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/str.go

@@ -18,20 +18,22 @@
 package etcd
 
 import (
+	"errors"
 	"fmt"
-	"strings"
-
-	"github.com/purpleidea/mgmt/util"
 
 	etcd "github.com/coreos/etcd/clientv3"
 	errwrap "github.com/pkg/errors"
 )
 
+// ErrNotExist is returned when GetStr can not find the requested key.
+// TODO: https://dave.cheney.net/2016/04/07/constant-errors
+var ErrNotExist = errors.New("errNotExist")
+
 // WatchStr 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 WatchStr(obj *EmbdEtcd, key string) chan error {
-	// new key structure is /$NS/strings/$key/$hostname = $data
+	// new key structure is /$NS/strings/$key = $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.
@@ -50,50 +52,38 @@ func WatchStr(obj *EmbdEtcd, key string) chan error {
 	return ch
 }
 
-// GetStr collects all of the strings which match a namespace in etcd.
-func GetStr(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 $key as the last token (old key structure), we
-	// can allow the key to contain the slash char, otherwise we need to
-	// verify that one isn't present in the input string.
+// GetStr collects the string which matches a gloabl namespace in etcd.
+func GetStr(obj *EmbdEtcd, key string) (string, error) {
+	// new key structure is /$NS/strings/$key = $data
 	path := fmt.Sprintf("/%s/strings/%s", NS, key)
-	keyMap, err := obj.Get(path, etcd.WithPrefix(), etcd.WithSort(etcd.SortByKey, etcd.SortAscend))
+	keyMap, err := obj.Get(path, etcd.WithPrefix())
 	if err != nil {
-		return nil, errwrap.Wrapf(err, "could not get strings in: %s", key)
+		return "", 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
+	if len(keyMap) == 0 {
+		return "", ErrNotExist
 	}
-	return result, nil
+
+	if count := len(keyMap); count != 1 {
+		return "", fmt.Errorf("returned %d entries", count)
+	}
+
+	val, exists := keyMap[path]
+	if !exists {
+		return "", fmt.Errorf("path `%s` is missing", path)
+	}
+
+	//log.Printf("Etcd: GetStr(%s): %s", key, val)
+	return val, nil
 }
 
-// SetStr 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.
+// SetStr 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 SetStr(obj *EmbdEtcd, hostname, key string, data *string) error {
-	// key structure is /$NS/strings/$key/$hostname = $data
-	path := fmt.Sprintf("/%s/strings/%s/%s", NS, key, hostname)
+func SetStr(obj *EmbdEtcd, key string, data *string) error {
+	// key structure is /$NS/strings/$key = $data
+	path := fmt.Sprintf("/%s/strings/%s", NS, key)
 	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)