Ensure Process.ProcessName doesn't change when setting Thread.Name on…

… Linux (#34064)

Fixes #33673

This issue is a side-effect of adding support for setting thread names on Linux in dotnet/coreclr#27182.

Co-authored-by: Dan Moseley <danmose@microsoft.com>
Co-authored-by: Aleksey Kliger (λgeek) <akliger@gmail.com>

src/coreclr/src/pal/src/thread/thread.cpp

@@ -1641,6 +1641,13 @@ CorUnix::InternalSetThreadDescription(
 
     pTargetThread->Lock(pThread);
 
+    // Ignore requests to set the main thread name because
+    // it causes the value returned by Process.ProcessName to change.
+    if ((pid_t)pTargetThread->GetThreadId() == getpid())
+    {
+        goto InternalSetThreadDescriptionExit;
+    }
+
     /* translate the wide char lpThreadDescription string to multibyte string */
     nameSize = WideCharToMultiByte(CP_ACP, 0, lpThreadDescription, -1, NULL, 0, NULL, NULL);
 

src/libraries/System.Threading.Thread/tests/ThreadTests.cs

@@ -651,6 +651,21 @@ public static void NameTest()
             });
         }
 
+        [Fact]
+        [ActiveIssue ("https://github.com/dotnet/runtime/issues/35908", TestRuntimes.Mono)]
+        public static void ThreadNameDoesNotAffectProcessName()
+        {
+            // On Linux, changing the main thread name affects ProcessName.
+            // To avoid that, .NET ignores requests to change the main thread name.
+            RemoteExecutor.Invoke(() =>
+            {
+                const string ThreadName = "my-thread";
+                Thread.CurrentThread.Name = ThreadName;
+                Assert.Equal(ThreadName, Thread.CurrentThread.Name);
+                Assert.NotEqual(ThreadName, Process.GetCurrentProcess().ProcessName);
+            }).Dispose();
+        }
+
         [Fact]
         public static void PriorityTest()
         {