Update operations are non atmoic across processes

[Alphasite]

What happened: Ran multiple instances of grype CLI and the database ended up invalid, failing the integrity checks.

What you expected to happen: It should be parallel safe.

How to reproduce it (as minimally and precisely as possible): Spin up multiple scan tasks in an environment without the database downloaded.

Anything else we need to know?:

Its likely this specific section of code:

	// activate the new db cache
	return file.CopyDir(c.fs, dbDirPath, c.dbDir)

I don't know if there is a complete solution but as i understand it using a symlink or a rename would probably be an atomic operation which doesn't have as much risk of concurrent issues which this current approach of delete and then copy has.

This covers a similar issue: https://stackoverflow.com/questions/307437/moving-a-directory-atomically for ideas.

Environment:

• Output of grype version:
• OS (e.g: cat /etc/os-release or similar): Ubuntu Trusty