[15?] RFR (S): 8249192: MonitorInfo stores raw oops across safepoints

[Thomas Schatzl]

Hi all,

   can I get some reviews to handle'ize some raw oops in the MonitorInfo 
class?

(Afaiu only) in LiveFrameStream::monitors_to_object_array() we try to 
allocate an objArray with raw oops held in the MonitorInfo class that 
are passed in a GrowableArray. This allocation can lead to a garbage 
collection, with the usual random crashes.

This change changes the raw oops in MonitorInfo to Handles, and adds a 
few HandleMarks along the way to make these handles go away asap.

This issue has been introduced in JDK-8140450: Implement Stack-Walking 
API in jdk9.

The CR has been triaged as P3, but I would like to ask whether it might 
be good to increase its priority to P2 and apply for inclusion in 15. My 
arguments are as follows:

- the original issue why I started looking at this were lots of 
seemingly random crashes (5 or 6 were reported and the change 
temporarily backed out for this reason) in tier8 with a g1 change that 
changed young gen sizing. These crashes including that young gen sizing 
change are all gone now with this bugfix.
I.e. this suggests that so far we seem to have not encountered this 
issue more frequently due to pure luck wrt to generation sizing.

- it affects all collectors (naturally).

- there are quite a few user reported random crashes with IntelliJ and 
variants, which due to the nature of IDEs tending to retrieve stack 
traces fairly frequently would be more affected than usual. So I suspect 
at least some of them to be caused by this issue, these are the only raw 
oops I am aware of.

My understanding of the cause and fix is fairly good, but I am no expert 
in this area, so I would like to defer to you about this suggestion. The 
change is imo important enough to be backported to 11 and 15 anyway, but 
the question is about the risk/reward tradeoff wrt to bringing it to 15 
and not 15.0.1.

CR:
https://bugs.openjdk.java.net/browse/JDK-8249192
Webrev:
http://cr.openjdk.java.net/~tschatzl/8249192/webrev/
Testing:
tier1-5,tier8 (with some unrelated changes), 1800+ runs of the reproducer

Thanks,
   Thomas