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/interfaces/client.go

@@ -60,4 +60,7 @@ type Client interface {
 	Txn(ctx context.Context, ifCmps []etcd.Cmp, thenOps, elseOps []etcd.Op) (*etcd.TxnResponse, error)
 	Watcher(ctx context.Context, path string, opts ...etcd.OpOption) (chan error, error)
 	ComplexWatcher(ctx context.Context, path string, opts ...etcd.OpOption) (*WatcherInfo, error)
+
+	// WatchMembers returns a channel of changing members in the cluster.
+	WatchMembers(context.Context) (<-chan *MembersResult, error)
 }

etcd/interfaces/const.go

@@ -0,0 +1,30 @@
+// Mgmt
+// Copyright (C) 2013-2023+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package interfaces
+
+import (
+	"time"
+)
+
+const (
+	// MemberChangePollingInterval is the polling interval to use when
+	// watching for member changes for client operations. This is basically
+	// the same idea in the core package named MemberChangeInterval, but
+	// held separately for now.
+	MemberChangePollingInterval = 1000 * time.Millisecond
+)

etcd/interfaces/structs.go

@@ -0,0 +1,57 @@
+// Mgmt
+// Copyright (C) 2013-2023+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package interfaces
+
+import (
+	etcdtypes "go.etcd.io/etcd/client/pkg/v3/types"
+)
+
+// MembersResult returns the expect result (including possibly an error) from a
+// WatchMembers operation.
+type MembersResult struct {
+
+	// Members is the list of members found in this result.
+	Members []*Member
+
+	// Err represents an error. If this is not nil, don't touch the other
+	// data in this struct.
+	Err error
+}
+
+// Member is our internal copy of the etcd member struct as found here:
+// https://godocs.io/github.com/coreos/etcd/etcdserver/etcdserverpb#Member but
+// which uses native types where possible.
+type Member struct {
+	// ID is unique in the cluster for each member.
+	ID uint64
+
+	// Name for the member which if not not started will be an empty string.
+	Name string
+
+	// IsLeader tells which member is leading the cluster. Expect this to
+	// change as time goes on.
+	// XXX: add when new version of etcd supports this
+	//IsLeader bool
+
+	// PeerURLs is the list of addresses peers servers can connect to.
+	PeerURLs etcdtypes.URLs
+
+	// ClientURLs is the list of addresses that clients can connect to. If
+	// the member is not started, then this will be a zero length list.
+	ClientURLs etcdtypes.URLs
+}