rpc: set a minimum long-polling interval for Events (#8050)

Since the goal of reading events at the head of the event log is to satisfy a
subscription style interface, there is no point in allowing head polling with
no wait interval. The pagination case already bypasses long polling, so the
extra option is unneessary.

Set a minimum default long-polling interval for the head case.
Add a test for minimum delay.

internal/rpc/core/events.go

@@ -165,8 +165,11 @@ func (env *Environment) Events(ctx context.Context,
 		maxItems = 100
 	}
 
+	const minWaitTime = 1 * time.Second
 	const maxWaitTime = 30 * time.Second
-	if waitTime > maxWaitTime {
+	if waitTime < minWaitTime {
+		waitTime = minWaitTime
+	} else if waitTime > maxWaitTime {
 		waitTime = maxWaitTime
 	}
 
@@ -185,7 +188,7 @@ func (env *Environment) Events(ctx context.Context,
 	accept := func(itm *eventlog.Item) error {
 		// N.B. We accept up to one item more than requested, so we can tell how
 		// to set the "more" flag in the response.
-		if len(items) > maxItems {
+		if len(items) > maxItems || itm.Cursor.Before(after) {
 			return eventlog.ErrStopScan
 		}
 		if cursorInRange(itm.Cursor, before, after) && query.Matches(itm.Events) {
@@ -194,7 +197,7 @@ func (env *Environment) Events(ctx context.Context,
 		return nil
 	}
 
-	if waitTime > 0 && before.IsZero() {
+	if before.IsZero() {
 		ctx, cancel := context.WithTimeout(ctx, waitTime)
 		defer cancel()