Automatically delete cache if zero-byte or corrupted files found

[asimihsan]

If the cache contains zero-byte or corrupted files we can no longer unpack tree pack sets, so we'll be unable to list directories or recover files. How to reproduce:

• Delete the cache directory
• Run:

arqinator \
    --backup-type s3 \
    --s3-region us-west-2 \
    --s3-bucket-name arq-akiajmthnhpkz2ixzrxq-us-west-2 \
    --verbose list-directory-contents \
    --backup-set-uuid 98DB38F8-B9C6-4296-9385-3C1BF858ED5D \
    --folder-uuid 8D4FAD2A-9E08-46F7-829D-E9601A65455D \
    --path /Users/ai/.ssh

• While it is caching tree pack sets CTRL-C it.
• Run the command again. You get the following output:

INFO[0006] Cached tree pack sets.
PANI[0006] Failed to mmap index file:  invalid argument
panic: (*logrus.Entry) (0x71c880,0xc8200112c0)

goroutine 1 [running]:
github.com/Sirupsen/logrus.Entry.log(0xc820072180, 0xc8203044b0, 0x0, 0x0, 0x0, 0xc820423b00, 0x0, 0x0, 0x0, 0xc820304510, ...)
    /Users/ai/Programming/golang/src/github.com/Sirupsen/logrus/entry.go:113 +0x618
github.com/Sirupsen/logrus.(*Entry).Panic(0xc820722550, 0xc8207224e0, 0x1, 0x1)
    /Users/ai/Programming/golang/src/github.com/Sirupsen/logrus/entry.go:158 +0x99
github.com/Sirupsen/logrus.(*Entry).Panicln(0xc820722550, 0xc820722750, 0x2, 0x2)
    /Users/ai/Programming/golang/src/github.com/Sirupsen/logrus/entry.go:253 +0x120
github.com/Sirupsen/logrus.(*Logger).Panicln(0xc820072180, 0xc820722750, 0x2, 0x2)
    /Users/ai/Programming/golang/src/github.com/Sirupsen/logrus/logger.go:204 +0xe7
github.com/Sirupsen/logrus.Panicln(0xc820722750, 0x2, 0x2)
    /Users/ai/Programming/golang/src/github.com/Sirupsen/logrus/exported.go:187 +0x41
github.com/asimihsan/arqinator/arq.(*ArqPackSetIndex).GetTreePackFile.func1(0xc820722870, 0xc8207229f0, 0xc8207227f8, 0xc820722860)
    /Users/ai/Programming/golang/src/github.com/asimihsan/arqinator/arq/pack_set_index.go:277 +0x3da

I've confirmed that there are zero-byte files in the cache directory.

I think we should be doing the following:

1. Scan the cache for zero-byte files. If any exist delete the cache directory and retry.
2. Scan the cache for corrupted files (how?). If any exist delete the cache directory and retry.

[asimihsan]

This reproduces the symptoms of #1 but I'm still not sure if this is the actual root cause of that issue.

[asimihsan]

Resolved by 8df12bf, which is released in v0.1.4.