Silent test failure: StackOverflowError in SpockComparisonFailure.toString() for objects with circular toString()

Describe the bug

Problem

When a Spock test fails with an equality assertion involving an object that has a circular toString(), SpockComparisonFailure.toString() triggers a StackOverflowError. This causes test runners like Surefire to silently drop the test result, making failures invisible. The build appears green.

Root cause

1. Spock test fails, SpockComparisonFailure embeds actual/expected values
2. SpockComparisonFailure.toString() calls getMessage() calls condition.getRendering()
3. Condition rendering calls toString() on the assertion values via ExpressionInfoValueRenderer -> RenderUtil.toStringOrDump()
4. ExpressionInfoValueRenderer.renderValue() catches Exception but not Error
5. StackOverflowError propagates, crashing the test runner's reporting

Comparison with JUnit 5

JUnit 5 does not have this issue. AssertionFailedError stores values without triggering recursive toString() during reporting.

Reproduction

An object with a circular toString() (e.g. self field pointing back to the same instance) used in a failing equality assertion is enough to trigger the bug.

AI disclosure

The analysis of the root cause was developed with significant assistance from an AI coding assistant. The bug was first identified in a separate MRE project using Surefire, then traced through the Spock source code with the AI's help.

To Reproduce

https://github.com/rmie/spock/tree/fix/silent-failure-circular-tostring

Expected behavior

at a minimum, the must not fail silently

Actual behavior

it fails silently

Java version

all tested version are affected

Buildtool version

all tested version are affected

What operating system are you using

Windows

Dependencies

https://github.com/rmie/spock/actions/runs/33632078870

Additional context

n/a

Use the linked reproduction to trigger a failing equality assertion with an object whose toString() is circular. Trace SpockComparisonFailure.toString() through getMessage(), condition.getRendering(), ExpressionInfoValueRenderer.renderValue(), and RenderUtil.toStringOrDump(); done means the failure is reported instead of silently terminating, with a regression test for the reproduction.