api/resolver: update resolver to enable subdomain routing (#1747)

* api/resolver: update domain / service prefix usage

* api/resolver/subdomain: implement subdomain resolver for domain resolution

* api/handler: fix tests

api/resolver/subdomain/subdomain.go

@@ -0,0 +1,86 @@
+// Package subdomain is a resolver which uses the subdomain to determine the domain to route to. It
+// offloads the endpoint resolution to a child resolver which is provided in New.
+package subdomain
+
+import (
+	"net"
+	"net/http"
+	"strings"
+
+	"github.com/micro/go-micro/v2/api/resolver"
+	"github.com/micro/go-micro/v2/logger"
+	"golang.org/x/net/publicsuffix"
+)
+
+func NewResolver(parent resolver.Resolver, opts ...resolver.Option) resolver.Resolver {
+	options := resolver.NewOptions(opts...)
+	return &Resolver{options, parent}
+}
+
+type Resolver struct {
+	opts resolver.Options
+	resolver.Resolver
+}
+
+func (r *Resolver) Resolve(req *http.Request) (*resolver.Endpoint, error) {
+	// resolve the endpoint using the provided resolver
+	endpoint, err := r.Resolver.Resolve(req)
+	if err != nil {
+		return nil, err
+	}
+
+	// override the domain
+	endpoint.Domain = r.resolveDomain(req)
+
+	// return the result
+	return endpoint, nil
+}
+
+func (r *Resolver) resolveDomain(req *http.Request) string {
+	// determine the host, e.g. foobar.m3o.app
+	host := req.URL.Hostname()
+	if len(host) == 0 {
+		if h, _, err := net.SplitHostPort(req.Host); err == nil {
+			host = h // host does contain a port
+		} else if strings.Contains(err.Error(), "missing port in address") {
+			host = req.Host // host does not contain a port
+		}
+	}
+
+	// check for an ip address
+	if net.ParseIP(host) != nil {
+		return r.opts.Domain
+	}
+
+	// check for dev enviroment
+	if host == "localhost" || host == "127.0.0.1" {
+		return r.opts.Domain
+	}
+
+	// extract the top level domain plus one (e.g. 'myapp.com')
+	domain, err := publicsuffix.EffectiveTLDPlusOne(host)
+	if err != nil {
+		logger.Debugf("Unable to extract domain from %v", host)
+		return r.opts.Domain
+	}
+
+	// there was no subdomain
+	if host == domain {
+		return r.opts.Domain
+	}
+
+	// remove the domain from the host, leaving the subdomain, e.g. "staging.foo.myapp.com" => "staging.foo"
+	subdomain := strings.TrimSuffix(host, "."+domain)
+
+	// return the reversed subdomain as the namespace, e.g. "staging.foo" => "foo-staging"
+	comps := strings.Split(subdomain, ".")
+	for i := len(comps)/2 - 1; i >= 0; i-- {
+		opp := len(comps) - 1 - i
+		comps[i], comps[opp] = comps[opp], comps[i]
+	}
+	return strings.Join(comps, "-")
+}
+
+func (r *Resolver) String() string {
+	return "subdomain"
+}