Incremental Java compile is disabled when empty source folders are added

[hungvietnguyen]

Environment

Gradle 5.1-rc-2 (in Gradle 5.1-rc-3 and above, the issue is masked by a different issue: #8194)

Steps to Reproduce

Use this sample Android project: MyApplication.tar.gz

1. Run the first build: ./gradlew :app:compileDebugJavaWithJavac, it should succeed.
2. In app/src/main/java, add package com.example.foo without any classes.
3. Run the second build: ./gradlew :app:compileDebugJavaWithJavac --info, observe that Gradle performs full recompilation:
   Full recompilation is required because A resource changed. Analysis took 0.053 secs.

Expected Behavior

Gradle should perform incremental compile in the second build.

Context

I saw this issue a while ago but can only manage to reproduce it now while debugging #8194 and #8197. It doesn't happen on a HelloWorld project, but it does on this particular project. (I haven't examined how this project exposes the issue yet.)

[hungvietnguyen]

Additional Request

The error message Full recompilation is required because A resource changed may be more helpful if it also points out exactly what resource has changed.

[wolfs]

@oehme It looks like changes to directories (i.e. adding directories in source folders) are processed as resource changes:

gradle/subprojects/language-java/src/main/java/org/gradle/api/internal/tasks/compile/incremental/recomp/InputChangeAction.java

Lines 45 to 53 in 8ce7ea5

	File file = input.getFile();
	if (hasExtension(file, ".java")) {
	javaChangeProcessor.processChange(input, spec);
	} else if (hasExtension(file, ".jar") || hasExtension(file, ".class")) {
	annotationProcessorChangeProcessor.processChange(input, spec);
	} else {
	resourceChangeProcessor.processChange(input, spec);
	}
	}

Is this intentional? I guess it would be fine to only process actual files. WDYT?

[oehme]

Yes, good catch!

[ljacomet]

@bigdaz Ended up taking a look at it and opened #8211 - feel free to take over after my day if that solution is not satisfactory