Propagate sshforward send side connection close

[aaronlehmann]

PR #3431 caused connections closed on the remote side of a sshforward session to not always result in the local side reading an EOF from the connection. This change restores that behavior by closing the write side of the forwarded connection after reading an EOF from the stream. Since only the write side is being closed, it doesn't prevent the remote side from continuing to read from the connection.

[sipsma]

Thanks for sending this @aaronlehmann, I just verified the original problem in dagger is still gone when running with this change, so we should be all good.

I did realize after my original suggestion that there's probably a slightly preferable change:

if conn, ok := conn.(interface {
	CloseWrite() error
}); ok {
	conn.CloseWrite()
}

Just because this is a public interface and callers could thus provide something besides a UnixConn technically. I tested that too and verified it still works for dagger.

Approving anyways because that's pretty obscure, don't care if we merge as is if you don't have time to test again before the v0.11.1 patch release comes out.

[aaronlehmann]

Updated to your new suggestion.

[tonistiigi]

Not sure I understand how this works. Afaics conn is always type *sock that is internal wrapper and never implements CloseWrite.

[aaronlehmann]

@tonistiigi: It seems like that wrapping only applies for SSH agent forwarding in ForwardAgent. The usage from MountSSHSocket does not seem to wrap the conn:

buildkit/session/sshforward/ssh.go

Line 51 in c521afd

go Copy(ctx, conn, stream, stream.CloseSend)