RFR: JDK-8300416 java.security.MessageDigestSpi clone can result in thread-unsafe clones [v4]

[Mark Powers]

> https://bugs.openjdk.org/browse/JDK-8300416

Mark Powers has updated the pull request incrementally with one additional commit since the last revision:

  added comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12348/files
  - new: https://git.openjdk.org/jdk/pull/12348/files/bcf95a20..d35bc54f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12348&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12348&range=02-03

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12348.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12348/head:pull/12348

PR: https://git.openjdk.org/jdk/pull/12348