RFR: 8292989: Avoid dynamic memory in AsyncLogWriter [v8]

[Xin Liu]

On Mon, 12 Sep 2022 05:55:01 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   get rid of TOKEN_SIZE.  use Message::calc_size(0) instead.
>>   
>>   it's still a compiler-time constant like TOKEN_SIZE. put the
>>   headroom logic in Buffer::push_back().
>
> src/hotspot/share/logging/logAsyncWriter.cpp line 52:
> 
>> 50: // Reserve space for a flush token, so 'push_flush_token' always succeeds.
>> 51: AsyncLogWriter::Buffer::Buffer(size_t capacity) : _pos(0), _capacity(capacity) {
>> 52:   _buf = NEW_C_HEAP_ARRAY(char, capacity, mtLogging);
> 
> I would give it a low minimal size. It needs to be at least token size, but small enough to serve as a good test for simulating buffer overflows. Maybe 2-3 x header size + ~64 bytes or so. E.g. 96 bytes?

HotSpot has a constraint in VMOption. The minimal number is 50K here. 
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/globals.hpp#L1905

I added a unit test in test_asynclog.cpp. Like you said, I feel it's more flexible to write tests if we don't limit size here. 

If buffer capacity is smaller than a token, it will hit assertion in `Buffer::push_flush_token`.

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PR: https://git.openjdk.org/jdk/pull/10092