Add Process.TryGetProcessById and SafeProcessHandle.TryOpen APIs

[Copilot]

Implements the approved API from #101582 — non-throwing alternatives to Process.GetProcessById and SafeProcessHandle.Open that return false when the target process doesn't exist, avoiding the inherent race condition.

// Returns false if process doesn't exist; throws UnauthorizedAccessException on permission errors
SafeProcessHandle.TryOpen(int processId, out SafeProcessHandle? processHandle);
Process.TryGetProcessById(int processId, out Process? process);

Changes

• Reference assembly (ref/System.Diagnostics.Process.cs): Added both new APIs with platform attributes and [NotNullWhen(true)]
• Windows implementation (SafeProcessHandle.Windows.cs): Extracted TryOpenCore — returns false on failure, throws UnauthorizedAccessException for ERROR_ACCESS_DENIED
• Unix implementation (SafeProcessHandle.Unix.cs): Same pattern — returns false on kill(pid, 0) failure, throws UnauthorizedAccessException on EPERM
• SafeProcessHandle.cs: Added public TryOpen with argument validation + PNSE guard; refactored Open to delegate directly to TryOpen; XML docs updated to reference UnauthorizedAccessException
• Process.cs: Added private Process(SafeProcessHandle processHandle) ctor to avoid re-acquiring the handle; TryGetProcessById delegates to SafeProcessHandle.TryOpen and passes the handle to the new ctor
• Tests (ProcessOpenTests.cs): Consolidated Open/TryOpen and GetProcessById/TryGetProcessById tests using [ConditionalTheory] with [InlineData]; added OS-specific UnauthorizedAccessException tests (Windows: lsass, Linux: PID 1) guarded by IsNotPrivilegedProcess; tests verify Kill + TryWaitForExit on opened handles; Open_ExitedProcess_BehaviorDependsOnPlatform uses (bool tryOpen, bool kill) parameters to verify both exit and kill produce the same platform-dependent behavior; replaced all CreateDefaultProcess usage with CreateProcess + explicit Kill() in finally blocks
• SafeProcessHandleTests.cs: Fixed build errors by cleanly removing only the Open_* tests that were moved to ProcessOpenTests.cs

[adamsitnik]

@copilot please address my feedback

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[adamsitnik]

@copilot address my feedback, build the product and run all process tests locally. Don't push any changes unless all process tests are passing

[Copilot]

@copilot address my feedback, build the product and run all process tests locally. Don't push any changes unless all process tests are passing

All feedback addressed. Built product and ran all process tests locally — 631 passed, 0 failed, 7 skipped (platform-specific).

[adamsitnik]

@copilot we are almost there, please address my feedback

[adamsitnik]

I would expect it to behave similar to Open_ExitedProcess_BehaviorDependsOnPlatform namely being able to open a process after it was killed/it exited on Windows.

Edit: I've run the test locally and confirmed that it fails on Windows:

      System.Diagnostics.Tests.ProcessOpenTests.GetProcessById_KilledProcess_Fails(tryGet: True) [FAIL]
        Assert.False() Failure
        Expected: False
        Actual:   True
        Stack Trace:
          D:\projects\runtime_copilot\src\libraries\System.Diagnostics.Process\tests\ProcessOpenTests.cs(234,0): at System.Diagnostics.Tests.ProcessOpenTests.GetProcessById_KilledProcess_Fails(Boolean tryGet)
             at System.RuntimeMethodHandle.InvokeMethod(ObjectHandleOnStack target, Void** arguments, ObjectHandleOnStack sig, BOOL isConstructor, ObjectHandleOnStack result)
          D:\projects\runtime_copilot\src\libraries\System.Private.CoreLib\src\System\Reflection\MethodBaseInvoker.cs(174,0): at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)

@copilot please merge GetProcessById_KilledProcess_Fails into Open_ExitedProcess_BehaviorDependsOnPlatform, add another boolean argument that determines whether the process should exit or be killed and then just use these asserts to verify that exit/killed produces the same output in terms of Open/TryOpen

[Copilot]

Done — merged GetProcessById_KilledProcess_Fails into Open_ExitedProcess_BehaviorDependsOnPlatform with [InlineData(bool tryOpen, bool kill)] (4 combinations). Both exit and kill scenarios now use the same platform-dependent assertions.

[adamsitnik]

LGTM now!

It's also passing for me locally on Windows:

  ----- start Thu 05/28/2026 22:05:31.74 ===============  To repro directly: =====================================================
  pushd D:\projects\runtime_copilot\artifacts\bin\System.Diagnostics.Process.Tests\Debug\net11.0-windows\
  "D:\projects\runtime_copilot\artifacts\bin\testhost\net11.0-windows-Debug-x64\dotnet.exe" exec --runtimeconfig System.Diagnostics.Process.Tests.runtimeconfig.json --depsfile System.Diagnostics.Process.Tests.deps.json C:\Users\adsitnik\.nuget\packages\microsoft.dotnet.xunitconsolerunner\2.9.3-beta.26257.113\build\..\tools\net\xunit.console.dll System.Diagnostics.Process.Tests.dll -xml testResults.xml -nologo -notrait category=OuterLoop -notrait category=failing
  popd
  ===========================================================================================================
    Discovering: System.Diagnostics.Process.Tests (method display = ClassAndMethod, method display options = None)
    Discovered:  System.Diagnostics.Process.Tests (found 409 of 436 test cases)
    Starting:    System.Diagnostics.Process.Tests (parallel test collections = on [24 threads], stop on fail = off)
      System.Diagnostics.Tests.ProcessStartInfoTests.TestWindowStyle [SKIP]
        Condition(s) not met: "IsAdmin_IsNotNano_RemoteExecutorIsSupported"
      System.Diagnostics.Tests.ProcessStartInfoTests.ShellExecute_Nano_Fails_Start [SKIP]
        Condition(s) not met: "IsWindowsNanoServer"
  0 File(s) copied
  Invalid number of parameters
      System.Diagnostics.Tests.ProcessOpenTests.TryOpen_ProcessOwnedByRoot_ThrowsUnauthorizedAccessException [SKIP]
        Condition(s) not met: "IsLinux"
      System.Diagnostics.Tests.ProcessOpenTests.Open_ProcessOwnedByRoot_ThrowsUnauthorizedAccessException [SKIP]
        Condition(s) not met: "IsLinux"
    Finished:    System.Diagnostics.Process.Tests
  === TEST EXECUTION SUMMARY ===
     System.Diagnostics.Process.Tests  Total: 628, Errors: 0, Failed: 0, Skipped: 4, Time: 81.868s
  ----- end Thu 05/28/2026 22:06:54.89 ----- exit code 0 ----------------------------------------------------------

[adamsitnik]

Overall it LGTM, we just need to remove the two tests that keep failing on Linux.