Make -NoCache skip reading packages from the global packages folder

[joelverhagen]

Fixes NuGet/Home#1406 and NuGet/Home#3074.

This PR brings -NoCache closer to sanity. Other related caching issues are tracked here:

• packages.config operations should use the HTTP cache Home#3132: packages.config scenarios should use the HTTP cache.
• Inconsistent -NoCache behavior Home#3244: -NoCache with a package missing on all sources should cause the operation to fail.
• Caching of version lists leads to not seeing a new package version for 30 minutes Home#3389: Caching of version lists in the HTTP cache should be smarter.
• .nupkg's are cached for only 30 minutes in the HTTP cache Home#3390: Packages in the HTTP cache should last for much longer than 30 minutes.
• Download/Restore operations should check the http cache before making http calls to all sources Home#3392: HTTP cache should be checked before multi-source HTTP requests.

Check the CachingTests.cs file for assertions on current caching functionality.

To summarize, the caching switches should behave as follows:

-NoCache means "don't read from the cache". If something is in the global packages folder and you are restoring packages.config (that is, the destination folder is something other than a global packages folder), don't use it. Use the version from the source. Same goes for the HTTP cache but the behavior should be consistent between packages.config and project.json.

-DirectDownload means "don't write to the cache". If something is in the global packages folder or HTTP cache, you can use it. The main idea here is to minimize disk usage.

-NoCache and -DirectDownload together should be "isolated mode". You neither read from nor write to the global packages folder (for packages.config) or HTTP cache (for both packages.config and project.json).

/cc @emgarten @yishaigalatzer @MarkOsborneMS @zarenner @zjrunner @pspill

[yishaigalatzer]

I would like to review 3132/3389/3390/3392 before executing on any of them. Mostly because we want a new unified cache across multiple feeds for package content (and keep the http cache for metadata only).

This is already on our backlog, and I would love to avoid further investment in the http cache

[joelverhagen]

@yishaigalatzer, the 4 issue numbers you mentioned are not addressed by this PR. My PR addresses 1406 and 3074, both of which are schedule for 3.6. I mentioned these issues to give a high level view of what should be considered for future caching changes.

[yishaigalatzer]

Yup and so was my comment

[joelverhagen]

🔔 could I get some eyes on this?

[jainaashish]

When it gets the latest content from server with NoCache argument, should it also update it in cache? My assumption is since user didn't specify the directDownload flag so it should write the latest content in cache.

[joelverhagen]

We can't overwrite packages in the global packages folder. Another process might be using it, something non-NuGet like MSBuild, so our global locking mechanism would not work.

[joelverhagen]

👀 plz?

[jainaashish]

LGTM