etcd: Add world API changes for watching member status

This adds some useful functionality so that anyone with access to the
world API, can learn information about the changing etcd cluster it's
using.

etcd/client/simple.go

@@ -24,6 +24,7 @@ import (
 	"time"
 
 	"github.com/purpleidea/mgmt/etcd/interfaces"
+	etcdUtil "github.com/purpleidea/mgmt/etcd/util"
 	"github.com/purpleidea/mgmt/util/errwrap"
 
 	etcd "go.etcd.io/etcd/client/v3"
@@ -480,3 +481,80 @@ func (obj *Simple) ComplexWatcher(ctx context.Context, path string, opts ...etcd
 		Events: eventsChan,
 	}, err
 }
+
+// WatchMembers returns a changing list of cluster membership. This is about the
+// server peers, not the connected clients.
+func (obj *Simple) WatchMembers(ctx context.Context) (<-chan *interfaces.MembersResult, error) {
+	if obj.client == nil { // etcd
+		return nil, fmt.Errorf("client is nil") // extra safety!
+	}
+
+	ch := make(chan *interfaces.MembersResult)
+	obj.wg.Add(1) // hook in to global wait group
+	go func() {
+		defer obj.wg.Done()
+		defer close(ch)
+		for {
+
+			members := []*interfaces.Member{}
+			result := &interfaces.MembersResult{}
+			resp, err := obj.client.MemberList(ctx)
+			if err != nil {
+				result.Err = err
+				goto Send
+			}
+			if resp == nil {
+				result.Err = fmt.Errorf("empty response")
+				goto Send
+			}
+
+			for _, m := range resp.Members {
+				if m == nil { // skip nil members
+					continue
+				}
+				// member: https://godocs.io/github.com/coreos/etcd/etcdserver/etcdserverpb#Member
+
+				purls, err := etcdUtil.FromStringListToURLs(m.PeerURLs)
+				if err != nil {
+					result.Err = errwrap.Wrapf(err, "invalid member peer URLs")
+					goto Send
+				}
+				curls, err := etcdUtil.FromStringListToURLs(m.ClientURLs)
+				if err != nil {
+					result.Err = errwrap.Wrapf(err, "invalid member client URLs")
+					goto Send
+				}
+
+				member := &interfaces.Member{
+					ID:   m.ID,
+					Name: m.Name,
+					//IsLeader: m.IsLeader, // XXX: add when new version of etcd supports this
+					PeerURLs:   purls,
+					ClientURLs: curls,
+				}
+				members = append(members, member)
+			}
+			result.Members = members
+
+		Send:
+			select {
+			case ch <- result: // send data
+			case <-ctx.Done():
+				return
+			}
+			if result.Err != nil {
+				return
+			}
+
+			// XXX: poll https://github.com/etcd-io/etcd/issues/5277
+			select {
+			case <-time.After(interfaces.MemberChangePollingInterval): // sleep before retry
+				// pass
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
+	return ch, nil
+}