Build fails when the .NET 11 CLI cannot connect to the MSBuild server during the restore of the compiler dependencies #156

The build fails with an error when PostSharp restores its compiler dependencies and the .NET 11 CLI cannot connect to its MSBuild server, although the restore itself succeeds:

PostSharp.targets(389,5): error : MSBuild server unavailable: could not connect to the server within the timeout window; the server may have failed to start. Falling back to an in-process build.

Observed

• TeamCity build 340008 (Build Artifacts of postsharp/PostSharp, .NET SDK 11.0.100-rc.1.26425.128, Windows agent) failed with this error while building PostSharp.Patterns.Common. The Patterns solution builds with PostSharp, so it used the product under test.
• The text is a resource of MSBuild.dll in the .NET 11 SDK. The CLI writes it to the standard error, then builds in-process.
• ToolInvocationHelper.InvokeTool (Core/PostSharp.MSBuild/ToolInvocationHelper.cs) reports every line of the standard error of a child process as an MSBuild error, whatever the exit code of the process.
• The same branch, on the same SDK, had built successfully in build 340000. The failure is intermittent.

Inferred, not measured

• The line came from the dotnet restore that BuildTimeDependencyRestorer.RestoreDependencyProject runs for the compiler dependencies. It is the only dotnet invocation of the MSBuild task that runs MSBuild and reports the standard error as errors: the crossgen dotnet msbuild of NativeImageManager downgrades errors to messages, and NetCoreResolver runs --list-runtimes only.
• The notice appears when the MSBuild server is slow to start, for example on a loaded build agent.

Expected

A restore that exits with code 0 does not fail the build. A notice on the standard error of dotnet is not an error.

To decide in the fix

• Whether the exit code, rather than the standard error, decides the success of the restore, and at which severity the standard error is logged.
• Whether the restore should run with the MSBuild server disabled (DOTNET_CLI_USE_MSBUILD_SERVER=0, MSBUILDUSESERVER=0). Otherwise, a build that has no server running starts one from PostSharp's task, and that server keeps running after the build.

Related: #92, which covers the validation of the MSBuild integration with the MSBuild server enabled by default.

— Claude for Gael Fraiteur