net: make cgo resolver work more accurately with network parameter

Unlike the go resolver, the existing cgo resolver exchanges both DNS A
and AAAA RR queries unconditionally and causes unreasonable connection
setup latencies to applications using the cgo resolver.

This change adds new argument (`network`) in all functions through the
series of calls: from Resolver.internetAddrList to cgoLookupIPCNAME.

Benefit: no redundant DNS calls if certain IP version is used IPv4/IPv6
(no `AAAA` DNS requests if used tcp4, udp4, ip4 network. And vice
versa: no `A` DNS requests if used tcp6, udp6, ip6 network)

Fixes #25947

Change-Id: I39edbd726d82d6133fdada4d06cd90d401e7e669
Reviewed-on: https://go-review.googlesource.com/c/120215
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/net/dial_test.go b/src/net/dial_test.go
index 3a45c0d2ec..983338885d 100644
--- a/src/net/dial_test.go
+++ b/src/net/dial_test.go
@@ -346,7 +346,7 @@ func TestDialParallel(t *testing.T) {
 	}
 }
 
-func lookupSlowFast(ctx context.Context, fn func(context.Context, string) ([]IPAddr, error), host string) ([]IPAddr, error) {
+func lookupSlowFast(ctx context.Context, fn func(context.Context, string, string) ([]IPAddr, error), network, host string) ([]IPAddr, error) {
 	switch host {
 	case "slow6loopback4":
 		// Returns a slow IPv6 address, and a local IPv4 address.
@@ -355,7 +355,7 @@ func lookupSlowFast(ctx context.Context, fn func(context.Context, string) ([]IPA
 			{IP: ParseIP("127.0.0.1")},
 		}, nil
 	default:
-		return fn(ctx, host)
+		return fn(ctx, network, host)
 	}
 }