[feautre request] Add a util method to get maxDirectMemory

[shoothzj]

Netty's PlatformDependent.maxDirectMemory() is powerful and general.
In other project, we want to read the maxDirectMemory too.
But we can't directly use PlatformDependent.maxDirectMemory() , because it will wrong if user set io.netty.maxDirectMemory greater than zero.
Instead of copy and paste netty's code(Under Apache License), I want a UtilityClass to get PlatformDependent.maxDirectMemory0 result.

I am not very familiar with netty code, I can work on this under guides.

Thanks for your help.

[normanmaurer]

@chrisvest what do you think... Not sure we want to expose just another public method. I guess we could do it tho

[franz1981]

@normanmaurer it's -1 to me, but I would like to understand why @shoothzj need it/the exact use case

[shoothzj]

@franz1981 It's started with apache/bookkeeper#2989 when locating a Out of DirectMemory Error. We found the the maxDirectMemory read by bookkeeper is not correct with huawei jdk 11, but the result read by netty is correct.
I want to improve the maxDirectMemory readed by bookkeepe but without the io.netty.maxDirectMemory.

Please point out if I didn't make it clear. My English is not good. Thanks
cc @normanmaurer

[franz1981]

wdyt @normanmaurer ?

I know that Netty's OutOfDirectMemoryError throwing condition is tied with the Netty's notion of max direct memory, hence if we allow the exception to leak ie OutOfDirectMemoryError is public, maybe makes sense our limits to leak as well.
Another option for @shoothzj could be to react to such information (if structured to be used) while the limit is reached, but we need to change OutOfDirectMemoryError in order to expose exception fields that represent our limits/current values ie
requestedCapacity, usedCapacity, DIRECT_MEMORY_LIMIT as max allowed capacity

[chrisvest]

@normanmaurer @franz1981 I don't see a big problem in exposing maxDirectMemory0 as estimateMaxDirectMemory.

[johnou]

technically it wouldn't even be part of the public API, last I recall PlatformDependent is in the "internal" package, that said could annotate it as UnstableApi?

[franz1981]

Let's wait what @normanmaurer answer on this, but I like both @chrisvest and @johnou answers