net: use Go's DNS resolver when system configuration permits

If the machine's network configuration files (resolv.conf,
nsswitch.conf) don't have any unsupported options, prefer Go's DNS
resolver, which doesn't have the cgo & thread over.

It means users can have more than 500 DNS requests outstanding (our
current limit for cgo lookups) and not have one blocked thread per
outstanding request.

Discussed in thread https://groups.google.com/d/msg/golang-dev/2ZUi792oztM/Q0rg_DkF5HMJ

Change-Id: I3f685d70aff6b47bec30b63e9fba674b20507f95
Reviewed-on: https://go-review.googlesource.com/8945
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dnsclient_unix.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"math/rand"
 	"os"
+	"strconv"
 	"sync"
 	"time"
 )
@@ -332,6 +333,35 @@ func lookup(name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
 	return
 }
 
+// hostLookupOrder specifies the order of LookupHost lookup strategies.
+// It is basically a simplified representation of nsswitch.conf.
+// "files" means /etc/hosts.
+type hostLookupOrder int
+
+const (
+	// hostLookupCgo means defer to cgo.
+	hostLookupCgo      hostLookupOrder = iota
+	hostLookupFilesDNS                 // files first
+	hostLookupDNSFiles                 // dns first
+	hostLookupFiles                    // only files
+	hostLookupDNS                      // only DNS
+)
+
+var lookupOrderName = map[hostLookupOrder]string{
+	hostLookupCgo:      "cgo",
+	hostLookupFilesDNS: "files,dns",
+	hostLookupDNSFiles: "dns,files",
+	hostLookupFiles:    "files",
+	hostLookupDNS:      "dns",
+}
+
+func (o hostLookupOrder) String() string {
+	if s, ok := lookupOrderName[o]; ok {
+		return s
+	}
+	return "hostLookupOrder=" + strconv.Itoa(int(o)) + "??"
+}
+
 // goLookupHost is the native Go implementation of LookupHost.
 // Used only if cgoLookupHost refuses to handle the request
 // (that is, only if cgoLookupHost is the stub in cgo_stub.go).
@@ -339,12 +369,18 @@ func lookup(name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
 // depending on our lookup code, so that Go and C get the same
 // answers.
 func goLookupHost(name string) (addrs []string, err error) {
-	// Use entries from /etc/hosts if they match.
-	addrs = lookupStaticHost(name)
-	if len(addrs) > 0 {
-		return
+	return goLookupHostOrder(name, hostLookupFilesDNS)
+}
+
+func goLookupHostOrder(name string, order hostLookupOrder) (addrs []string, err error) {
+	if order == hostLookupFilesDNS || order == hostLookupFiles {
+		// Use entries from /etc/hosts if they match.
+		addrs = lookupStaticHost(name)
+		if len(addrs) > 0 || order == hostLookupFiles {
+			return
+		}
 	}
-	ips, err := goLookupIP(name)
+	ips, err := goLookupIPOrder(name, order)
 	if err != nil {
 		return
 	}
@@ -355,25 +391,30 @@ func goLookupHost(name string) (addrs []string, err error) {
 	return
 }
 
+// lookup entries from /etc/hosts
+func goLookupIPFiles(name string) (addrs []IPAddr) {
+	for _, haddr := range lookupStaticHost(name) {
+		haddr, zone := splitHostZone(haddr)
+		if ip := ParseIP(haddr); ip != nil {
+			addr := IPAddr{IP: ip, Zone: zone}
+			addrs = append(addrs, addr)
+		}
+	}
+	return
+}
+
 // goLookupIP is the native Go implementation of LookupIP.
 // Used only if cgoLookupIP refuses to handle the request
 // (that is, only if cgoLookupIP is the stub in cgo_stub.go).
-// Normally we let cgo use the C library resolver instead of
-// depending on our lookup code, so that Go and C get the same
-// answers.
 func goLookupIP(name string) (addrs []IPAddr, err error) {
-	// Use entries from /etc/hosts if possible.
-	haddrs := lookupStaticHost(name)
-	if len(haddrs) > 0 {
-		for _, haddr := range haddrs {
-			haddr, zone := splitHostZone(haddr)
-			if ip := ParseIP(haddr); ip != nil {
-				addr := IPAddr{IP: ip, Zone: zone}
-				addrs = append(addrs, addr)
-			}
-		}
-		if len(addrs) > 0 {
-			return
+	return goLookupIPOrder(name, hostLookupFilesDNS)
+}
+
+func goLookupIPOrder(name string, order hostLookupOrder) (addrs []IPAddr, err error) {
+	if order == hostLookupFilesDNS || order == hostLookupFiles {
+		addrs = goLookupIPFiles(name)
+		if len(addrs) > 0 || order == hostLookupFiles {
+			return addrs, nil
 		}
 	}
 	type racer struct {
@@ -409,8 +450,13 @@ func goLookupIP(name string) (addrs []IPAddr, err error) {
 			}
 		}
 	}
-	if len(addrs) == 0 && lastErr != nil {
-		return nil, lastErr
+	if len(addrs) == 0 {
+		if lastErr != nil {
+			return nil, lastErr
+		}
+		if order == hostLookupDNSFiles {
+			addrs = goLookupIPFiles(name)
+		}
 	}
 	return addrs, nil
 }