swarm: add CLI --ens-endpoint flag (#14386)

Implement a CLI flag that can be repeated to allow multiple ENS
resolvers for different TLDs.

swarm/api/api.go

@@ -17,6 +17,7 @@
 package api
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -40,6 +41,69 @@ type Resolver interface {
 	Resolve(string) (common.Hash, error)
 }
 
+// errNoResolver is returned by MultiResolver.Resolve if no resolver
+// can be found for the address.
+var errNoResolver = errors.New("no resolver")
+
+// MultiResolver is used to resolve URL addresses based on their TLDs.
+// Each TLD can have multiple resolvers, and the resoluton from the
+// first one in the sequence will be returned.
+type MultiResolver struct {
+	resolvers map[string][]Resolver
+}
+
+// MultiResolverOption sets options for MultiResolver and is used as
+// arguments for its constructor.
+type MultiResolverOption func(*MultiResolver)
+
+// MultiResolverOptionWithResolver adds a Resolver to a list of resolvers
+// for a specific TLD. If TLD is an empty string, the resolver will be added
+// to the list of default resolver, the ones that will be used for resolution
+// of addresses which do not have their TLD resolver specified.
+func MultiResolverOptionWithResolver(r Resolver, tld string) MultiResolverOption {
+	return func(m *MultiResolver) {
+		if _, ok := m.resolvers[tld]; !ok {
+			m.resolvers[tld] = []Resolver{}
+		}
+		m.resolvers[tld] = append(m.resolvers[tld], r)
+	}
+}
+
+// NewMultiResolver creates a new instance of MultiResolver.
+func NewMultiResolver(opts ...MultiResolverOption) (m *MultiResolver) {
+	m = &MultiResolver{
+		resolvers: map[string][]Resolver{},
+	}
+	for _, o := range opts {
+		o(m)
+	}
+	return m
+}
+
+// Resolve resolves address by choosing a Resolver by TLD.
+// If there are more default Resolvers, or for a specific TLD,
+// the Hash from the the first one which does not return error
+// will be returned.
+func (m MultiResolver) Resolve(addr string) (h common.Hash, err error) {
+	rs, _ := m.resolvers[""]
+	if i := strings.LastIndex(addr, "."); i >= 0 {
+		rstld, ok := m.resolvers[addr[i+1:]]
+		if ok {
+			rs = rstld
+		}
+	}
+	if rs == nil {
+		return h, errNoResolver
+	}
+	for _, r := range rs {
+		h, err = r.Resolve(addr)
+		if err == nil {
+			return
+		}
+	}
+	return
+}
+
 /*
 Api implements webserver/file system related content storage and retrieval
 on top of the dpa