Allow setting a QgsFeedback object in QgsExpressionContext

This can be checked by expression functions which are costly to
evaluate (e.g. those which fetch features from a layer) and which
would benefit from early exits when the results of the expression
evaluation are no longer needed (e.g. due to canceling a layer
rendering, etc)

python/core/auto_generated/qgsexpressioncontext.sip.in

@@ -783,6 +783,29 @@ Clears all cached values from the context.
 .. seealso:: :py:func:`cachedValue`
 
 .. versionadded:: 2.16
+%End
+
+    void setFeedback( QgsFeedback *feedback );
+%Docstring
+Attach a ``feedback`` object that can be queried regularly by the expression engine to check
+if expression evaluation should be canceled.
+
+Ownership of ``feedback`` is NOT transferred, and the caller must take care that it exists
+for the lifetime of the expression context.
+
+.. seealso:: :py:func:`feedback`
+
+.. versionadded:: 3.20
+%End
+
+    QgsFeedback *feedback() const;
+%Docstring
+Returns the feedback object that can be queried regularly by the expression to check
+if evaluation should be canceled, if set.
+
+.. seealso:: :py:func:`setFeedback`
+
+.. versionadded:: 3.20
 %End
 
     static const QString EXPR_FIELDS;

src/core/qgsexpressioncontext.cpp

@@ -592,3 +592,13 @@ void QgsExpressionContext::clearCachedValues() const
 {
   mCachedValues.clear();
 }
+
+void QgsExpressionContext::setFeedback( QgsFeedback *feedback )
+{
+  mFeedback = feedback;
+}
+
+QgsFeedback *QgsExpressionContext::feedback() const
+{
+  return mFeedback;
+}

src/core/qgsexpressioncontext.h

@@ -715,6 +715,29 @@ class CORE_EXPORT QgsExpressionContext
      */
     void clearCachedValues() const;
 
+    /**
+     * Attach a \a feedback object that can be queried regularly by the expression engine to check
+     * if expression evaluation should be canceled.
+     *
+     * Ownership of \a feedback is NOT transferred, and the caller must take care that it exists
+     * for the lifetime of the expression context.
+     *
+     * \see feedback()
+     *
+     * \since QGIS 3.20
+     */
+    void setFeedback( QgsFeedback *feedback );
+
+    /**
+     * Returns the feedback object that can be queried regularly by the expression to check
+     * if evaluation should be canceled, if set.
+     *
+     * \see setFeedback()
+     *
+     * \since QGIS 3.20
+     */
+    QgsFeedback *feedback() const;
+
     //! Inbuilt variable name for fields storage
     static const QString EXPR_FIELDS;
     //! Inbuilt variable name for value original value variable
@@ -748,6 +771,8 @@ class CORE_EXPORT QgsExpressionContext
     QStringList mHighlightedVariables;
     QStringList mHighlightedFunctions;
 
+    QgsFeedback *mFeedback = nullptr;
+
     // Cache is mutable because we want to be able to add cached values to const contexts
     mutable QMap< QString, QVariant > mCachedValues;
 

src/core/qgsfeaturerequest.cpp

@@ -340,7 +340,7 @@ void QgsFeatureRequest::setFeedback( QgsFeedback *feedback )
   mFeedback = feedback;
 }
 
-QgsFeedback *QgsFeatureRequest::feedback()
+QgsFeedback *QgsFeatureRequest::feedback() const
 {
   return mFeedback;
 }