net: enable pure Go resolver for wasip1

Top-level functions in the net package that only read files,
for example LookupPort(...), or LookupIP(host) where host resides
in /etc/hosts, now work on wasip1.

If the application has the ability to create sockets (for example,
when using a sockets extension to WASI preview 1), it's now
possible to do name resolution by passing a custom Dial function
to a Resolver instance.

Change-Id: I923886f67e336820bc89f09ea1855387c8dac61a
Reviewed-on: https://go-review.googlesource.com/c/go/+/500579
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Randy Reddig <ydnar@shaderlab.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/net/cgo_stub.go

@@ -4,11 +4,12 @@
 
 // This file holds stub versions of the cgo functions called on Unix systems.
 // We build this file if using the netgo build tag, or if cgo is not
-// enabled and we are using a Unix system other than Darwin.
+// enabled and we are using a Unix system other than Darwin, or if it's
+// wasip1 where cgo is never available.
 // Darwin is exempted because it always provides the cgo routines,
 // in cgo_unix_syscall.go.
 
-//go:build netgo || (!cgo && unix && !darwin)
+//go:build netgo || (!cgo && unix && !darwin) || wasip1
 
 package net
 

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 && !wasip1
+//go:build !js
 
 package net
 

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 && !wasip1
+//go:build !js
 
 // DNS client: see RFC 1035.
 // Has to be linked into package net for Dial.

src/net/dnsconfig_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 && !wasip1 && !windows
+//go:build !js && !windows
 
 // Read system DNS config from /etc/resolv.conf
 

src/net/lookup_fake.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 && wasm) || wasip1
+//go:build js && wasm
 
 package net
 

src/net/lookup_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 unix
+//go:build unix || wasip1
 
 package net
 

src/net/net_fake.go

@@ -15,8 +15,6 @@ import (
 	"sync"
 	"syscall"
 	"time"
-
-	"golang.org/x/net/dns/dnsmessage"
 )
 
 var listenersMu sync.Mutex
@@ -406,7 +404,3 @@ func (fd *fakeNetFD) writeMsg(p []byte, oob []byte, sa syscall.Sockaddr) (n int,
 func (fd *fakeNetFD) dup() (f *os.File, err error) {
 	return nil, syscall.ENOSYS
 }
-
-func (r *Resolver) lookup(ctx context.Context, name string, qtype dnsmessage.Type, conf *dnsConfig) (dnsmessage.Parser, string, error) {
-	panic("unreachable")
-}

src/net/net_fake_js.go

@@ -8,7 +8,12 @@
 
 package net
 
-import "internal/poll"
+import (
+	"context"
+	"internal/poll"
+
+	"golang.org/x/net/dns/dnsmessage"
+)
 
 // Network file descriptor.
 type netFD struct {
@@ -25,3 +30,7 @@ type netFD struct {
 	pfd         poll.FD
 	isConnected bool // handshake completed or use of association with peer
 }
+
+func (r *Resolver) lookup(ctx context.Context, name string, qtype dnsmessage.Type, conf *dnsConfig) (dnsmessage.Parser, string, error) {
+	panic("unreachable")
+}