FinalizableReferenceQueue might keep ClassLoaders around

[netdpb]

Based on #92, in 2012 we made sure that using FinalizableReferenceQueue didn't stop ClassLoaders from being destroyed once they were no longer used.

Testing this behavior in JDK 9 revealed some strange failures. FinalizableReferenceQueueClassLoaderUnloadingTest has three tests:

1. testUnloadableWithoutSecurityManager
2. testUnloadableWithSecurityManager
3. testUnloadableInStaticFieldIfClosed

(3) fails in JDK 9, but the others pass. During testing, we tried ignoring (3) in JDK 9. Then, (2) fails in JDK 9 but (1) passes. If we ignore (2) and (3) in JDK 9, then (1) fails.

Tracking down stray references to ClassLoaders for this behavior is difficult, and this heisenbug-like behavior will make it even more difficult.

Since Guava v10, MapMaker no longer uses FinalizableReferenceQueue, and we believe it to be infrequently used in general. Because of that, we're going to ignore all of these tests in JDK 9 and treat this as a known, but low-priority, bug.

Please comment here if you are impacted by this bug.

Please also feel free to work on tracking down the underlying issue. If you fix it, we would welcome a PR.

[YuryYaroshevich]

After running mvn clean package inside of guava-tests dir I didn't see failed tests from FinalizableReferenceQueueClassLoaderUnloadingTest but instead I saw a bunch of failing tests from MoreFilesFileTraverserTest.
My machine setup:

administrators-MacBook-Pro-3:guava yyar$ java -version
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

[jrtom]

@YuryYaroshevich please clarify (a) what version of Guava you're using and (b) what test failures you're seeing; this might be a completely separate issue.

[YuryYaroshevich]

@jrtom ,
I forked guava repo and then I did:

1. cd guava-tests
2. mvn clean test
   and I got these results:

Tests in error: 
  testFileTraverser_multipleDirectoryLayers_breadthFirstStartsWithTopLayer(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017334-0: unable to guarantee security of recursive delete
  testFileTraverser_singleDirectory(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017360-0: unable to guarantee security of recursive delete
  testFileTraverser_emptyDirectory(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017361-0: unable to guarantee security of recursive delete
  testFileTraverser_multipleFilesAndDirectories(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017362-0: unable to guarantee security of recursive delete
  testFileTraverser_nonExistingFile(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017364-0: unable to guarantee security of recursive delete
  testFileTraverser_file(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017366-0: unable to guarantee security of recursive delete
  testFileTraverser_singleFile(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017366-1: unable to guarantee security of recursive delete
  testFileTraverser_multipleDirectoryLayers_traversalReturnsAll(com.google.common.io.MoreFilesFileTraverserTest): /var/folders/5_/f590cn0n36s2sml54ffv3xc4n9rn41/T/1522830017368-0: unable to guarantee security of recursive delete

Tests run: 849670, Failures: 0, Errors: 8, Skipped: 0

[cpovirk]

#3100