Regression in 8.6.0: groovyGradle/greclipse fails with "roundtripStateInternal or equalityStateInternal should be non-null" during input fingerprinting

Start by reproducing the failure with a clean Gradle user home using ./gradlew spotlessCheck --no-daemon --no-build-cache --stacktrace. Read FormatterStepSerializationRoundtrip.java, ConfigurationCacheHackList.java, and the related serialization classes along the shown stack trace. Done means the freshly provisioned greclipse task no longer fails during input fingerprinting.

Summary

Starting with Spotless 8.6.0, the groovyGradle { greclipse() } task fails with:

If the initializer was null, then one of roundtripStateInternal or equalityStateInternal should be non-null, and neither was

This is a regression: 8.4.0 and 8.5.0 work with the exact same project/config/Gradle version; 8.6.0 fails. Pinning back to 8.5.0 is our current workaround.

It is closely related to (and may be the same root cause as) #2927 and #2502, but we can pinpoint it to the 8.5.0 → 8.6.0 bump, and it does not require the configuration cache (it happens with config cache disabled / not configured).

Environment

• Spotless Gradle plugin 8.6.0 (8.4.0 and 8.5.0 OK)
• Gradle 9.5.1
• JDK 26 (Temurin) — also reproduces on the same setup regardless
• Formatter: groovyGradle { greclipse() } (default greclipse → Groovy-Eclipse 5.7.0/e4.35, Groovy 5.0.0)
• Configuration cache: not enabled
• Build cache: disabled on the command line (--no-build-cache) — does not affect the outcome

Spotless config (relevant part)

spotless {
    groovyGradle {
        toggleOffOn()
        greclipse()
        leadingTabsToSpaces()
    }
}

How to reproduce

Run on a clean Gradle user home (e.g. a fresh CI container where greclipse is provisioned from scratch):

./gradlew spotlessCheck --no-daemon --no-build-cache --stacktrace

:spotlessGroovyGradle fails. It does not reproduce against a warm local cache where greclipse is already provisioned — which suggests the failure is tied to the freshly-built greclipse step state rather than to any formatting violation.

Stack trace

The exception is thrown while Gradle fingerprints the task's input properties (before execution), when it calls hashCode() on Spotless's ConfigurationCacheHackList:

Caused by: java.lang.IllegalStateException: If the initializer was null, then one of roundtripStateInternal or equalityStateInternal should be non-null, and neither was
	at com.diffplug.spotless.FormatterStepSerializationRoundtrip.writeObject(FormatterStepSerializationRoundtrip.java:74)
	at com.diffplug.spotless.FormatterStepSerializationRoundtrip$HackClone.writeObject(FormatterStepSerializationRoundtrip.java:121)
	at com.diffplug.spotless.ConfigurationCacheHackList.writeObject(ConfigurationCacheHackList.java:78)
	at com.diffplug.spotless.LazyForwardingEquality.toBytes(LazyForwardingEquality.java:110)
	at com.diffplug.spotless.FormatterStepEqualityOnStateSerialization.lambda$serializedState$0(FormatterStepEqualityOnStateSerialization.java:95)
	at com.diffplug.spotless.ThrowingEx.get(ThrowingEx.java:67)
	at com.diffplug.spotless.FormatterStepEqualityOnStateSerialization.serializedState(FormatterStepEqualityOnStateSerialization.java:95)
	at com.diffplug.spotless.FormatterStepEqualityOnStateSerialization.hashCode(FormatterStepEqualityOnStateSerialization.java:75)
	at com.diffplug.spotless.FormatterStepSerializationRoundtrip$HackClone.hashCode(FormatterStepSerializationRoundtrip.java:142)
	at com.diffplug.spotless.ConfigurationCacheHackList.hashCode(ConfigurationCacheHackList.java:151)
	at org.gradle.internal.execution.impl.DefaultInputFingerprinter$InputCollectingVisitor.visitInputProperty(DefaultInputFingerprinter.java:131)
	at org.gradle.api.internal.tasks.execution.TaskExecution.visitMutableInputs(TaskExecution.java:336)
	at org.gradle.internal.execution.impl.DefaultInputFingerprinter.fingerprintInputProperties(DefaultInputFingerprinter.java:77)
	at org.gradle.internal.execution.steps.CaptureMutableStateBeforeExecutionStep.captureExecutionStateWithOutputs(CaptureMutableStateBeforeExecutionStep.java:124)
	...
	at org.gradle.internal.execution.steps.LoadPreviousExecutionStateStep.executeMutable(LoadPreviousExecutionStateStep.java:36)

Analysis / likely cause

• The failure is in FormatterStepSerializationRoundtrip.writeObject (line 74), which asserts that if initializer == null then one of roundtripStateInternal / equalityStateInternal must be non-null — and here both are null.
• It is reached via ConfigurationCacheHackList.hashCode() during Gradle's input fingerprinting (DefaultInputFingerprinter / CaptureMutableStateBeforeExecutionStep), i.e. before the task action runs — so it triggers for ordinary up-to-date checking, independent of the build cache and without the configuration cache.
• Because it only reproduces with a freshly provisioned greclipse (clean Gradle user home) and not against a warm cache, the greclipse step's state appears to be left without a computed roundtripStateInternal/equalityStateInternal (initializer already consumed) at the point Gradle serializes it for the fingerprint.

Workaround

Pin the Spotless Gradle plugin to 8.5.0.

Happy to provide a minimal reproducer repo or extra logs (--info/--debug) if helpful.