Put current method behind a build tag

Author: chriso

README.md

@@ -84,19 +84,39 @@ func main() {
 
 ## Name Resolution
 
-Go has a built-in name resolver that sidesteps CGO (e.g. `getaddrinfo(3)`)
-calls.
+There are two methods available for resolving a set of IP addresses
+for a hostname.
 
-This library will automatically configure the `net.DefaultResolver`
-from the standard library to use the `Dial` function from this library.
-You just need the following import somewhere:
+### getaddrinfo
+
+The `sock_getaddrinfo` host function is used to implement name resolution.
+This requires WasmEdge, or a WasmEdge compatible WASI layer
+(e.g. [wasi-go](http://github.com/stealthrocket/wasi-go)).
+
+When using this method, the standard library resolver **will not work**. You
+_cannot_ use `net.DefaultResolver`, `net.LookupIP`, etc. with this approach
+because the standard library does not allow us to patch it with an alternative
+implementation.
+
+Note that `sock_getaddrinfo` may block!
+
+### Pure Go Resolver
+
+The pure Go name resolver is not currently enabled for GOOS=wasip1.
+
+The following series of CLs will change this: https://go-review.googlesource.com/c/go/+/500576.
+This will hopefully land in Go v1.22 in ~February 2024.
+
+If you're using a version of Go that has the CL's included, you can
+instruct this library to use the pure Go resolver by including the
+`purego` build tag.
+
+The library will then automatically configure the `net.DefaultResolver`.
+All you need is the following import somewhere in your application:
 
 ```go
 import _ "github.com/stealthrocket/net"
 ```
 
 You should then be able to use the lookup functions from the standard
 library (e.g. `net.LookupIP(host)`).
-
-Note that name resolution currently depends on the following series of CLs:
-https://go-review.googlesource.com/c/go/+/500576

dial_wasip1.go

@@ -11,8 +11,6 @@ import (
 )
 
 func init() {
-	net.DefaultResolver.Dial = DialContext
-
 	if t, ok := http.DefaultTransport.(*http.Transport); ok {
 		t.DialContext = DialContext
 	}

lookup_wasip1_purego.go

@@ -0,0 +1,75 @@
+//go:build wasip1 && purego
+
+package net
+
+import (
+	"fmt"
+	"net"
+)
+
+func init() {
+	net.DefaultResolver.Dial = DialContext
+}
+
+func lookupAddr(context, network, address string) (net.Addr, error) {
+	switch network {
+	case "tcp", "tcp4", "tcp6":
+	case "udp", "udp4", "udp6":
+	case "unix", "unixgram":
+		return &net.UnixAddr{Name: address, Net: network}, nil
+	default:
+		return nil, fmt.Errorf("not implemented: %s", network)
+	}
+	host, portstr, err := net.SplitHostPort(address)
+	if err != nil {
+		return nil, err
+	}
+	port, err := net.LookupPort(network, portstr)
+	if err != nil {
+		return nil, err
+	}
+	if host == "" {
+		if context == "listen" {
+			switch network {
+			case "tcp", "tcp4":
+				return &net.TCPAddr{IP: net.IPv4zero, Port: port}, nil
+			case "tcp6":
+				return &net.TCPAddr{IP: net.IPv6zero, Port: port}, nil
+			}
+		}
+		return nil, fmt.Errorf("invalid address %q for %s", address, context)
+	}
+	ips, err := net.LookupIP(host)
+	if err != nil {
+		return nil, err
+	}
+	if network == "tcp" || network == "tcp4" {
+		for _, ip := range ips {
+			if len(ip) == net.IPv4len {
+				return &net.TCPAddr{IP: ip, Port: port}, nil
+			}
+		}
+	}
+	if network == "tcp" || network == "tcp6" {
+		for _, ip := range ips {
+			if len(ip) == net.IPv6len {
+				return &net.TCPAddr{IP: ip, Port: port}, nil
+			}
+		}
+	}
+	if network == "udp" || network == "udp4" {
+		for _, ip := range ips {
+			if len(ip) == net.IPv4len {
+				return &net.UDPAddr{IP: ip, Port: port}, nil
+			}
+		}
+	}
+	if network == "udp" || network == "udp6" {
+		for _, ip := range ips {
+			if len(ip) == net.IPv6len {
+				return &net.UDPAddr{IP: ip, Port: port}, nil
+			}
+		}
+	}
+	return nil, fmt.Errorf("cannot listen on %q", host)
+}