RFR: 8332065: Calling readLine(null...) or readPassword(null...) on System.console() hangs jshell

[Jan Lahoda]

JShell supports using `System.console()` when running snippets in a separate VM (the agent). This works by re-sending requests from the agent to the main process.

For some `Console` methods, like `readLine`, this has two phases - first a prompt is sent, and then input is received. But, there's a mistake on the way this is handled: the protocol starts a "transaction", and then goes to process the prompt. And if the prompt processing fails, the "transaction" never ends, and the communication locks.

The proposal here is to do the prompt processing before starting the communication, so if it fails, there are no problems with the communication.

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Commit messages:
 - Improving tests and implementation.
 - 8332065: Calling readLine(null...) or readPassword(null...) on System.console() hangs jshell

Changes: https://git.openjdk.org/jdk/pull/19420/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19420&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332065
  Stats: 30 lines in 2 files changed: 24 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19420.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19420/head:pull/19420

PR: https://git.openjdk.org/jdk/pull/19420