[erts, kernel] reuseaddr/reuseport/exclusiveaddruse support/fixes

[rickard-green]

• Introduce socket option reuseport which may or may not have load balancing.

• Introduce socket option reuseport_lb with load balancing.

• Introduce socket option exclusiveaddruse. This socket option is Windows specific and will silently be ignored on other systems.

• Change behavior on Windows so that SO_REUSEADDR is only set if both reusaddr and reuseport have been set. This since SO_REUSEADDR on Windows behaves as BSD behaves if both SO_REUSEADDR and SO_REUSEPORT have been set.

[github-actions]

CT Test Results

       4 files     186 suites   1h 48m 1s ⏱️
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3 449 runs  3 109 ✔️ 339 💤 1 ❌

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// Erlang/OTP Github Action Bot

[okeuday]

@rickard-green There is a problem with using SO_REUSEPORT regarding its meaning on Linux and FreeBSD. FreeBSD had SO_REUSEPORT in the past, before it was present on Linux (mainly for UDP multicast usage probably) and Linux added the SO_REUSEPORT with extra load-balancing functionality. After that, FreeBSD added SO_REUSEPORT_LB to be equivalent to the SO_REUSEPORT option on Linux. Other BSDs also have the older meaning of SO_REUSEPORT, so all BSDs would need to have support for the SO_REUSEPORT_LB option to have the equivalent functionality to SO_REUSEPORT on Linux. So, you likely need to support reuseport and reuseportlb (or some other name) as separate options to avoid confusion.

[rickard-green]

@okeuday Yes, your right. We probably need to expose the load balancing option as well.

[rickard-green]

@okeuday

Added an reuseport_lb option as well (using FreeBSD's name for it).

reuseport is documented as may or may not be load balancing while reuseport_lb is documented as with load balancing. On Linux they will both set SO_REUSEPORT.

[okeuday]

@rickard-green You may want to consider reuseport on Linux causing an error, due to lack of support, to ensure it represents the BSD-meaning of SO_REUSEPORT because the use-case of the BSD SO_REUSEPORT should be very different. The potential for the use of the BSD SO_REUSEPORT in Erlang is interesting too, even though load-balancing is not provided, because it could be used to have a backup TCP connection with a backup Erlang process to own the backup connection (it would be only used if the other Erlang process dies). However, doing something like that, would be clearly different from needing the BSD SO_REUSEPORT_LB functionality which is what people would normally use for ensuring TCP traffic is evenly distributed among Erlang processes. So, the reuseport_lb option use is likely more common to ensure efficient load-balancing can occur.

[rickard-green]

@okeuday We've decided not to error out when reuseport without load balancing is not available in this API. The socket API is expected to get a more fine-grained option API.