etcd: Rewrite embed etcd implementation

This is a giant cleanup of the etcd code. The earlier version was
written when I was less experienced with golang.

This is still not perfect, and does contain some races, but at least
it's a decent base to start from. The automatic elastic clustering
should be considered an experimental feature. If you need a more
battle-tested cluster, then you should manage etcd manually and point
mgmt at your existing cluster.

etcd/interfaces/client.go

@@ -0,0 +1,63 @@
+// Mgmt
+// Copyright (C) 2013-2019+ 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 (
+	"context"
+
+	etcd "github.com/coreos/etcd/clientv3" // "clientv3"
+	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
+)
+
+// WatcherData is the structure of data passed to a callback from any watcher.
+type WatcherData struct {
+	// Created is true if this event is the initial event sent on startup.
+	Created bool
+
+	// XXX: what goes here... this? or a more processed version?
+	Path   string // the path we're watching
+	Header pb.ResponseHeader
+	Events []*etcd.Event
+	Err    error
+}
+
+// WatcherInfo is what is returned from a Watcher. It contains everything you
+// might need to get information about the running watch.
+type WatcherInfo struct {
+	// Cancel must be called to shutdown the Watcher when we are done with
+	// it. You can alternatively call cancel on the input ctx.
+	Cancel func()
+
+	// Events returns a channel of any events that occur. This happens on
+	// watch startup, watch event, and watch failure. This channel closes
+	// when the Watcher shuts down. If you block on these reads, then you
+	// will block the entire Watcher which is usually not what you want.
+	Events <-chan *WatcherData
+}
+
+// Client provides a simple interface specification for client requests. Both
+// EmbdEtcd.MakeClient and client.Simple implement this.
+type Client interface {
+	GetClient() *etcd.Client
+	Set(ctx context.Context, key, value string, opts ...etcd.OpOption) error
+	Get(ctx context.Context, path string, opts ...etcd.OpOption) (map[string]string, error)
+	Del(ctx context.Context, path string, opts ...etcd.OpOption) (int64, error)
+	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)
+}

etcd/interfaces/error.go

@@ -0,0 +1,33 @@
+// Mgmt
+// Copyright (C) 2013-2019+ 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
+
+// Error is a constant error type that implements error.
+type Error string
+
+// Error fulfills the error interface of this type.
+func (e Error) Error() string { return string(e) }
+
+const (
+	// ErrNotExist is returned when GetStr or friends can not find the
+	// requested key.
+	ErrNotExist = Error("ErrNotExist")
+
+	// ErrShutdown is returned when we're exiting during a shutdown.
+	ErrShutdown = Error("ErrShutdown")
+)