Summary:
In D28521 we removed GarbageCollect() from BackupEngine's constructor. The reason was that opening BackupEngine on HDFS was very slow and in most cases we didn't have any garbage. We allowed the user to call GarbageCollect() when it detects some garbage files in his backup directory.
Unfortunately, this left us vulnerable to an interesting issue. Let's say we started a backup and copied files {1, 3} but the backup failed. On another host, we restore DB from backup and generate {1, 3, 5}. Since {1, 3} is already there, we will not overwrite. However, these files might be from a different database so their contents might be different. See internal task t6781803 for more info.
Now, when we're copying files and we discover a file already there, we check:
1. if the file is not referenced from any backups, we overwrite the file.
2. if the file is referenced from other backups AND the checksums don't match, we fail the backup. This will only happen if user is using a single backup directory for backing up two different databases.
3. if the file is referenced from other backups AND the checksums match, it's all good. We skip the copy and go copy the next file.
Test Plan: Added new test to backupable_db_test. The test fails before this patch.
Reviewers: sdong, rven, yhchiang
Reviewed By: yhchiang
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D37599
50eab9cf30