lang: Initial implementation of the mgmt language

This is an initial implementation of the mgmt language. It is a
declarative (immutable) functional, reactive, domain specific
programming language. It is intended to be a language that is:

* safe
* powerful
* easy to reason about

With these properties, we hope this language, and the mgmt engine will
allow you to model the real-time systems that you'd like to automate.

This also includes a number of other associated changes. Sorry for the
large size of this patch.

etcd/scheduler/rrstrategy.go

@@ -0,0 +1,84 @@
+// 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 scheduler // TODO: i'd like this to be a separate package, but cycles!
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/purpleidea/mgmt/util"
+)
+
+func init() {
+	Register("rr", func() Strategy { return &rrStrategy{} }) // must register the func and name
+}
+
+type rrStrategy struct {
+	// some stored state
+	hosts []string
+}
+
+// Schedule returns hosts in round robin style from the available hostnames.
+func (obj *rrStrategy) Schedule(hostnames map[string]string, opts *schedulerOptions) ([]string, error) {
+	if len(hostnames) <= 0 {
+		return nil, fmt.Errorf("strategy: cannot schedule from zero hosts")
+	}
+	if opts.maxCount <= 0 {
+		return nil, fmt.Errorf("strategy: cannot schedule with a max of zero")
+	}
+
+	// always get a deterministic list of current hosts first...
+	sortedHosts := []string{}
+	for key := range hostnames {
+		sortedHosts = append(sortedHosts, key)
+	}
+	sort.Strings(sortedHosts)
+
+	if obj.hosts == nil {
+		obj.hosts = []string{} // initialize if needed
+	}
+
+	// add any new hosts we learned about, to the end of the list
+	for _, x := range sortedHosts {
+		if !util.StrInList(x, obj.hosts) {
+			obj.hosts = append(obj.hosts, x)
+		}
+	}
+
+	// remove any hosts we previouly knew about from the list
+	for ix := len(obj.hosts) - 1; ix >= 0; ix-- {
+		if !util.StrInList(obj.hosts[ix], sortedHosts) {
+			// delete entry at this index
+			obj.hosts = append(obj.hosts[:ix], obj.hosts[ix+1:]...)
+		}
+	}
+
+	// get the maximum number of hosts to return
+	max := len(obj.hosts)    // can't return more than we have
+	if opts.maxCount < max { // found a smaller limit
+		max = opts.maxCount
+	}
+
+	result := []string{}
+	// now return the number of needed hosts from the list
+	for i := 0; i < max; i++ {
+		result = append(result, obj.hosts[i])
+	}
+
+	return result, nil
+}