Revert "net: permit use of Resolver.PreferGo, netgo on Windows and Plan 9"

This reverts CL 400654.

Reason for revert: broke net.TestGoLookupIP on the darwin-amd64-nocgo builder.

Updates #33097.

Change-Id: Idaf94eda88c9d4401e667a4d31c00ce376d91909
Reviewed-on: https://go-review.googlesource.com/c/go/+/401754
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: Bryan Mills

src/net/addrselect.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build unix
+
 // Minimal RFC 6724 address selection.
 
 package net

src/net/cgo_stub.go

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

src/net/conf.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !js
+//go:build unix
 
 package net
 
@@ -21,7 +21,7 @@ type conf struct {
 	forceCgoLookupHost bool
 
 	netGo  bool // go DNS resolution forced
-	netCgo bool // non-go DNS resolution forced (cgo, or win32)
+	netCgo bool // cgo DNS resolution forced
 
 	// machine has an /etc/mdns.allow file
 	hasMDNSAllow bool
@@ -49,23 +49,9 @@ func initConfVal() {
 	confVal.dnsDebugLevel = debugLevel
 	confVal.netGo = netGo || dnsMode == "go"
 	confVal.netCgo = netCgo || dnsMode == "cgo"
-	if !confVal.netGo && !confVal.netCgo && (runtime.GOOS == "windows" || runtime.GOOS == "plan9") {
-		// Neither of these platforms actually use cgo.
-		//
-		// The meaning of "cgo" mode in the net package is
-		// really "the native OS way", which for libc meant
-		// cgo on the original platforms that motivated
-		// PreferGo support before Windows and Plan9 got support,
-		// at which time the GODEBUG=netdns=go and GODEBUG=netdns=cgo
-		// names were already kinda locked in.
-		confVal.netCgo = true
-	}
 
 	if confVal.dnsDebugLevel > 0 {
 		defer func() {
-			if confVal.dnsDebugLevel > 1 {
-				println("go package net: confVal.netCgo =", confVal.netCgo, " netGo =", confVal.netGo)
-			}
 			switch {
 			case confVal.netGo:
 				if netGo {
@@ -89,10 +75,6 @@ func initConfVal() {
 		return
 	}
 
-	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
-		return
-	}
-
 	// If any environment-specified resolver options are specified,
 	// force cgo. Note that LOCALDOMAIN can change behavior merely
 	// by being specified with the empty string.
@@ -147,19 +129,7 @@ func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrde
 	}
 	fallbackOrder := hostLookupCgo
 	if c.netGo || r.preferGo() {
-		switch c.goos {
-		case "windows":
-			// TODO(bradfitz): implement files-based
-			// lookup on Windows too? I guess /etc/hosts
-			// kinda exists on Windows. But for now, only
-			// do DNS.
-			fallbackOrder = hostLookupDNS
-		default:
-			fallbackOrder = hostLookupFilesDNS
-		}
-	}
-	if c.goos == "windows" || c.goos == "plan9" {
-		return fallbackOrder
+		fallbackOrder = hostLookupFilesDNS
 	}
 	if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {
 		return fallbackOrder

src/net/dnsclient_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !js
+//go:build unix
 
 // DNS client: see RFC 1035.
 // Has to be linked into package net for Dial.
@@ -20,7 +20,6 @@ import (
 	"internal/itoa"
 	"io"
 	"os"
-	"runtime"
 	"sync"
 	"time"
 
@@ -379,21 +378,12 @@ func (conf *resolverConfig) tryUpdate(name string) {
 	}
 	conf.lastChecked = now
 
-	switch runtime.GOOS {
-	case "windows":
-		// There's no file on disk, so don't bother checking
-		// and failing.
-		//
-		// The Windows implementation of dnsReadConfig (called
-		// below) ignores the name.
-	default:
-		var mtime time.Time
-		if fi, err := os.Stat(name); err == nil {
-			mtime = fi.ModTime()
-		}
-		if mtime.Equal(conf.dnsConfig.mtime) {
-			return
-		}
+	var mtime time.Time
+	if fi, err := os.Stat(name); err == nil {
+		mtime = fi.ModTime()
+	}
+	if mtime.Equal(conf.dnsConfig.mtime) {
+		return
 	}
 
 	dnsConf := dnsReadConfig(name)

src/net/dnsconfig.go

@@ -2,17 +2,40 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !js && !windows
+//go:build unix
 
 // Read system DNS config from /etc/resolv.conf
 
 package net
 
 import (
 	"internal/bytealg"
+	"os"
+	"sync/atomic"
 	"time"
 )
 
+var (
+	defaultNS   = []string{"127.0.0.1:53", "[::1]:53"}
+	getHostname = os.Hostname // variable for testing
+)
+
+type dnsConfig struct {
+	servers       []string      // server addresses (in host:port form) to use
+	search        []string      // rooted suffixes to append to local name
+	ndots         int           // number of dots in name to trigger absolute lookup
+	timeout       time.Duration // wait before giving up on a query, including retries
+	attempts      int           // lost packets before giving up on server
+	rotate        bool          // round robin among servers
+	unknownOpt    bool          // anything unknown was encountered
+	lookup        []string      // OpenBSD top-level database "lookup" order
+	err           error         // any error that occurs during open of resolv.conf
+	mtime         time.Time     // time of resolv.conf modification
+	soffset       uint32        // used by serverOffset
+	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
+	useTCP        bool          // force usage of TCP for DNS resolutions
+}
+
 // See resolv.conf(5) on a Linux machine.
 func dnsReadConfig(filename string) *dnsConfig {
 	conf := &dnsConfig{
@@ -133,6 +156,17 @@ func dnsReadConfig(filename string) *dnsConfig {
 	return conf
 }
 
+// serverOffset returns an offset that can be used to determine
+// indices of servers in c.servers when making queries.
+// When the rotate option is enabled, this offset increases.
+// Otherwise it is always 0.
+func (c *dnsConfig) serverOffset() uint32 {
+	if c.rotate {
+		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
+	}
+	return 0
+}
+
 func dnsDefaultSearch() []string {
 	hn, err := getHostname()
 	if err != nil {