etcd, resources, world: Add World API for shared keys

It's up to the end user to decide who is writing and/or overwriting
them.

It could also be useful to reimplement (refactor) some of the existing
World API's to be implemented in terms of these primitives.

etcd/strmap.go

@@ -0,0 +1,115 @@
+// Mgmt
+// Copyright (C) 2013-2017+ 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 Affero 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 Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package etcd
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/purpleidea/mgmt/util"
+
+	etcd "github.com/coreos/etcd/clientv3"
+	errwrap "github.com/pkg/errors"
+)
+
+// WatchStrMap returns a channel which spits out events on key activity.
+// FIXME: It should close the channel when it's done, and spit out errors when
+// something goes wrong.
+func WatchStrMap(obj *EmbdEtcd, key string) chan error {
+	// new key structure is /$NS/strings/$key/$hostname = $data
+	path := fmt.Sprintf("/%s/strings/%s", NS, key)
+	ch := make(chan error, 1)
+	// FIXME: fix our API so that we get a close event on shutdown.
+	callback := func(re *RE) error {
+		// TODO: is this even needed? it used to happen on conn errors
+		//log.Printf("Etcd: Watch: Path: %v", path) // event
+		if re == nil || re.response.Canceled {
+			return fmt.Errorf("watch is empty") // will cause a CtxError+retry
+		}
+		if len(ch) == 0 { // send event only if one isn't pending
+			ch <- nil // event
+		}
+		return nil
+	}
+	_, _ = obj.AddWatcher(path, callback, true, false, etcd.WithPrefix()) // no need to check errors
+	return ch
+}
+
+// GetStrMap collects all of the strings which match a namespace in etcd.
+func GetStrMap(obj *EmbdEtcd, hostnameFilter []string, key string) (map[string]string, error) {
+	// old key structure is /$NS/strings/$hostname/$key = $data
+	// new key structure is /$NS/strings/$key/$hostname = $data
+	// FIXME: if we have the $key as the last token (old key structure), we
+	// can allow the key to contain the slash char, otherwise we need to
+	// verify that one isn't present in the input string.
+	path := fmt.Sprintf("/%s/strings/%s", NS, key)
+	keyMap, err := obj.Get(path, etcd.WithPrefix(), etcd.WithSort(etcd.SortByKey, etcd.SortAscend))
+	if err != nil {
+		return nil, errwrap.Wrapf(err, "could not get strings in: %s", key)
+	}
+	result := make(map[string]string)
+	for key, val := range keyMap {
+		if !strings.HasPrefix(key, path) { // sanity check
+			continue
+		}
+
+		str := strings.Split(key[len(path):], "/")
+		if len(str) != 2 {
+			return nil, fmt.Errorf("unexpected chunk count of %d", len(str))
+		}
+		_, hostname := str[0], str[1]
+
+		if hostname == "" {
+			return nil, fmt.Errorf("unexpected chunk length of %d", len(hostname))
+		}
+
+		// FIXME: ideally this would be a server side filter instead!
+		if len(hostnameFilter) > 0 && !util.StrInList(hostname, hostnameFilter) {
+			continue
+		}
+		//log.Printf("Etcd: GetStr(%s): (Hostname, Data): (%s, %s)", key, hostname, val)
+		result[hostname] = val
+	}
+	return result, nil
+}
+
+// SetStrMap sets a key and hostname pair to a certain value. If the value is
+// nil, then it deletes the key. Otherwise the value should point to a string.
+// TODO: TTL or delete disconnect?
+func SetStrMap(obj *EmbdEtcd, hostname, key string, data *string) error {
+	// key structure is /$NS/strings/$key/$hostname = $data
+	path := fmt.Sprintf("/%s/strings/%s/%s", NS, key, hostname)
+	ifs := []etcd.Cmp{} // list matching the desired state
+	ops := []etcd.Op{}  // list of ops in this transaction (then)
+	els := []etcd.Op{}  // list of ops in this transaction (else)
+	if data == nil {    // perform a delete
+		// TODO: use https://github.com/coreos/etcd/pull/7417 if merged
+		//ifs = append(ifs, etcd.KeyExists(path))
+		ifs = append(ifs, etcd.Compare(etcd.Version(path), ">", 0))
+		ops = append(ops, etcd.OpDelete(path))
+	} else {
+		data := *data                                                // get the real value
+		ifs = append(ifs, etcd.Compare(etcd.Value(path), "=", data)) // desired state
+		els = append(els, etcd.OpPut(path, data))
+	}
+
+	// it's important to do this in one transaction, and atomically, because
+	// this way, we only generate one watch event, and only when it's needed
+	_, err := obj.Txn(ifs, ops, els) // TODO: do we need to look at response?
+	return errwrap.Wrapf(err, "could not set strings in: %s", key)
+}