[processing] Fix crash in extract by attribute when field name does not exist

Fixes #19531

python/plugins/processing/algs/qgis/SelectByAttribute.py

@@ -130,6 +130,9 @@ class SelectByAttribute(QgisAlgorithm):
         fields = layer.fields()
 
         idx = layer.fields().lookupField(fieldName)
+        if idx < 0:
+            raise QgsProcessingException(self.tr("Field '{}' was not found in layer").format(fieldName))
+
         fieldType = fields[idx].type()
 
         if fieldType != QVariant.String and operator in self.STRING_OPERATORS:

python/plugins/processing/tests/AlgorithmsTestBase.py

@@ -119,6 +119,9 @@ class AlgorithmsTest(object):
             exec(('\n'.join(defs['expectedFailure'][:-1])), globals(), locals())
             expectFailure = eval(defs['expectedFailure'][-1])
 
+        if 'expectedException' in defs:
+            expectFailure = True
+
         # ignore user setting for invalid geometry handling
         context = QgsProcessingContext()
         context.setProject(QgsProject.instance())

python/plugins/processing/tests/testdata/qgis_algorithm_tests.yaml

@@ -2086,6 +2086,20 @@ tests:
         name: expected/extract_by_attribute_not_null.gml
         type: vector
 
+  - algorithm: native:extractbyattribute
+    name: Extract by attribute (bad field)
+    params:
+      FIELD: notafield
+      INPUT:
+        name: polys.gml
+        type: vector
+      OPERATOR: '9'
+    results:
+      OUTPUT:
+        name: expected/failure.gml
+        type: vector
+    expectedException: true
+
   - algorithm: native:extractbyattribute
     name: Extract by attribute (starts with)
     params:

src/analysis/processing/qgsalgorithmextractbyattribute.cpp

@@ -101,8 +101,10 @@ QVariantMap QgsExtractByAttributeAlgorithm::processAlgorithm( const QVariantMap
   std::unique_ptr< QgsFeatureSink > nonMatchingSink( parameterAsSink( parameters, QStringLiteral( "FAIL_OUTPUT" ), context, nonMatchingSinkId, source->fields(),
       source->wkbType(), source->sourceCrs() ) );
 
-
   int idx = source->fields().lookupField( fieldName );
+  if ( idx < 0 )
+    throw QgsProcessingException( QObject::tr( "Field '%1' was not found in INPUT source" ).arg( fieldName ) );
+
   QVariant::Type fieldType = source->fields().at( idx ).type();
 
   if ( fieldType != QVariant::String && ( op == BeginsWith || op == Contains || op == DoesNotContain ) )