etcd: Switch to buffered channel to remove duplicates

Since we don't return the actual values and instead only tell about
events (which leaves the `Get` of the value as a second operation) then
we don't have to use a channel with backpressure since all the events
are identical.

etcd/str.go

@@ -33,7 +33,7 @@ import (
 func WatchStr(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)
+	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
@@ -41,7 +41,9 @@ func WatchStr(obj *EmbdEtcd, key string) chan error {
 		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