Add license header to files that have no header

[tlakollo]

This PR adds the following message as header

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

for files that dont contain any license information

[tlakollo]

The runtime repository has license headers for tests and files. Currently, the linker repo does not have a license displayed in all files, with most testing files not having any header. Some concern brought by @MichalStrehovsky is that some of these files may precede the acquisition made by Microsoft and in that case is not clear if we can copyright them.
Having said that, the files under some other license haven't been changed and this PR only adds to files with no license explicitly displayed

[marek-safar]

I'd not be too concerned with copyright for test files, especially if they are small.

@mrvoorhe could you comment with explicit approval too?

[vitek-karas]

I'm perfectly OK adding the license header to everything - makes it simple and consistent.
That said, maybe we should change the editor config to make this into a warning:

linker/.editorconfig

Lines 103 to 105 in 1a6468f

	# IDE0073: File header
	dotnet_diagnostic.IDE0073.severity = suggestion
	file_header_template = Licensed to the .NET Foundation under one or more agreements.\nThe .NET Foundation licenses this file to you under the MIT license.

In that case linter would catch missing headers and fail the CI (and running it locally should actually add the header there for you).

[tlakollo]

I'm perfectly OK adding the license header to everything - makes it simple and consistent.

What about files that currently have a different license header for example AssemblyResolver.cs which has a Novell license?

[vitek-karas]

I guess we will need to involve somebody who understands licensing. The LICENSE.txt in the root doesn't mention Novell at all, it's purely a .NET Foundation license. I assume that we would need to either change the license for the file or add something to the main license.

[MichalStrehovsky]

I would not touch the files that have existing license headers - see dotnet/runtime#63146 (comment).

I don't know if we have existing guidance from CELA on source files that are old, but don't have any licensing header whatsoever.

It should be fine to add the header to any files added in the past ~5 years (and it is our policy to have those headers).

[tlakollo]

Following CELA guidance we can:

• Add Novel, Xamarin, and others to our notice file
• Add the DFN license information at the start of the file once Microsoft had made changes to the file

[tlakollo]

Files that still have previous license headers and haven't been modified are:

• linker\src\linker\Linker\TypePreserve.cs
• linker\src\linker\Linker\Tracer.cs
• linker\src\linker\Linker\MethodAction.cs
• linker\src\linker\Linker.Steps\RegenerateGuidStep.cs

Files from project:

• linker\src\analyzer\analyzer.csproj

[vitek-karas]

If these files are not modified, then we should keep them as-is.
For the rest I think this makes sense. But please get a review from @richlander at least.

[richlander]

The LICENSE.txt in the root doesn't mention Novell at all

That's what the 3PN is for.

Here are some good rules:

• Do: Add .NET Foundation copyright notice to all files, including those with no notice.
• Do: Add .NET Foundation notice to the top even if there are other notices. That's just a sanity win for consistency for our project.
• Do: Add .NET Foundation notice for files we never modified. It makes it clear that these files are covered by our MIT license and our 3PN. Also, it just helps with our sanity. Let's avoid special rules where we can. Also, if someone complains about this practice, we can address it at the time. We can beg for forgiveness based on a legit intent to improve clarity.
• Do not: remove/modify the Novell notice (ever).
• Do: If something feels odd of wrong, ask.

[sbomer]

Thanks! It's probably worth re-doing the changes on top of the latest commit from a fresh state to make sure there are no unrelated changes introduced by accident.

[tlakollo]

@richlander I think this PR is ready for review, PTAL

[tlakollo]

Every commit now is potentially a merge conflict, so it will be nice to merge this as soon as possible to prevent unnecessary rework

[sbomer]

LGTM, thanks a lot!