TestKit is broken when tests execute Java compilation and debugging is turned on

[hegyibalint]

Current behavior

When:

• TestKit is used to execute a build
• The build does something related to a Java compilation
• And the executor is used with debugging on

The test will fail with an IllegalAccessError:

Caused by: java.lang.IllegalAccessError: class org.gradle.api.internal.tasks.compile.JdkJavaCompiler (in unnamed module @0x25377fa1) cannot access class com.sun.tools.javac.util.Context (in module jdk.compiler) because module jdk.compiler does not export com.sun.tools.javac.util to unnamed module @0x25377fa1	
	at org.gradle.api.internal.tasks.compile.JdkJavaCompiler.<init>(JdkJavaCompiler.java:59)	
	at org.gradle.api.internal.tasks.compile.DefaultJavaCompilerFactory.createTargetCompiler(DefaultJavaCompilerFactory.java:93)

Expected Behavior

TestKit-driven tests should not fail when debugging is on.

Context

The modifications done in order to extract more structured compiler information (#28281) made it necessary to have new --add-exports flags:

gradle/platforms/core-runtime/base-services/src/main/java/org/gradle/internal/jvm/JpmsConfiguration.java

Lines 40 to 41 in b47610e

	"--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED", // Required by JdkTools and JdkJavaCompiler
	"--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED" // Required by JdkTools and JdkJavaCompiler

The modification above made sure that normally executed builds were executed with the correct flags.
We've made a mistake and not expanded the list of JVM flags passed to the test executor set by java-gradle-plugin.

This made the behavior differ:

• When debugging is off, the Gradle runner is forked, and our standard mechanism adds the flags to the forked java process.
• When debugging is on, the Gradle runner is embedded inside the test JVM (i.e. no forking happens), and this necessitates that you already have a JVM with the correct flags applied; otherwise, the IllegalAccessError presented above will happen.