RFR: 8072701: resume001 failed due to ERROR: timeout for waiting for a BreakpintEvent [v3]

[Alex Menkov]

On Fri, 12 Jul 2024 04:08:25 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> test/hotspot/jtreg/vmTestbase/nsk/jdi/ThreadReference/resume/resume001.java line 382:
>> 
>>> 380:             if (expresult != returnCode0) {
>>> 381:                 vm.resume();
>>> 382:                 vm.resume();  // for case error when both VirtualMachine and the thread2 were suspended
>> 
>> Pre-existing but I don't understand the comment. Why would you need 2 `vm.resume()` here? If `thread2` was suspended directly don't you need a `thread2.resume()`?
>
> First just to clarify a general JDI feature about thread suspending and resuming. You can undo a ThreadReference.suspend() or a thread suspended as the result of an event by dong a vm.resume(). This is documented in the JDI API spec, which talks about suspendCounts and how various APIs and event delivery affect them.
> 
> I was  tempted to clean up these vm.resume() calls but opted not to. The point being made in the comment is that worse case thread2 was suspended by a breakpoint or thread2.suspend() and all threads were suspended by a vm.resume() (meaning thread2 could have a suspendCount of 2). Two vm.resumes() take are done to make sure thread2 gets resumed under this situation. However, one of the vm.resume calls could instead be a thread2.resume(). Doing two vm.resume() calls was probably just laziness by the original  authors. It works though.
> 
> However, by my accounting at any failure point thread2 never has a suspendCount > 1, so really just one vm.resume() would be enough.
> 
> The original code did these two vm.resume() calls unconditionally, but they are not needed if there was no error.

The original code had 2 vm.resume() - one on them to match vm.suspend() and 2nd one to allow debugee to continue on error.
Now we have 3 vm.resume() - one is to match vm.suspend() (line 377) and 2 conditional (on error).
Theoretically we can get an error when both vm and thread2 are suspended, so 2 vm.resume() looks reasonable.
Anyway resume() is a nop if the thread is not suspended

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PR Review Comment: https://git.openjdk.org/jdk/pull/20088#discussion_r1675474511