCoordinateTransformPrivate::threadLocalProjData()
   locker.changeMode( QgsReadWriteLocker::Write );
 
 #if PROJ_VERSION_MAJOR>=6
+  // use a temporary proj error collector
+  QStringList projErrors;
+  proj_log_func( context, &projErrors, proj_collecting_logger );
+
   QgsProjUtils::proj_pj_unique_ptr transform;
   if ( !mProjCoordinateOperation.isEmpty() )
   {
@@ -333,7 +357,10 @@ ProjData QgsCoordinateTransformPrivate::threadLocalProjData()
   if ( !transform ) // fallback on default proj pathway
   {
     if ( !mSourceCRS.projObject() || ! mDestCRS.projObject() )
+    {
+      proj_log_func( context, nullptr, nullptr );
       return nullptr;
+    }
 
     PJ_OPERATION_FACTORY_CONTEXT *operationContext = proj_create_operation_factory_context( context, nullptr );
 
@@ -481,10 +508,20 @@ ProjData QgsCoordinateTransformPrivate::threadLocalProjData()
     {
       nonAvailableError = QString( proj_errno_string( errNo ) );
     }
-    else
+    else if ( !projErrors.empty() )
+    {
+      nonAvailableError = projErrors.constLast();
+    }
+
+    if ( nonAvailableError.isEmpty() )
     {
       nonAvailableError = QObject::tr( "No coordinate operations are available between these two reference systems" );
     }
+    else
+    {
+      // strip proj prefixes from error string, so that it's nicer for users
+      nonAvailableError = nonAvailableError.remove( QStringLiteral( "internal_proj_create_operations: " ) );
+    }
   }
 
   if ( !nonAvailableError.isEmpty() )
@@ -502,6 +539,9 @@ ProjData QgsCoordinateTransformPrivate::threadLocalProjData()
     }
   }
 
+  // reset logger to terminal output
+  proj_log_func( context, nullptr, proj_logger );
+
   if ( !transform )
   {
     // ouch!