This PR provides a regression test for [JDK-8325179](https://bugs.openjdk.org/browse/JDK-8325179): _Race in BasicDirectoryModel.validateFileCache_ reviewed in #18111.
The test is inspired and based on `ConcurrentModification` that I wrote for [JDK-8327137](https://bugs.openjdk.org/browse/JDK-8327137) / [JDK-8323670](https://bugs.openjdk.org/browse/JDK-8323670). **How the test works** The test creates a temporary directory in the current directory and creates a number of files in it. (The number of files is controlled by `NUMBER_OF_THREADS` constant). Then the test creates `JFileChooser` in the temporary directory. The test starts several scanner threads, the number is controlled by `NUMBER_OF_THREADS`, which repeatedly call `fileChooser.rescanCurrentDirectory()`. This results in calling `BasicDirectoryModel.validateFileCache` which starts a background thread — "Basic L&F File Loading Thread" — to enumerate the files. The test runner thread and scanner threads are synchronised with `CyclicBarrier`, this ensures all the scanner threads start at the same time. After a short delay, the runner thread takes a snapshot of live threads. After a longer delay, the operation is repeated. See the `getThreadSnapshot` method. (The number of snapshots is controlled by `SNAPSHOTS` constant.) The number of File Loading Threads in each snapshot is counted. There should be no more than two threads. It is possible that thread two such threads after JDK-8325179 is fixed: the existing thread is interrupted but hasn't exited yet, and a new thread is already created. The test fails consistently without the fix for JDK-8325179. On Windows, the output looks like this: Number of snapshots: 20 Number of snapshots where number of loader threads: = 1: 0 = 2: 0 > 2: 20 java.lang.RuntimeException: Detected 20 snapshots with several loading threads at LoaderThreadCount.runTest(LoaderThreadCount.java:132) at LoaderThreadCount.wrapper(LoaderThreadCount.java:72) at java.base/java.lang.Thread.run(Thread.java:1583) On Linux and macOS, there's more variation, for example I got the following output on one of Linux systems: Number of snapshots: 15 Number of snapshots where number of loader threads: = 1: 7 = 2: 2 > 2: 6 The test passes on the builds where JDK-8325179 is present. ------------- Commit messages: - Add copyright header; ensure headless mode (for macOS) - Merge master - Remove printing number of loader threads in each snapshot - Add comments to clarify how thread snapshots are captured - Use > 2 condition for error case - Add a test for JDK-8325179: count FilesLoader threads Changes: https://git.openjdk.org/jdk/pull/18957/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18957&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8331142 Stats: 263 lines in 1 file changed: 263 ins; 0 del; 0 mod Patch: https://git.openjdk.org/jdk/pull/18957.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/18957/head:pull/18957 PR: https://git.openjdk.org/jdk/pull/18957
