[11u] RFR: 8257828 SafeFetch may crash if invoked in non-JavaThreads

[Thomas Stüfe]

Hi,

I'd like reviews for this downport, please.

Issue: https://bugs.openjdk.java.net/browse/JDK-8257828
Original patch: https://git.openjdk.java.net/jdk/commit/3ab1dfeb
Original Review: https://git.openjdk.java.net/jdk/pull/1695

This patch does two things:

1) On Posix platforms it fixes SafeFetch to work in all threads, not only
in JavaThreads. In non-JavaThreads before this patch it would crash.

The patch is totally trivial. In the signal handler, when handling
SafeFetch, there was a condition that Thread != NULL and be a JavaThread.
This condition was unnecessary. All SafeFetch handling needs is a valid
context. The Patch moves SafeFetch handling out of the is-JavaThread
condition.

2) On Windows we did not have the problem; however there is the
`Handle_Exception` function handling SafeFetch among other things, which
may be called with a non-java thread, but then forcibly casts the Thread*
to JavaThread.

11u patch:
http://cr.openjdk.java.net/~stuefe/webrevs/backports/8257828-safefetch-non-java-threads

The original patch does not apply, at all. The reason for this is that we
consolidated common functionality of the many cpu+arch signal handlers to
one function, in os_posix.cpp. This happened in JDK16 and I do not want to
backport this cleanup change to 11. For this patch, it means that the
posix-related fixes practically had to be rewritten for each and every
signal handler. Each fix is primitive however, since it just moves
SafeFetch handling around, but every signal handler is a tiny bit different.

I tested this patch for several weeks in our systems on Linux
ppc/ppcle/x64/s390/aarch64, AIX, Solaris and Mac, no problems surfaced.

I did not test on 32bit arm.

Thanks, Thomas