proposal: net: export supportsIPv4, supportsIPv6, supportsIPv4map

[database64128]

Proposal Details

The net package queries and caches the capabilities of the system IP stack, and calls these functions to determine the type of socket to open when user code calls Dial, Listen and friends. It'd be nice if user code that opens sockets by hand can conveniently call these existing functions, instead of having to duplicate the feature in user code, or resorting to linkname.

The proposal is to add the following:

// SupportsIPv4 reports whether the platform supports IPv4 networking
// functionality.
func SupportsIPv4() bool

// SupportsIPv6 reports whether the platform supports IPv6 networking
// functionality.
func SupportsIPv6() bool

// SupportsIPv4map reports whether the platform supports mapping an
// IPv4 address inside an IPv6 address at transport layer
// protocols. See RFC 4291, RFC 4038 and RFC 3493.
func SupportsIPv4map() bool

This proposal, if accepted, will allow tfo-go to drop its linkname against net.favoriteAddrFamily.

For #67401 (comment).

[xiaokentrl]

Very Very Very good

[seankhliao]

it's already available in x/net/nettest
https://pkg.go.dev/golang.org/x/net/nettest#SupportsIPv4

[database64128]

@seankhliao x/net/nettest is missing SupportsIPv4map, and it imports the testing package, which makes it unsuitable for use in non-testing packages.

[seankhliao]

in general we also deem these methods unsuitable for non testing cases as the availability of different proposals can change at run time

[database64128]

in general we also deem these methods unsuitable for non testing cases as the availability of different proposals can change at run time

This does not sound right. The functions I'm proposing to export are extensively used by the net package for non-testing purposes. Please read the first sentence of my proposal.