Sync the breadcrumbs writer

[victorhurdugaci]

Fixes: #1733 and #1771

The root cause is this change:

dnx/src/Microsoft.Framework.Runtime/ExportProviders/ProjectLibraryExportProvider.cs

Line 102 in be5bb7b

context.DependencyWalker.Walk(project.Name, project.Version, target.TargetFramework);

. We write the breadcrumbs during the dependency graph walking. That code was called only once before and it happened before breadcrumb writing.

Now that is it called multiple times, it can happen to run at the same time as the breadcrumb writing that that's running on a different thread.

This change will allow Add to be called at anytime and only throw if you try to add a new breadcrumb (one that didn't exists when write was called).

Also, the change includes a lock for multi-process breadcrumb writing. It locks the breadcrumb file and doesn't allow other processes to access it while writing.

Please review: @davidfowl @pranavkm

[Eilon]

Two things we discussed that require follow-up:

1. Why is the dependency tree being walked twice? Is that a bug? Or on purpose? Are the results the same? Cache?
2. Is it allowed/supported for user code to load NuGet packages while the app is running? If so, we should theoretically breadcrumb those, even though it might be technically challenging.

[victorhurdugaci]

@davidfowl the two questions above are for you. @Eilon and I discussed offline

[davidfowl]

Hah! My change caused those problem 😊

[Eilon]

@davidfowl you owe @victorhurdugaci $10 plus a dozen donuts.

[victorhurdugaci]

@davidfowl we can make than just a dozen donuts if you answer the Eilon's two questions 😄

[davidfowl]

I'm wondering if we can turn of breadcrumbing for compilation purposes or move where it is done so it doesn't happen more than once?

[victorhurdugaci]

We can do that. It would be the same ignoring any breadcrumb add once writing has started. That leads to @Eilon 's second question: is it possible to load NuGet packages dynamically while the app is running?

[davidfowl]

Ok here's what we need to do:

• Make the servicing classes services that are passed into the NuGetDependencyResolver instead of being a singleton (a singleton may be passed in but don't assume it is one).
• Make it so that a bool can be passed in that will disable ball bread crumbing

[victorhurdugaci]

•Make it so that a bool can be passed in that will disable ball bread crumbing

Are you okay with simply ignoring adds after writing has started?

[davidfowl]

You can absolutely call Assembly.Load or write your own loader that does something similar to what we do but that's not our problem

[davidfowl]

Yes we actually have this in the wrong layer. Everyone uses this api to reason about the dependency graph. We really shouldn't be bread crumbing anything that wouldn't get loaded. As much, it should only ever be true when the ApplicationHost runs

[victorhurdugaci]

Updated. No more adds after write

[moozzyk]

nit: remove whitespaces

[davidfowl]

Whats the deal with this?

[dnfclas]

@victorhurdugaci, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;

[davidfowl]

Why is this public?

[victorhurdugaci]

Because it is used by the package manager too. Please note that I moved the file in a different project

[davidfowl]