RFR(xs): 8181419: Race in jdwp invoker handling may lead to crashes or invalid results

[Thomas Stüfe]

Hi all,

please take a look at this proposed fix for a theoretical race in the jdwp
library.

Issue: https://bugs.openjdk.java.net/browse/JDK-8181419
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8181419-Race-in-jdwp-invoker-handling-may-lead-to-crashes-or-invalid-results/webrev.00/webrev/

In short, this is an addition to Severin's fix to the jdwp invoke handling (
https://bugs.openjdk.java.net/browse/JDK-8153711).

We have a potential race condition where the delayed cleanup of the saved
returnvalue object reference and the exception reference (released
in deletePotentiallySavedGlobalRefs() ) may be overtaken by a new request
which populates the thread request structure anew. If this happens,
deletePotentiallySavedGlobalRefs() may actually release the return value /
exception references of the follow up request, if that one was already
processed.

The solution I choose is safe and conservative. We still release both
references, but use the locally saved JNI references. We just avoid
accessing the thread local request structure after it has been cleared for
reuse. This keeps timing and locking behaviour unchanged.

I am currently running jtreg tests for com/sun/jdi on AIX and Linux.

Kind Regards, Thomas
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