fix: handle empty variable list in PrepareWhereClause (#9126)

pkg/querybuilder/where_clause_visitor.go

@@ -437,6 +437,10 @@ func (v *filterExpressionVisitor) VisitComparison(ctx *grammar.ComparisonContext
 					}
 					switch varValues := varItem.Value.(type) {
 					case []any:
+						if len(varValues) == 0 {
+							v.errors = append(v.errors, fmt.Sprintf("malformed request payload: variable `%s` used in expression has an empty list value", strings.TrimPrefix(var_, "$")))
+							return ""
+						}
 						values = varValues
 					case any:
 						values = []any{varValues}
@@ -516,6 +520,10 @@ func (v *filterExpressionVisitor) VisitComparison(ctx *grammar.ComparisonContext
 			if ok {
 				switch varValues := varItem.Value.(type) {
 				case []any:
+					if len(varValues) == 0 {
+						v.errors = append(v.errors, fmt.Sprintf("malformed request payload: variable `%s` used in expression has an empty list value", strings.TrimPrefix(var_, "$")))
+						return ""
+					}
 					value = varValues[0]
 				case any:
 					value = varValues

pkg/querybuilder/where_clause_visitor_test.go

@@ -0,0 +1,75 @@
+package querybuilder
+
+import (
+	"strings"
+	"testing"
+
+	qbtypes "github.com/SigNoz/signoz/pkg/types/querybuildertypes/querybuildertypesv5"
+	"github.com/SigNoz/signoz/pkg/types/telemetrytypes"
+)
+
+// testFieldKey returns a mock TelemetryFieldKey for the given name
+func testFieldKey(name string) *telemetrytypes.TelemetryFieldKey {
+	return &telemetrytypes.TelemetryFieldKey{
+		Name:          name,
+		Signal:        telemetrytypes.SignalLogs,
+		FieldContext:  telemetrytypes.FieldContextAttribute,
+		FieldDataType: telemetrytypes.FieldDataTypeString,
+	}
+}
+
+// TestPrepareWhereClause_EmptyVariableList ensures PrepareWhereClause errors when a variable has an empty list value
+func TestPrepareWhereClause_EmptyVariableList(t *testing.T) {
+	tests := []struct {
+		name        string
+		expr        string
+		variables   map[string]qbtypes.VariableItem
+		expectError bool
+		wantInError string
+	}{
+		{
+			name: "Empty []any for equality",
+			expr: "service = $service",
+			variables: map[string]qbtypes.VariableItem{
+				"service": {Value: []any{}},
+			},
+			expectError: true,
+			wantInError: "Found 1 errors while parsing the search expression",
+		},
+		{
+			name: "Empty []any for IN clause",
+			expr: "service IN $service",
+			variables: map[string]qbtypes.VariableItem{
+				"service": {Value: []any{}},
+			},
+			expectError: true,
+			wantInError: "Found 1 errors while parsing the search expression",
+		},
+	}
+
+	keys := map[string][]*telemetrytypes.TelemetryFieldKey{
+		"service": {testFieldKey("service")},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			opts := FilterExprVisitorOpts{
+				FieldKeys: keys,
+				Variables: tt.variables,
+			}
+
+			_, err := PrepareWhereClause(tt.expr, opts, 0, 0)
+
+			if tt.expectError {
+				if err == nil {
+					t.Fatalf("expected error, got nil")
+				}
+				if tt.wantInError != "" && !strings.Contains(err.Error(), tt.wantInError) {
+					t.Fatalf("expected error to contain %q, got %q", tt.wantInError, err.Error())
+				}
+			} else if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+		})
+	}
+}