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, 28 insertions, 2 deletions

diff --git a/src/net/lookup.go b/src/net/lookup.go
index e0f21fa9a8..cb810dea26 100644
--- a/src/net/lookup.go
+++ b/src/net/lookup.go
@@ -97,6 +97,19 @@ func lookupPortMap(network, service string) (port int, error error) {
 	return 0, &AddrError{Err: "unknown port", Addr: network + "/" + service}
 }
 
+// ipVersion returns the provided network's IP version: '4', '6' or 0
+// if network does not end in a '4' or '6' byte.
+func ipVersion(network string) byte {
+	if network == "" {
+		return 0
+	}
+	n := network[len(network)-1]
+	if n != '4' && n != '6' {
+		n = 0
+	}
+	return n
+}
+
 // DefaultResolver is the resolver used by the package-level Lookup
 // functions and by Dialers without a specified Resolver.
 var DefaultResolver = &Resolver{}
@@ -189,6 +202,12 @@ func LookupIP(host string) ([]IP, error) {
 // LookupIPAddr looks up host using the local resolver.
 // It returns a slice of that host's IPv4 and IPv6 addresses.
 func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, error) {
+	return r.lookupIPAddr(ctx, "ip", host)
+}
+
+// lookupIPAddr looks up host using the local resolver and particular network.
+// It returns a slice of that host's IPv4 and IPv6 addresses.
+func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IPAddr, error) {
 	// Make sure that no matter what we do later, host=="" is rejected.
 	// parseIP, for example, does accept empty strings.
 	if host == "" {
@@ -205,7 +224,7 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 	// can be overridden by tests. This is needed by net/http, so it
 	// uses a context key instead of unexported variables.
 	resolverFunc := r.lookupIP
-	if alt, _ := ctx.Value(nettrace.LookupIPAltResolverKey{}).(func(context.Context, string) ([]IPAddr, error)); alt != nil {
+	if alt, _ := ctx.Value(nettrace.LookupIPAltResolverKey{}).(func(context.Context, string, string) ([]IPAddr, error)); alt != nil {
 		resolverFunc = alt
 	}
 
@@ -218,7 +237,7 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 	dnsWaitGroup.Add(1)
 	ch, called := r.getLookupGroup().DoChan(host, func() (interface{}, error) {
 		defer dnsWaitGroup.Done()
-		return testHookLookupIP(lookupGroupCtx, resolverFunc, host)
+		return testHookLookupIP(lookupGroupCtx, resolverFunc, network, host)
 	})
 	if !called {
 		dnsWaitGroup.Done()
@@ -289,6 +308,13 @@ func LookupPort(network, service string) (port int, err error) {
 func (r *Resolver) LookupPort(ctx context.Context, network, service string) (port int, err error) {
 	port, needsLookup := parsePort(service)
 	if needsLookup {
+		switch network {
+		case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
+		case "": // a hint wildcard for Go 1.0 undocumented behavior
+			network = "ip"
+		default:
+			return 0, &AddrError{Err: "unknown network", Addr: network}
+		}
 		port, err = r.lookupPort(ctx, network, service)
 		if err != nil {
 			return 0, err