engine: Resources package rewrite

This giant patch makes some much needed improvements to the code base.

* The engine has been rewritten and lives within engine/graph/
* All of the common interfaces and code now live in engine/
* All of the resources are in one package called engine/resources/
* The Res API can use different "traits" from engine/traits/
* The Res API has been simplified to hide many of the old internals
* The Watch & Process loops were previously inverted, but is now fixed
* The likelihood of package cycles has been reduced drastically
* And much, much more...

Unfortunately, some code had to be temporarily removed. The remote code
had to be taken out, as did the prometheus code. We hope to have these
back in new forms as soon as possible.

util/sync.go

@@ -0,0 +1,87 @@
+// Mgmt
+// Copyright (C) 2013-2018+ 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 util
+
+import (
+	"sync"
+)
+
+// EasyOnce is a wrapper for the sync.Once functionality which lets you define
+// and register the associated `run once` function at declaration time. It may
+// be copied at any time.
+type EasyOnce struct {
+	Func func()
+
+	once *sync.Once
+}
+
+// Done runs the function which was defined in `Func` a maximum of once.
+func (obj *EasyOnce) Done() {
+	if obj.once == nil {
+		// we must initialize it!
+		obj.once = &sync.Once{}
+	}
+	if obj.Func != nil {
+		obj.once.Do(obj.Func)
+	}
+}
+
+// EasyExit is a struct that helps you build a close switch and signal which can
+// be called multiple times safely, and used as a signal many times in parallel.
+type EasyExit struct {
+	exit chan struct{}
+	once *sync.Once
+	err  error
+}
+
+// NewEasyExit builds an easy exit struct.
+func NewEasyExit() *EasyExit {
+	return &EasyExit{
+		exit: make(chan struct{}),
+		once: &sync.Once{},
+	}
+}
+
+// Done triggers the exit signal. It associates an error condition with it too.
+func (obj *EasyExit) Done(err error) {
+	if obj.once == nil {
+		// we must initialize it!
+		obj.once = &sync.Once{}
+	}
+	if err != nil {
+		// TODO: we could add a mutex, and turn this into a multierr
+		obj.err = err
+	}
+	obj.once.Do(func() { close(obj.exit) })
+}
+
+// Signal returns the channel that we watch for the exit signal on. It will
+// close to signal us when triggered by Exit().
+func (obj *EasyExit) Signal() chan struct{} {
+	return obj.exit
+}
+
+// Error returns the error condition associated with the Done signal. It blocks
+// until Done is called at least once. It then returns any of the errors or nil.
+// It is only guaranteed to at least return the error from the first Done error.
+func (obj *EasyExit) Error() error {
+	select {
+	case <-obj.exit:
+	}
+	return obj.err
+}