The test is failing because it is detecting an extra instance of `TestClass1`.
The test (the debugger side) first tells the debuggee to create 10 instances of
`TestClass1`. The debugger then uses JDI `ClassType.newInstance()` to create
100 more instances. It then resumes the debuggee and uses
`RefrenceType.instances()` to find out how many instances of `TestClass1` are
reachable. Since the 100 created by `ClassType.newInstance()` should not have
any references keeping them live, the answer should be 10, but sometimes it
ends up being 11, so there is an extra instance.
I determined that this extra instance is always the last of the 100 that are
created with `ClassType.newInstance()`. It uses the JDI/JDWP invoker interface.
I found the following code in the debug agent invoker.c to be the problem:
if (!detached) {
outStream_initReply(&out, id);
(void)outStream_writeValue(env, &out, tag, returnValue);
(void)outStream_writeObjectTag(env, &out, exc);
(void)outStream_writeObjectRef(env, &out, exc);
outStream_sendReply(&out);
}
…
if (mustReleaseReturnValue && returnValue.l != NULL) {
tossGlobalRef(env, &returnValue.l);
}
The first block is responsible for sending the reply to the debugger for the
JDI `ClassType.newInstance()` call. `returnValue` is a JNI global ref to the
object that was just created, and `tossGlobalRef()` frees it after the reply
packet has been sent. The problem is that once the reply packet has been
received by the debugger (for the 100th `TestClass1` allocation), it resumes
the debuggee and issues the `ReferenceType.instances()` call. This might be
handled by the debug agent before it ever gets to the `tossGlobalRef()` call.
So there will still be a reference to the 100th `TestClass1` object.
The fix is to call `tossGlobalRef()` after we are done with `returnValue`, but
before sending out the packet. We are done with `returnValue` once the
`outStream_writeValue()` call has been made. I decided to handle `exc` (the
exception object) in the same manner. Although no tests were failing as a
result of releasing it after sending the reply, I think you could write a test
that triggered an exception and verified that the exception was not still
considered live after doing the resume.
Regarding any concerns you might have for moving `tossGlobalRef()` code from
outside the `if (!detached)` to inside, if you follow the logic of this
function you'll see that `mustReleaseReturnValue` can only be set true if
`detached` is false. You'll also see that `exc` can only be non-null if
`detached` is false. Thus these two `tossGlobalRef()` calls were only ever made
when `detached` was false, and that remains true after my changes.
Regarding any concerns you might have for making the `tossGlobalRef()` calls
outside of the locks, the locking is a remnant from when the `exception` and
`returnValue` fields were referenced directly out of the `InvokeRequest`
struct, which could be accessed by other threads. That is no longer the case
after changes were made for
[JDK-8181419](https://bugs.openjdk.java.net/browse/JDK-8181419), which copied
the fields into local variables. This code actually has been subject to a
pretty long bug tail. See the last couple of long comments by me in
[JDK-8176567](https://bugs.openjdk.java.net/browse/JDK-8176567) for details.
-------------
Commit messages:
- Free invoker result and exception refs before sending the reply packet.
Changes: https://git.openjdk.java.net/jdk/pull/6943/files
Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6943&range=00
Issue: https://bugs.openjdk.java.net/browse/JDK-8176567
Stats: 30 lines in 1 file changed: 14 ins; 15 del; 1 mod
Patch: https://git.openjdk.java.net/jdk/pull/6943.diff
Fetch: git fetch https://git.openjdk.java.net/jdk pull/6943/head:pull/6943
PR: https://git.openjdk.java.net/jdk/pull/6943
