How volatile is the cache?

[trusktr]

Suppose we install a package from NPM, it gets cached in sinopia, then the npm user decides to delete the package from existence. What guarantee do we have that the package stays in sinopia?

f.e., can we restart the server? What can and can't we do?

[trusktr]

I was originally going to ask if it is possible for sinopia to copy over packages from NPM so that they become essentially like private packages in sinopia, but then I saw the cache detail.