Global lock/mutex to prevent multiple MSBuild instances building the same project concurrently

Suppose you have more than one MSBuild invocations (either both command line, or a build from an IDE such as VS that is concurrent with a command-line build).

Currently we have no defense mechanisms against concurrent builds originating from different processes.

Let's brainstorm about perhaps taking a global mutex around the proj file being built (at per-project granularity). It's not clear at all what granularity should be used (around a project, around a solution?). Also the same project can be built multiple times with different targets, global properties, etc. Some of these builds have no side-effects, but some of them do. Do we only lock around builds that have side-effects?

Need to be careful here to allow multiple MSBuild.exe nodes belonging to the same build invocation to bypass the locking.

I understand it's a very fuzzy feature request, and it's going to be hard to get right, but I feel like we need some long term investment in this space. Otherwise if someone accidentally starts a build from the IDE while a command-line build is running, we can run into corruption and race conditions that are very hard to diagnose.

cc @xoofx