ProcessThreadTests.TestStartTimeProperty - Get spawned thread by ID f…

…rom Process.Threads to avoid mismatch (#104972)

* select the thread based on current thread native id, remove the try catch block:

- it can hide LINQ issues as LINQ may throw InvalidOperationException
- it's not needed for the current thread, because the code is still running so we know the thread is alive

fixes #103448

src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.Unix.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.ComponentModel;
+using System.Runtime.InteropServices;
 using System.Threading;
 using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
@@ -30,5 +31,26 @@ public void TestPriorityLevelProperty_Unix()
 
             Assert.Throws<PlatformNotSupportedException>(() => thread.PriorityLevel = level);
         }
+
+        private static int GetCurrentThreadId()
+        {
+            // The magic values come from https://github.com/torvalds/linux.
+            int SYS_gettid = RuntimeInformation.ProcessArchitecture switch
+            {
+                Architecture.Arm => 224,
+                Architecture.Arm64 => 178,
+                Architecture.X86 => 224,
+                Architecture.X64 => 186,
+                Architecture.S390x => 236,
+                Architecture.Ppc64le => 207,
+                Architecture.RiscV64 => 178,
+                _ => 178,
+            };
+
+            return syscall(SYS_gettid);
+        }
+
+        [DllImport("libc")]
+        private static extern int syscall(int nr);
     }
 }

src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.cs

@@ -7,6 +7,7 @@
 using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
 using System.Threading.Tasks;
+using System.Runtime.InteropServices;
 
 namespace System.Diagnostics.Tests
 {
@@ -107,8 +108,8 @@ public void TestStartTimeProperty_OSX()
             }
         }
 
-        [Fact]
-        [PlatformSpecific(TestPlatforms.Linux|TestPlatforms.Windows)] // OSX and FreeBSD throw PNSE from StartTime
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
+        [PlatformSpecific(TestPlatforms.Linux | TestPlatforms.Windows)] // OSX and FreeBSD throw PNSE from StartTime
         public async Task TestStartTimeProperty()
         {
             TimeSpan allowedWindow = TimeSpan.FromSeconds(2);
@@ -148,15 +149,13 @@ public async Task TestStartTimeProperty()
                 await Task.Factory.StartNew(() =>
                 {
                     p.Refresh();
-                    try
-                    {
-                        var newest = p.Threads.Cast<ProcessThread>().OrderBy(t => t.StartTime.ToUniversalTime()).Last();
-                        Assert.InRange(newest.StartTime.ToUniversalTime(), curTime - allowedWindow, DateTime.Now.ToUniversalTime() + allowedWindow);
-                    }
-                    catch (InvalidOperationException)
-                    {
-                        // A thread may have gone away between our getting its info and attempting to access its StartTime
-                    }
+
+                    int newThreadId = GetCurrentThreadId();
+
+                    ProcessThread[] processThreads = p.Threads.Cast<ProcessThread>().ToArray();
+                    ProcessThread newThread = Assert.Single(processThreads, thread => thread.Id == newThreadId);
+
+                    Assert.InRange(newThread.StartTime.ToUniversalTime(), curTime - allowedWindow, DateTime.Now.ToUniversalTime() + allowedWindow);
                 }, CancellationToken.None, TaskCreationOptions.LongRunning, TaskScheduler.Default);
             }
         }