Adapt constant folding to ignore functions returning Unknown

Some functions are intended to have side effects and therefore should
not be folded even if all parameters are constants. Currently the
constant folding optimizer throws an error if such a function does
not provide an implementation. This change allows functions with
side effects to return Unknown.

Author: zeitgeist87

cel/folding.go

@@ -134,6 +134,10 @@ func tryFold(ctx *OptimizerContext, a *ast.AST, expr ast.Expr) error {
 	if err != nil {
 		return err
 	}
+	// Don't update the expression if it's an unknown value.
+	if out.Type() == types.UnknownType {
+		return nil
+	}
 	// Update the fold expression to be a literal.
 	ctx.UpdateExpr(expr, ctx.NewLiteral(out))
 	return nil

cel/folding_test.go

@@ -17,12 +17,15 @@ package cel
 import (
 	"reflect"
 	"sort"
+	"strings"
 	"testing"
 
 	"google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/proto"
 
 	"github.com/google/cel-go/common/ast"
+	"github.com/google/cel-go/common/types"
+	"github.com/google/cel-go/common/types/ref"
 
 	proto3pb "github.com/google/cel-go/test/proto3pb"
 	exprpb "google.golang.org/genproto/googleapis/api/expr/v1alpha1"
@@ -313,6 +316,83 @@ func TestConstantFoldingOptimizer(t *testing.T) {
 	}
 }
 
+func TestConstantFoldingCallsWithSideEffects(t *testing.T) {
+	tests := []struct {
+		expr   string
+		folded string
+		error  string
+	}{
+		{
+			expr:   `noSideEffect(3)`,
+			folded: `3`,
+		},
+		{
+			expr:   `withSideEffect(3)`,
+			folded: `withSideEffect(3)`,
+		},
+		{
+			expr:  `noImpl(3)`,
+			error: `constant-folding evaluation failed: no such overload: noImpl`,
+		},
+	}
+	e, err := NewEnv(
+		OptionalTypes(),
+		EnableMacroCallTracking(),
+		Function("noSideEffect",
+			Overload("noSideEffect_int_int",
+				[]*Type{IntType},
+				IntType, FunctionBinding(func(args ...ref.Val) ref.Val {
+					return args[0]
+				}))),
+		Function("withSideEffect",
+			Overload("withSideEffect_int_int",
+				[]*Type{IntType},
+				IntType, FunctionBinding(func(args ...ref.Val) ref.Val {
+					return &types.Unknown{}
+				}))),
+		Function("noImpl",
+			Overload("noImpl_int_int",
+				[]*Type{IntType},
+				IntType)),
+	)
+	if err != nil {
+		t.Fatalf("NewEnv() failed: %v", err)
+	}
+	for _, tst := range tests {
+		tc := tst
+		t.Run(tc.expr, func(t *testing.T) {
+			checked, iss := e.Compile(tc.expr)
+			if iss.Err() != nil {
+				t.Fatalf("Compile() failed: %v", iss.Err())
+			}
+			folder, err := NewConstantFoldingOptimizer()
+			if err != nil {
+				t.Fatalf("NewConstantFoldingOptimizer() failed: %v", err)
+			}
+			opt := NewStaticOptimizer(folder)
+			optimized, iss := opt.Optimize(e, checked)
+			if tc.error != "" {
+				if iss.Err() == nil {
+					t.Errorf("got nil, wanted error containing %q", tc.error)
+				} else if !strings.Contains(iss.Err().Error(), tc.error) {
+					t.Errorf("got %q, wanted error containing %q", iss.Err().Error(), tc.error)
+				}
+				return
+			}
+			if iss.Err() != nil {
+				t.Fatalf("Optimize() generated an invalid AST: %v", iss.Err())
+			}
+			folded, err := AstToString(optimized)
+			if err != nil {
+				t.Fatalf("AstToString() failed: %v", err)
+			}
+			if folded != tc.folded {
+				t.Errorf("got %q, wanted %q", folded, tc.folded)
+			}
+		})
+	}
+}
+
 func TestConstantFoldingOptimizerMacroElimination(t *testing.T) {
 	tests := []struct {
 		expr       string