Upgrading VS can cause projects to appear out-of-date, triggering lots of builds

[drewnoakes]

Steps

1. Enable fast up-to-date check logging
2. Create a .NET project and build it a few times, ensuring the project is up-to-date
3. Close VS
4. Upgrade VS
5. Re-open the project
6. Build the project

Expected

Project is still up-to-date.

Actual

Changes to MSBuild in the VS directory cause project(s) to appear out of date.

Logs show:

FastUpToDate: Input 'C:\Program Files\Microsoft Visual Studio\2022\Preview\MSBuild\Current\Microsoft.Common.targets\ImportAfter\Microsoft.LiveUnitTesting.targets' is newer (2024-07-29 09:35:47.859) than earliest output 'C:\myproject\some-file.cs' (2024-07-26 10:25:17.495), not up-to-date.

Analysis

We already have code that attempts to exclude certain inputs from the FUTDC based on folder location. These are files that we do not reasonably expect to change, such as files in NuGet packages and files in Program Files. That feature has either regressed, or should be extended to handle this case.

[drewnoakes]

A more detailed log, showing why this file is being checked:

FastUpToDate: Comparing timestamps of inputs and outputs:
FastUpToDate:     Adding UpToDateCheckBuilt outputs:
FastUpToDate:         C:\MyProject\bin\MyProject\Debug\net8.0\MyProject.dll
FastUpToDate:         C:\MyProject\obj\MyProject\Debug\net8.0\MyProject.dll
FastUpToDate:         C:\MyProject\bin\MyProject\Debug\net8.0\MyProject.xml
FastUpToDate:         C:\MyProject\obj\MyProject\Debug\net8.0\MyProject.xml
FastUpToDate:     Adding newest import input:
FastUpToDate:         C:\Program Files\Microsoft Visual Studio\2022\Preview\MSBuild\Current\Microsoft.Common.targets\ImportAfter\Microsoft.LiveUnitTesting.targets
FastUpToDate: Input 'C:\Program Files\Microsoft Visual Studio\2022\Preview\MSBuild\Current\Microsoft.Common.targets\ImportAfter\Microsoft.LiveUnitTesting.targets' is newer (2024-09-19 10:15:54.924) than earliest output 'C:\MyProject\bin\MyProject\Debug\net8.0\MyProject.xml' (2024-09-19 09:47:41.324), not up-to-date.
FastUpToDate: Up-to-date check completed in 1.4 ms
------ Build started: Project: MyProject, Configuration: Debug Any CPU ------

The newest import input is the most recently edited project file (i.e. .props/.targets/.csproj/etc), and MSBuild tells us this during DTBs (via the first item in MSBuildAllProjects). This means that we don't have to check every single project file, only the newest. Unfortunately, if the newest file is one of these "read-only" files, it may be that another important change was made to a different project file that is invisible to the FUTD check.

All that to say that unfortunately it won't be 100% reliable to ignore a "read-only" in this case, purely with changes to the FUTDC.

To pursue this, we could change MSBuild to optionally exclude these items when computing the newest project file, but that'd require a change to MSBuild.

The relevant code in MSbuild is:

https://github.com/dotnet/msbuild/blob/5bab860b2477f03050f170473a2563c5217d546f/src/Build/Evaluation/Evaluator.cs#L2170-L2173

And we would apply the policy defined in:

project-system/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/ProjectFileClassifier.cs

Lines 156 to 168 in 8d5a9b8

	/// <summary>
	/// Gets whether a file is expected to not be modified in place on disk once it has been created.
	/// </summary>
	/// <param name="filePath">The path to the file to test.</param>
	/// <returns><see langword="true"/> if the file is non-modifiable, otherwise <see langword="false"/>.</returns>
	public bool IsNonModifiable(string filePath)
	{
	return (s_programFiles64 is not null && filePath.StartsWith(s_programFiles64, StringComparisons.Paths))
	|| (s_programFiles86 is not null && filePath.StartsWith(s_programFiles86, StringComparisons.Paths))
	|| (s_windows is not null && filePath.StartsWith(s_windows, StringComparisons.Paths))
	|| _nuGetPackageFolders.Any(static (nuGetFolder, filePath) => filePath.StartsWith(nuGetFolder, StringComparisons.Paths), filePath)
	|| (s_vsInstallationDirectory is not null && filePath.StartsWith(s_vsInstallationDirectory, StringComparisons.Paths));
	}