tailer: account for every fetched entry and log skips

fetchRange counted an entry as consumed before attempting to decode
it, so undecodable entries were skipped permanently with no record,
and the position could overshoot the requested range if a log served
more entries than asked. The position now advances by exactly the
number of received entries (clamped to the range) and skips are
logged with their index window.

internal/tailer/tailer.go

@@ -156,11 +156,11 @@ func (t *Tailer) fetchRange(ctx context.Context, client *rfc6962.Client, logID s
 		if len(entries) == 0 {
 			return total, nil
 		}
-		decoded := int64(0)
+		skipped := int64(0)
 		for _, e := range entries {
 			leaf, err := rfc6962.DecodeLeafEntry(e)
-			decoded++
 			if err != nil {
+				skipped++
 				continue
 			}
 			fs := leaf.Timestamp
@@ -170,6 +170,7 @@ func (t *Tailer) fetchRange(ctx context.Context, client *rfc6962.Client, logID s
 				}
 			}
 			if fs <= 0 {
+				skipped++
 				continue
 			}
 			for _, name := range leaf.Names {
@@ -182,8 +183,15 @@ func (t *Tailer) fetchRange(ctx context.Context, client *rfc6962.Client, logID s
 				}
 			}
 		}
-		total += decoded
-		pos += decoded
+		if skipped > 0 {
+			log.Printf("tail %s: skipped %d undecodable entries in [%d,%d]", client.ShortID(), skipped, pos, end)
+		}
+		n := int64(len(entries))
+		if pos+n > to {
+			n = to - pos
+		}
+		total += n
+		pos += n
 	}
 	return total, nil
 }