Fix BigTable multi-entry row cached Tail.

logdog/common/storage/bigtable/cache.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package bigtable
 
 import (
         "bytes"
         "encoding/binary"
         "time"
@@ skipping 29 matching lines @@
         v, err := binary.ReadVarint(bytes.NewReader(itm.Data))
         if err != nil {
                 log.Fields{
                         log.ErrorKey: err,
                         "project":    project,
                         "path":       path,
                 }.Warningf(c, "Could not decode last tail cache.")
                 return 0
         }
 
+        log.Fields{
+                "index": v,
+        }.Infof(c, "Using cached tail index.")
         return v
 }
 
 func putLastTailIndex(c context.Context, cache caching.Cache, project config.ProjectName, path types.StreamPath, v int64) {
         buf := make([]byte, binary.MaxVarintLen64)
         buf = buf[:binary.PutVarint(buf, v)]
 
         itm := mkLastTailItem(project, path)
         itm.Data = buf
         cache.Put(c, lastTailIndexCacheDuration, itm)
 }
 
 func mkLastTailItem(project config.ProjectName, path types.StreamPath) *caching.Item {
         return &caching.Item{
                 Schema: cacheSchema,
                 Type:   "bt_tail_idx",
                 Key:    caching.HashKey(string(project), string(path)),
         }
 }