RFR: JDK-8325530: Vague error message when com.sun.tools.attach.VirtualMachine fails to load agent library [v3]

[Chris Plummer]

On Thu, 22 Feb 2024 21:43:23 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

>> 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:
> 
>   feedback

In invoke_Agent_OnAttach(), I think the second print_cr() should start with a space:


      st->print_cr("%s was not loaded.", agent->name());
      if (*ebuf != '\0') {
        st->print_cr("%s", &ebuf[0]);
      }
      return false;


Otherwise in the output you see the reason for the failure start right after the period:

`com.sun.tools.attach.AgentLoadException: Failed to load agent library: FailedLoadAgentTestNotExists was not loaded.libFailedLoadAgentTestNotExists.so: cannot open shared object file: No such file or directory`

Also, ebuf is of size 1024 and is getting truncated sometimes. See your macosx log files on mdash. This doesn't necessarily have to be fixed, but something to consider.

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PR Comment: https://git.openjdk.org/jdk/pull/17954#issuecomment-1960431370