TFS online build error in NuGet.targets

Question

Just yesterday, all of our TFS online builds started failing with the same NuGet.targets error
C:\a\src\.nuget\NuGet.targets (92): The process cannot access the file 'C:\Users\buildguest\AppData\Roaming\NuGet\NuGet.Config' because it is being used by another process.
This started happening right after a performance related TFS outage on 12/18/2014. Don't know how it could have changed anything, maybe just coincidence.
No changes to solutions, structure, packages or otherwise - just source changes checked in.
Can this be corrected?

Answer 1

You are likely still using the msbuild based NuGet package restore from prior to NuGet 2.7 and you are still likely using an old version of NuGet.exe with that as that
process checked a version of NuGet.exe in with it.  This is no longer a recommend approach because there are certain NuGet packages that now include msbuild targets as part of them because MSBuild needs all of the refereced targets files on disk when
it starts it is not possible to add targets files as part of the NuGet restore. 
My guess is the error you are now seeing is because we have moved to dual proc build machines and this a multiproc MSBuild with legacy old style msbuild hack package restore bug. 
It is likely you can upgrade to a newer version of nuget.exe and check that version in with their solution and it will work around this issue.
There are detailed instructions on the NuGet.org site about the various ways to do package restore.
http://docs.nuget.org/docs/reference/package-restore
http://docs.nuget.org/docs/reference/Package-Restore-with-Team-Build