net: add mechanisms to force go or cgo lookup, and to debug default strategy

GODEBUG=netdns=1 prints a one-time strategy decision. (cgo or go DNS lookups)
GODEBUG=netdns=2 prints the per-lookup strategy as a function of the hostname.

The new "netcgo" build tag forces cgo DNS lookups.

GODEBUG=netdns=go (or existing build tag "netgo") forces Go DNS resolution.
GODEBUG=netdns=cgo (or new build tag "netcgo") forces libc DNS resolution.

Options can be combined with e.g. GODEBUG=netdns=go+1 or GODEBUG=netdns=2+cgo.

Fixes #11322
Fixes #11450

Change-Id: I7a67e9f759fd0a02320e7803f9ded1638b19e861
Reviewed-on: https://go-review.googlesource.com/11584Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/cgo_stub.go

@@ -6,6 +6,8 @@
 
 package net
 
+func init() { netGo = true }
+
 type addrinfoErrno int
 
 func (eai addrinfoErrno) Error() string   { return "<nil>" }

src/net/conf.go

@@ -9,6 +9,7 @@ package net
 import (
 	"os"
 	"runtime"
+	"strconv"
 	"sync"
 	"syscall"
 )
@@ -18,10 +19,14 @@ type conf struct {
 	// forceCgoLookupHost forces CGO to always be used, if available.
 	forceCgoLookupHost bool
 
+	netGo  bool // "netgo" build tag in use (or no cgo)
+	netCgo bool // cgo DNS resolution forced
+
 	// machine has an /etc/mdns.allow file
 	hasMDNSAllow bool
 
 	goos          string // the runtime.GOOS, to ease testing
+	dnsDebugLevel int
 
 	nss    *nssConf
 	resolv *dnsConfig
@@ -39,6 +44,28 @@ func systemConf() *conf {
 }
 
 func initConfVal() {
+	dnsMode, debugLevel := goDebugNetDNS()
+	confVal.dnsDebugLevel = debugLevel
+	confVal.netGo = netGo || dnsMode == "go"
+	confVal.netCgo = netCgo || dnsMode == "cgo"
+
+	if confVal.dnsDebugLevel > 0 {
+		defer func() {
+			switch {
+			case confVal.netGo:
+				if netGo {
+					println("go package net: built with netgo build tag; using Go's DNS resolver")
+				} else {
+					println("go package net: GODEBUG setting forcing use of Go's resolver")
+				}
+			case confVal.forceCgoLookupHost:
+				println("go package net: using cgo DNS resolver")
+			default:
+				println("go package net: dynamic selection of DNS resolver")
+			}
+		}()
+	}
+
 	// Darwin pops up annoying dialog boxes if programs try to do
 	// their own DNS requests. So always use cgo instead, which
 	// avoids that.
@@ -51,7 +78,9 @@ func initConfVal() {
 	// force cgo. Note that LOCALDOMAIN can change behavior merely
 	// by being specified with the empty string.
 	_, localDomainDefined := syscall.Getenv("LOCALDOMAIN")
-	if os.Getenv("RES_OPTIONS") != "" || os.Getenv("HOSTALIASES") != "" ||
+	if os.Getenv("RES_OPTIONS") != "" ||
+		os.Getenv("HOSTALIASES") != "" ||
+		netCgo ||
 		localDomainDefined {
 		confVal.forceCgoLookupHost = true
 		return
@@ -84,7 +113,15 @@ func initConfVal() {
 }
 
 // hostLookupOrder determines which strategy to use to resolve hostname.
-func (c *conf) hostLookupOrder(hostname string) hostLookupOrder {
+func (c *conf) hostLookupOrder(hostname string) (ret hostLookupOrder) {
+	if c.dnsDebugLevel > 1 {
+		defer func() {
+			print("go package net: hostLookupOrder(", hostname, ") = ", ret.String(), "\n")
+		}()
+	}
+	if c.netGo {
+		return hostLookupFilesDNS
+	}
 	if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {
 		return hostLookupCgo
 	}
@@ -232,3 +269,34 @@ func (c *conf) hostLookupOrder(hostname string) hostLookupOrder {
 	// Something weird. Let libc deal with it.
 	return hostLookupCgo
 }
+
+// goDebugNetDNS parses the value of the GODEBUG "netdns" value.
+// The netdns value can be of the form:
+//    1       // debug level 1
+//    2       // debug level 2
+//    cgo     // use cgo for DNS lookups
+//    go      // use go for DNS lookups
+//    cgo+1   // use cgo for DNS lookups + debug level 1
+//    1+cgo   // same
+//    cgo+2   // same, but debug level 2
+// etc.
+func goDebugNetDNS() (dnsMode string, debugLevel int) {
+	goDebug := goDebugString("netdns")
+	parsePart := func(s string) {
+		if s == "" {
+			return
+		}
+		if '0' <= s[0] && s[0] <= '9' {
+			debugLevel, _ = strconv.Atoi(s)
+		} else {
+			dnsMode = s
+		}
+	}
+	if i := byteIndex(goDebug, '+'); i != -1 {
+		parsePart(goDebug[:i])
+		parsePart(goDebug[i+1:])
+		return
+	}
+	parsePart(goDebug)
+	return
+}

src/net/conf_netcgo.go

+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build netcgo
+
+package net
+
+/*
+
+// Fail if cgo isn't available.
+
+*/
+import "C"
+
+// The build tag "netcgo" forces use of the cgo DNS resolver.
+// It is the opposite of "netgo".
+func init() { netCgo = true }

src/net/conf_test.go

@@ -295,3 +295,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 	}
 
 }
+
+func TestSystemConf(t *testing.T) {
+	systemConf()
+}

src/net/net.go

@@ -46,6 +46,14 @@ import (
 	"time"
 )
 
+// netGo and netCgo contain the state of the build tags used
+// to build this binary, and whether cgo is available.
+// conf.go mirrors these into conf for easier testing.
+var (
+	netGo  bool // set true in cgo_stub.go for build tag "netgo" (or no cgo)
+	netCgo bool // set true in conf_netcgo.go for build tag "netcgo"
+)
+
 func init() {
 	sysInit()
 	supportsIPv4 = probeIPv4Stack()

src/net/parse.go

@@ -361,3 +361,26 @@ func readFull(r io.Reader) (all []byte, err error) {
 		}
 	}
 }
+
+// goDebugString returns the value of the named GODEBUG key.
+// GODEBUG is of the form "key=val,key2=val2"
+func goDebugString(key string) string {
+	s := os.Getenv("GODEBUG")
+	for i := 0; i < len(s)-len(key)-1; i++ {
+		if i > 0 && s[i-1] != ',' {
+			continue
+		}
+		afterKey := s[i+len(key):]
+		if afterKey[0] != '=' || s[i:i+len(key)] != key {
+			continue
+		}
+		val := afterKey[1:]
+		for i, b := range val {
+			if b == ',' {
+				return val[:i]
+			}
+		}
+		return val
+	}
+	return ""
+}

src/net/parse_test.go

@@ -50,3 +50,30 @@ func TestReadLine(t *testing.T) {
 		byteno += len(line) + 1
 	}
 }
+
+func TestGoDebugString(t *testing.T) {
+	defer os.Setenv("GODEBUG", os.Getenv("GODEBUG"))
+	tests := []struct {
+		godebug string
+		key     string
+		want    string
+	}{
+		{"", "foo", ""},
+		{"foo=", "foo", ""},
+		{"foo=bar", "foo", "bar"},
+		{"foo=bar,", "foo", "bar"},
+		{"foo,foo=bar,", "foo", "bar"},
+		{"foo1=bar,foo=bar,", "foo", "bar"},
+		{"foo=bar,foo=bar,", "foo", "bar"},
+		{"foo=", "foo", ""},
+		{"foo", "foo", ""},
+		{",foo", "foo", ""},
+		{"foo=bar,baz", "loooooooong", ""},
+	}
+	for _, tt := range tests {
+		os.Setenv("GODEBUG", tt.godebug)
+		if got := goDebugString(tt.key); got != tt.want {
+			t.Errorf("for %q, goDebugString(%q) = %q; want %q", tt.godebug, tt.key, got, tt.want)
+		}
+	}
+}