Improve StringBuilder getChar and putChar robustness with racy updates to StringBuilder. Test racing threads with append, insert, and reading characters.
------------- Commit messages: - 8351000: StringBuilder getChar and putChar robustness Changes: https://git.openjdk.org/jdk/pull/24951/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24951&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8351000 Stats: 314 lines in 2 files changed: 309 ins; 1 del; 4 mod Patch: https://git.openjdk.org/jdk/pull/24951.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/24951/head:pull/24951 PR: https://git.openjdk.org/jdk/pull/24951
