Fix macOS Process hangs caused by stopped children

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessWaitState.Unix.cs

@@ -622,18 +622,18 @@ internal static void CheckChildren(bool reapAll, bool configureConsole)
                     pid = Interop.Sys.WaitIdAnyExitedNoHangNoWait();
                     if (pid > 0)
                     {
-                        if (s_childProcessWaitStates.TryGetValue(pid, out ProcessWaitState? pws))
+                        if (s_childProcessWaitStates.TryGetValue(pid, out ProcessWaitState? pws) &&
+                            pws.TryReapChild(configureConsole))
                         {
-                            // Known Process.
-                            if (pws.TryReapChild(configureConsole))
-                            {
-                                pws.ReleaseRef();
-                            }
+                            pws.ReleaseRef();
                         }
                         else
                         {
-                            // unlikely: This is not a managed Process, so we are not responsible for reaping.
-                            // Fall back to checking all Processes.
+                            // The child may be unmanaged, or stopped rather than exited. On macOS,
+                            // waitid can report stopped children despite omitting WSTOPPED;
+                            // the SIGCHLD handler is also registered with SA_NOCLDSTOP.
+                            // Scan all managed children instead of repeatedly observing
+                            // the same child without reaping it.
                             checkAll = true;
                             break;
                         }

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

@@ -3,10 +3,4 @@
 
 using Xunit;
 
-#if TARGET_OSX
-// Retain serialization until process-management hangs under concurrent test classes on macOS are resolved.
-// https://github.com/dotnet/runtime/issues/135294
-[assembly: CollectionBehavior(CollectionBehavior.CollectionPerAssembly)]
-#endif
-
 [assembly: SkipOnPlatform(TestPlatforms.Browser, "System.Diagnostics.Process is not supported on Browser.")]

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

@@ -769,6 +769,64 @@ public async Task TestChildProcessCleanupAfterDispose(bool shortProcess, bool en
             Assert.True(processReaped);
         }
 
+        [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        [PlatformSpecific(TestPlatforms.OSX)]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void WaitForExit_StoppedChild_DoesNotPreventReapingOtherChildren(bool useAsync)
+        {
+            RemoteExecutor.Invoke(async useAsyncString =>
+            {
+                const uint StoppedProcessStatus = 4; // SSTOP in sys/proc.h.
+                using Process stopped = Process.Start("/bin/sleep", "300");
+                using Process exiting = Process.Start("/bin/sleep", "300");
+                int stoppedPid = stopped.Id;
+                int exitingPid = exiting.Id;
+                int killSignal = Interop.Sys.GetPlatformSignalNumber(PosixSignal.SIGKILL);
+                using var sigChildReceived = new ManualResetEventSlim();
+                using PosixSignalRegistration registration = PosixSignalRegistration.Create(PosixSignal.SIGCHLD, context =>
+                {
+                    context.Cancel = true;
+                    sigChildReceived.Set();
+                });
+
+                try
+                {
+                    Assert.Equal(0, Interop.Sys.Kill(stoppedPid, Interop.Sys.GetPlatformSIGSTOP()));
+
+                    Assert.True(SpinWait.SpinUntil(
+                        () => Interop.libproc.GetProcessInfoById(stoppedPid) is { } info &&
+                            info.pbsd.pbi_status == StoppedProcessStatus, WaitInMS));
+
+                    sigChildReceived.Reset();
+                    Assert.Equal(0, Interop.Sys.Kill(Environment.ProcessId, Interop.Sys.GetPlatformSignalNumber(PosixSignal.SIGCHLD)));
+                    Assert.True(sigChildReceived.Wait(WaitInMS));
+
+                    Assert.Equal(0, Interop.Sys.Kill(exitingPid, killSignal));
+                    if (bool.Parse(useAsyncString))
+                    {
+                        using var cts = new CancellationTokenSource(WaitInMS);
+                        await exiting.WaitForExitAsync(cts.Token);
+                    }
+                    else
+                    {
+                        Assert.True(exiting.WaitForExit(WaitInMS));
+                    }
+                    Assert.True(exiting.HasExited);
+                }
+                finally
+                {
+                    // Bypass managed process locks so cleanup also works if the reaper is stuck.
+                    Assert.Equal(0, Interop.Sys.Kill(stoppedPid, killSignal));
+                    Assert.True(stopped.WaitForExit(WaitInMS));
+                    exiting.Kill();
+                    Assert.True(exiting.WaitForExit(WaitInMS));
+                }
+
+                return RemoteExecutor.SuccessExitCode;
+            }, useAsync.ToString()).Dispose();
+        }
+
         private static Process CreateShortProcess()
         {
             Process process = new Process();

src/libraries/System.Diagnostics.Process/tests/System.Diagnostics.Process.Tests.csproj

@@ -10,7 +10,6 @@
   <PropertyGroup>
     <TargetPlatformIdentifier>$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)'))</TargetPlatformIdentifier>
     <DefineConstants Condition="'$(TargetPlatformIdentifier)' == 'windows'">$(DefineConstants);TargetsWindows</DefineConstants>
-    <DefineConstants Condition="'$(TargetOS)' == 'osx' and '$(TargetPlatformIdentifier)' == 'unix'">$(DefineConstants);TARGET_OSX</DefineConstants>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonPath)System\IO\StringParser.cs"
@@ -93,6 +92,8 @@
              Link="Common\Interop\OSX\Interop.Libraries.cs" />
     <Compile Include="$(CommonPath)Interop\OSX\Interop.libproc.cs"
              Link="Common\Interop\OSX\Interop.libproc.cs" />
+    <Compile Include="$(CommonPath)Interop\OSX\Interop.libproc.GetProcessInfoById.cs"
+             Link="Common\Interop\OSX\Interop.libproc.GetProcessInfoById.cs" />
     <Compile Include="$(CommonPath)Interop\OSX\Interop.libSystem.cs"
              Link="Common\Interop\OSX\Interop.libSystem.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\Interop.Libraries.cs"