RFR: JDK-8313804: JDWP support for -Djava.net.preferIPv6Addresses=system [v3]

[Alex Menkov]

On Tue, 19 Sep 2023 23:54:31 GMT, Liam Miller-Cushon <cushon at openjdk.org> wrote:

> > I think Alex has commented that this can be made more obvious with a check for AF_UNSPEC.
> 
> Thanks, yes, I have applied that suggestion.
> 
> > > For 748, it's handling a situation where IPv4 isn't specifically preferred. The implementation comment notes 'if preferredAddressFamily is AF_INET6 or not set'.
> > 
> > 
> > What does "specifically preferred" mean? By default it will be set to AF_INET. If it is set to AF_INET, you can't tell if `java.net.preferIPv6Addresses=false` was specified (which implies AF_INET), or if nothing was specified and it just defaulted to AF_INET. The comment and the code read as if preferredAddressFamily will be something other than AF_INET unless `java.net.preferIPv6Addresses=false` is used. Maybe the code is correct and the comment needs some cleaning up. TBH I don't understand what this code is suppose to do.
> 
> I agree 'preferredAddressFamily is AF_INET6 or not set' is unclear, since `preferredAddressFamily` was only ever `AF_INET` or `AF_INET6`. I wondered if the use of `if (preferredAddressFamily != AF_INET) {` instead of `if (preferredAddressFamily == AF_INET6) {` and the comment about 'not set' was referring to the possibility of `AF_UNSPEC`. The history for this logic is in
> 
>     * https://bugs.openjdk.org/browse/JDK-8250630
> 
>     * https://mail.openjdk.org/pipermail/serviceability-dev/2020-August/thread.html#32673
> 
> 
> The bug mentions it fixed an issue on Alpine Linux, and I don't have convenient access to a test environment for that.
> 
> I could update L750 to `if (preferredAddressFamily == AF_INET6) {` to prevent this logic from re-ordering address when `java.net.preferIPv6Addresses=system` is set.
> 
> What do you think?

I took some time to understand what this code for.
I think the condition is not correct. it should be 

    if (!allowOnlyIPv4) {

allowOnlyIPv4 (reflects "java.net.preferIPv4Stack" sys.prop) means that only IPv4 connections are allowed (even if IPv6 available)

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PR Comment: https://git.openjdk.org/jdk/pull/15796#issuecomment-1726708238