`copier.Copy` inside `util.CopyDNSClient` defeat the DCE

[ldez]

The dead code elimination (DCE) is defeated by the usage of copier.Copy inside util.CopyDNSClient.

copier.Copy uses reflect.Value.MethodByName:

https://github.com/jinzhu/copier/blob/c6b47b092d9840406d0abc347e68a28a7b812643/copier.go#L396-L401

I think I understand why the dns.Client is copied, but this is not always useful.

The defeat of the DCE has a huge impact on binary size.

I can think to several way to fix the problem, but I want your feedback before making proposals.

[bodgit]

Thanks for raising this issue.

The primary purpose of util.CopyDNSClient is to get a copy of the passed *dns.Client with the network transport forced to tcp but honouring if it was originally dual-stack, IPv4-only or IPv6-only. Any timeouts that are set are also important to keep.

I think I originally used copier.Copy as it made my code look simpler and then I didn't have to worry about most of the internals of dns.Client{} but I'm happy to replace that with something else; just explicitly copying the fields if that's simpler.

[ldez]

I think a simple shallow copy can work:

diff --git i/internal/util/util.go w/internal/util/util.go
index 2d32dc4..76f2cc2 100644
--- i/internal/util/util.go
+++ w/internal/util/util.go
@@ -10,7 +10,6 @@ import (
        "time"
 
        "github.com/bodgit/tsig"
-       "github.com/jinzhu/copier"
        "github.com/miekg/dns"
 )
 
@@ -37,10 +36,8 @@ type Exchanger interface {
 // TCP. If the existing network is configured to only use IPv4 or IPv6 then
 // the appropriate network is chosen to maintain this choice.
 func CopyDNSClient(dnsClient *dns.Client) (*dns.Client, error) {
-       client := new(dns.Client)
-       if err := copier.Copy(client, dnsClient); err != nil {
-               return nil, err
-       }
+       client := &dns.Client{}
+       *client = *dnsClient
 
        switch client.Net {
        case "tcp", "tcp4", "tcp6":