Save cached releases to standard directories

[Siilwyn]

Instead of placing the release archives in the home directory they should be placed in the appropriate directory per OS. I've done the same for Greenkeeper's RC so I'd like to create a PR for it.

Before doing that I'd like to know if there are any special things I should take into consideration and if the core contributors are on board with this so I don't do it in vain. :)

[malept]

What do you propose to do with the existing cache directory? Is that still taken into account?

[Siilwyn]

We could use the old location if it exists or move the old location to the new location. At Greenkeeper the moving is done. Both have their own pros and cons.

[Siilwyn]

@malept any preference for one of the solutions?

[malept]

I would prefer moving it.

[develar]

@Siilwyn I see error in your merged changes, that leads to #47 (comment)

Code

const oldLocation = path.join(os.homedir(), './.electron', this.filename)
    if (pathExists.sync(oldLocation)) {
      return oldLocation
    }

in the cachedZip

But! We call cachedZip in the ensureCacheDir twice — one for check, and another one in the call downloadZip. So, 2 sync calls are performed, and latter can lead to race conditions — downloadZip must download only to a new location.

In version of electron-download that used by electron-builder, I will remove this code for now.

[Siilwyn]

Oh that is bad. :/

@develar could you expand your explanation? I don't quite follow: cachedZip is not being called directly from ensureCacheDir and downloadZip is called once.

[develar]

@Siilwyn downloadZip calls cachedZip, but cachedZip will check and return old location. Not evident how does it lead to error, but in any case it is not good — cachedZip should return constant result (based on options) and old location should be checked in another place.

[Siilwyn]

The function that checks if the zip file is already cached sounds like the appropriate place for this check to me. Where would you place it?

Edit: I replicated the error @develar, will debug it!
Edit 2: might have to do with the checksum file location handling.
Edit 3: definitely something to do with checksum validating, downloading a version below 1.3.2 works fine.

[Siilwyn]

@develar moving the logic to the directory location seems to fix it. Could you confirm this?