[RFR] 8196969: JTreg Failure: serviceability/sa/ClhsdbJstack.java causes NPE

[Severin Gehwolf]

Anyone? Chris maybe?

On Fri, 2019-09-27 at 14:12 +0200, Severin Gehwolf wrote:
> Hi,
> 
> Could I please get reviews for this SA fix? The issue only happens
> intermittently and with -Xcomp. The new regression test reproduces the
> issue somewhat reliably. I got 10/10 fails for unpatched, but I've seen
> it pass as well.
> 
> When the issue happens, PCDesc's getScopeDecodeOffset() returs 0
> (DebugInformationRecorder.SERIALIZED_NULL). The current SA code doesn't
> handle this case and goes on and tries to read ScopeDesc from the
> DebugInfoReadStream at the bogus offset. From then on, bad things
> happen. A NPE in StackTrace could be one symptom.
> 
> The same code in hotspot deals with serialized null differently. It
> doesn't read from the debug info stream, and manually sets up a
> reasonable frame. Note decode_body is called from ScopeDesc's
> constructor where decode_offset might have been set to 0:
> 
> void ScopeDesc::decode_body() {
>   if (decode_offset() == DebugInformationRecorder::serialized_null) {
>     // This is a sentinel record, which is only relevant to
>     // approximate queries.  Decode a reasonable frame.
>     _sender_decode_offset = DebugInformationRecorder::serialized_null;
>     _method = _code->method();
>     _bci = InvocationEntryBci;
>     _locals_decode_offset = DebugInformationRecorder::serialized_null;
>     _expressions_decode_offset = DebugInformationRecorder::serialized_null;
>     _monitors_decode_offset = DebugInformationRecorder::serialized_null;
>   } else {
>     // decode header
>     DebugInfoReadStream* stream  = stream_at(decode_offset());
> 
>     _sender_decode_offset = stream->read_int();
>     _method = stream->read_method();
>     _bci    = stream->read_bci();
> 
>     // decode offsets for body and sender
>     _locals_decode_offset      = stream->read_int();
>     _expressions_decode_offset = stream->read_int();
>     _monitors_decode_offset    = stream->read_int();
>   }
> }
> 
> The proposed patch handles serialized null scopes similar to the
> hotspot side of things, by returning a null scope. CompiledVFrame
> already deals with null scopes when in debugging mode.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8196969
> webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8196969/03/webrev/
> 
> Testing: tier 1 tests on Linux x86_64 (release/fastdebug). jdk-submit
> and ran various reproducer tests including 1000 interations of the
> added regression test. All pass.
> 
> Thoughts?
> 
> Thanks,
> Severin