Golang DB.Update方法代碼示例

// dropBlockIDIndex drops the internal block id index.
func dropBlockIDIndex(db database.DB) error {
	return db.Update(func(dbTx database.Tx) error {
		meta := dbTx.Metadata()
		err := meta.DeleteBucket(idByHashIndexBucketName)
		if err != nil {
			return err
		}

		return meta.DeleteBucket(hashByIDIndexBucketName)
	})
}

// dropIndex drops the passed index from the database.  Since indexes can be
// massive, it deletes the index in multiple database transactions in order to
// keep memory usage to reasonable levels.  It also marks the drop in progress
// so the drop can be resumed if it is stopped before it is done before the
// index can be used again.
func dropIndex(db database.DB, idxKey []byte, idxName string) error {
	// Nothing to do if the index doesn't already exist.
	var needsDelete bool
	err := db.View(func(dbTx database.Tx) error {
		indexesBucket := dbTx.Metadata().Bucket(indexTipsBucketName)
		if indexesBucket != nil && indexesBucket.Get(idxKey) != nil {
			needsDelete = true
		}
		return nil
	})
	if err != nil {
		return err
	}
	if !needsDelete {
		log.Infof("Not dropping %s because it does not exist", idxName)
		return nil
	}

	// Mark that the index is in the process of being dropped so that it
	// can be resumed on the next start if interrupted before the process is
	// complete.
	log.Infof("Dropping all %s entries.  This might take a while...",
		idxName)
	err = db.Update(func(dbTx database.Tx) error {
		indexesBucket := dbTx.Metadata().Bucket(indexTipsBucketName)
		return indexesBucket.Put(indexDropKey(idxKey), idxKey)
	})
	if err != nil {
		return err
	}

	// Since the indexes can be so large, attempting to simply delete
	// the bucket in a single database transaction would result in massive
	// memory usage and likely crash many systems due to ulimits.  In order
	// to avoid this, use a cursor to delete a maximum number of entries out
	// of the bucket at a time.
	const maxDeletions = 2000000
	var totalDeleted uint64
	for numDeleted := maxDeletions; numDeleted == maxDeletions; {
		numDeleted = 0
		err := db.Update(func(dbTx database.Tx) error {
			bucket := dbTx.Metadata().Bucket(idxKey)
			cursor := bucket.Cursor()
			for ok := cursor.First(); ok; ok = cursor.Next() &&
				numDeleted < maxDeletions {

				if err := cursor.Delete(); err != nil {
					return err
				}
				numDeleted++
			}
			return nil
		})
		if err != nil {
			return err
		}

		if numDeleted > 0 {
			totalDeleted += uint64(numDeleted)
			log.Infof("Deleted %d keys (%d total) from %s",
				numDeleted, totalDeleted, idxName)
		}
	}

	// Call extra index specific deinitialization for the transaction index.
	if idxName == txIndexName {
		if err := dropBlockIDIndex(db); err != nil {
			return err
		}
	}

	// Remove the index tip, index bucket, and in-progress drop flag now
	// that all index entries have been removed.
	err = db.Update(func(dbTx database.Tx) error {
		meta := dbTx.Metadata()
		indexesBucket := meta.Bucket(indexTipsBucketName)
		if err := indexesBucket.Delete(idxKey); err != nil {
			return err
		}

		if err := meta.DeleteBucket(idxKey); err != nil {
			return err
		}

		return indexesBucket.Delete(indexDropKey(idxKey))
	})
	if err != nil {
		return err
	}

	log.Infof("Dropped %s", idxName)
	return nil
}