SharedMemoryHelpers.CreateOrOpenFile throws IOException 'already exists' for NuGet-Migrations named mutex on Linux #125632

Description

dotnet CLI first-run experience fails intermittently on Linux with:

System.IO.IOException: The file '/tmp/.dotnet/shm/session1/NuGet-Migrations' already exists.
   at System.IO.SharedMemoryHelpers.CreateOrOpenFile(String sharedMemoryFilePath, SharedMemoryId id, Boolean createIfNotExist, Boolean& createdFile)
   at System.IO.SharedMemoryProcessDataHeader`1.CreateOrOpen(...)
   at System.Threading.NamedMutexProcessDataBase.CreateOrOpen(...)
   at System.Threading.WaitSubsystem.NamedMutex.CreateNamedMutex(...)
   at System.Threading.Mutex.CreateMutexCore(...)
   at System.Threading.Mutex..ctor(Boolean initiallyOwned, String name)
   at NuGet.Common.Migrations.MigrationRunner.Run(...)
   at Microsoft.DotNet.Configurer.DotnetFirstTimeUseConfigurer.Configure()
   at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, TimeSpan startupTime)

This is blocking the VMR official build (dotnet-unified-build definition 1330) on main. Observed in:

• Build 20260316.1 — ValidateInstallers_Linux_x64 job, DebScenarioTest(repo: "mcr.microsoft.com/dotnet/runtime-deps", tag: "10.0-noble")

The VerifyConsoleTemplateComplex(CSharp) scenario test runs dotnet new console inside a Docker container. A prior dotnet invocation in the same container created the shared memory file at /tmp/.dotnet/shm/session1/NuGet-Migrations. The second invocation's CreateOrOpenFile throws IOException instead of opening the existing file.

Expected behavior

SharedMemoryHelpers.CreateOrOpenFile should handle the case where the shared memory file already exists from a previous process, either by opening it or by recreating it.

Actual behavior

Throws System.IO.IOException: The file ... already exists and the dotnet command fails with exit code 1.

Relation to prior issue

dotnet/runtime#80619 tracked a similar NuGet-Migrations mutex failure on Windows (The system cannot open the device or file specified) in .NET 7/8 timeframe. That issue was fixed and closed in Aug 2023. This appears to be a different codepath — the new SharedMemoryProcessDataHeader / SharedMemoryHelpers implementation on Linux.