RFR: 7903441: System.out and System.err messages are missing in jtr file when a test times out in agentvm mode [v2]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to fix the issue reported in https://bugs.openjdk.org/browse/CODETOOLS-7903441?
> 
> As noted in that issue, when a test timeouts when run in the agent vm mode, it has been noticed that the System.out/System.err messages that were written by the test are lost and not present in the .jtr file of that test.
> 
> There are 2 parts to this issue and this PR thus has 2 separate commits to help review this.
> 
> Agent VM test execution involves communicating between processes - the jtreg process (a.k.a `Agent`) and the process that is executing the test case (a.k.a `AgentServer`), through sockets. One part of this communication involves sending across the System.out and System.err generated messages from the `AgentServer` VM to the `Agent`. As part of https://bugs.openjdk.org/browse/CODETOOLS-7902198, work was done to help send across these messages in a timely fashion. One part of that change involves converting the byte oriented messages into characters. This is done using a `CharsetDecoder` which decodes `ByteBuffer` contents to a `CharBuffer`. Once the decoded content lands into the `charBuffer` it stays in that buffer until either the OutputStream is closed or the `charBuffer` capacity has been reached. Even a `flush()` call doesn't push out these decoded characters into the target/underlying OutputStream and that results in lost messages. The commit https://github.com/openjdk/jtreg/c
 ommit/578e103dcb911c07c59007138e62a78c3abba6d7 in this PR addresses that by making sure `flush()` operation on the `OutputStream` does indeed flush the decoded characters to the underlying stream, thus allowing it to reach the other process.
> 
> In the current jtreg code, the compile action and main action implementation on the `AgentServer` side both use `autoFlush=true` for `System.err` but not for `System.out`, I couldn't find an explanation why `System.out` explicitly sets `autoFlush=false`. The additional commit https://github.com/openjdk/jtreg/commit/1d08b7656d46c124730c6844fefc595804266d36 in this PR addresses this by setting `System.out` as `autoFlush=true`. Auto-flush is defined by `java.io.PrintStream` as:
> 
>> Optionally, a {@code PrintStream} can be created so as to flush automatically; this means that the {@code flush} method of the underlying output stream is automatically invoked after a byte array is written, one of the {@code println} methods is invoked, or a newline character or byte ({@code '\n'}) is written.
> ...

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  review suggestion - fix grammar in code comment

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

Changes:
  - all: https://git.openjdk.org/jtreg/pull/155/files
  - new: https://git.openjdk.org/jtreg/pull/155/files/1d08b765..e062605b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=155&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=155&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/155.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/155/head:pull/155

PR: https://git.openjdk.org/jtreg/pull/155