RFR: JDK-8325530: Vague error message when com.sun.tools.attach.VirtualMachine fails to load agent library [v4]

[Alex Menkov]

> VirtualMachine.loadAgentPath/loadAgentLibrary can fail with AgentLoadException in 2 cases:
> - attach listener returns error; in the case the exception is thrown from HotSpotVirtualMachine.processCompletionStatus (called from HotSpotVirtualMachine.execute);
> - attach listener returns success, but reply does not contain Agent_onAttach return code ("return code: %d" message).
> 
> before jdk21 if attach listener fails to load required library, it returned error (case 1)
> from jdk21 attach listener always returns success (case 2)
> Both cases are ok, but for 2nd case HotSpotVirtualMachine.loadAgentLibrary read only single line of the response message, so exception message didn't contain error details.
> 
> The fix updates HotSpotVirtualMachine.loadAgentLibrary to read the whole response message.
> Walking through the code, fixed some other minor things in attach listener and attach API implementation (commented in the code)
> 
> Testing:
>   - test/jdk/com/sun/tools;
>   - tier1,tier2,tier5-svc

Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:

  Fixed readErrorMessage()

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17954/files
  - new: https://git.openjdk.org/jdk/pull/17954/files/dcb1db0d..66b24479

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17954&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17954&range=02-03

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17954.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17954/head:pull/17954

PR: https://git.openjdk.org/jdk/pull/17954