New complexity package

complexity/complexity.go

@@ -0,0 +1,85 @@
+package complexity
+
+import (
+	"github.com/99designs/gqlgen/graphql"
+	"github.com/vektah/gqlparser/ast"
+)
+
+func Calculate(es graphql.ExecutableSchema, op *ast.OperationDefinition, vars map[string]interface{}) int {
+	walker := complexityWalker{
+		es:   es,
+		vars: vars,
+	}
+	typeName := ""
+	switch op.Operation {
+	case ast.Query:
+		typeName = es.Schema().Query.Name
+	case ast.Mutation:
+		typeName = es.Schema().Mutation.Name
+	case ast.Subscription:
+		typeName = es.Schema().Subscription.Name
+	}
+	return walker.selectionSetComplexity(typeName, op.SelectionSet)
+}
+
+type complexityWalker struct {
+	es   graphql.ExecutableSchema
+	vars map[string]interface{}
+}
+
+func (cw complexityWalker) selectionSetComplexity(typeName string, selectionSet ast.SelectionSet) int {
+	var complexity int
+	for _, selection := range selectionSet {
+		switch s := selection.(type) {
+		case *ast.Field:
+			var childComplexity int
+			switch s.ObjectDefinition.Kind {
+			case ast.Object, ast.Interface, ast.Union:
+				childComplexity = cw.selectionSetComplexity(s.ObjectDefinition.Name, s.SelectionSet)
+			}
+
+			args := s.ArgumentMap(cw.vars)
+			if customComplexity, ok := cw.es.Complexity(typeName, s.Name, childComplexity, args); ok {
+				complexity = safeAdd(complexity, customComplexity)
+			} else {
+				// default complexity calculation
+				complexity = safeAdd(complexity, safeAdd(1, childComplexity))
+			}
+
+		case *ast.FragmentSpread:
+			complexity = safeAdd(complexity, cw.selectionSetComplexity(typeName, s.Definition.SelectionSet))
+
+		case *ast.InlineFragment:
+			complexity = safeAdd(complexity, cw.selectionSetComplexity(typeName, s.SelectionSet))
+		}
+	}
+	return complexity
+}
+
+// safeAdd is a saturating add of a and b that ignores negative operands.
+// If a + b would overflow through normal Go addition,
+// it returns the maximum integer value instead.
+//
+// Adding complexities with this function prevents attackers from intentionally
+// overflowing the complexity calculation to allow overly-complex queries.
+//
+// It also helps mitigate the impact of custom complexities that accidentally
+// return negative values.
+func safeAdd(a, b int) int {
+	// Ignore negative operands.
+	if a <= 0 {
+		if b < 0 {
+			return 0
+		}
+		return b
+	} else if b <= 0 {
+		return a
+	}
+
+	c := a + b
+	if c < a {
+		// Set c to maximum integer instead of overflowing.
+		c = int(^uint(0) >> 1)
+	}
+	return c
+}