RFR: 8325179: Race in BasicDirectoryModel.validateFileCache [v2]

[Alexey Ivanov]

> Ensure access to the `filesLoader` field of `BasicDirectoryModel` is synchronized.
> 
> Without synchronization, a thread checks if `filesLoader` is not null and creates a new `FilesLoader` thread. If the thread is pre-empted between these two operations, another thread or even several threads can see the `null` value and create new `FilesLoader` threads.
> 
> The same way, `BasicDirectoryModel.invalidateFileCache` needs to be synchornized to interrupt the current `filesLoader` and assign `null`.
> 
> This bug allows reproducing `ConcurrentModificationException` seen in [JDK-8323670](https://bugs.openjdk.org/browse/JDK-8323670) and [JDK-8307091](https://bugs.openjdk.org/browse/JDK-8307091) using the test in PR #18109.

Alexey Ivanov has updated the pull request incrementally with four additional commits since the last revision:

 - Replace synchronized invalidateFileCache with synchronized block inside
 - Declare DoChangeContents constructor private, wrap its parameters
 - Space after synchronized in DoChangeContents.run
 - Convert runnable to local variable

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18111/files
  - new: https://git.openjdk.org/jdk/pull/18111/files/4215925b..a0659236

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18111&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18111&range=00-01

  Stats: 13 lines in 1 file changed: 3 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/18111.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18111/head:pull/18111

PR: https://git.openjdk.org/jdk/pull/18111