[browser] Compile MT native libraries with pthread support

docs/workflow/building/libraries/webassembly-instructions.md

@@ -113,6 +113,10 @@ The libraries build contains some native code. This includes shims over libc, op
 ./build.sh libs.native -os browser -c Debug|Release
 ```
 
+For multithreaded Browser builds, pass `/p:WasmEnableThreads=true` to both runtime and
+library builds. This enables `-pthread` for native library compilation and linking,
+including minipal. Do not mix these binaries with a single-threaded build.
+
 ## Building individual libraries
 
 Individual projects and libraries can be build by specifying the build configuration.

src/libraries/System.Threading.Thread/tests/System.Threading.Thread.Tests.csproj

@@ -21,4 +21,8 @@
     <ProjectReference Include="MTAMain\MTAMain.csproj" />
     <ProjectReference Include="DefaultApartmentStateMain\DefaultApartmentStateMain.csproj" />
   </ItemGroup>
+  <ItemGroup>
+    <Compile Include="$(CommonPath)Interop\Unix\Interop.Libraries.cs" />
+    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.Threading.cs" />
+  </ItemGroup>
 </Project>

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

@@ -32,6 +32,53 @@ public static class ThreadTests
         private static object t_threadStaticReference;
         private static int s_globalStaticValue;
 
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
+        [PlatformSpecific(TestPlatforms.AnyUnix | TestPlatforms.Browser)]
+        [InlineData(1)]
+        [InlineData(4)]
+        public static void NativeThreadIds_AreDistinctAndStable(int workerCount)
+        {
+            using var ready = new CountdownEvent(workerCount);
+            using var release = new ManualResetEventSlim();
+            ulong currentThreadId = Interop.Sys.GetCurrentThreadId();
+            ulong[] threadIds = new ulong[workerCount];
+            Action[] waitForThreads = new Action[workerCount];
+
+            for (int i = 0; i < workerCount; i++)
+            {
+                int index = i;
+                Thread thread = ThreadTestHelpers.CreateGuardedThread(out waitForThreads[i], () =>
+                {
+                    threadIds[index] = Interop.Sys.GetCurrentThreadId();
+                    ready.Signal();
+                    Assert.True(release.Wait(UnexpectedTimeoutMilliseconds));
+                    Assert.Equal(threadIds[index], Interop.Sys.GetCurrentThreadId());
+                });
+                thread.IsBackground = true;
+                thread.Start();
+            }
+
+            try
+            {
+                Assert.True(ready.Wait(UnexpectedTimeoutMilliseconds));
+                var uniqueIds = new HashSet<ulong> { currentThreadId };
+                Assert.NotEqual(0UL, currentThreadId);
+                foreach (ulong threadId in threadIds)
+                {
+                    Assert.NotEqual(0UL, threadId);
+                    Assert.True(uniqueIds.Add(threadId), $"Duplicate native thread ID: {threadId}");
+                }
+            }
+            finally
+            {
+                release.Set();
+                foreach (Action waitForThread in waitForThreads)
+                {
+                    waitForThread();
+                }
+            }
+        }
+
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         public static void ConstructorTest()
         {

src/libraries/tests.proj

@@ -647,6 +647,7 @@
     <SmokeTestProject Remove="@(SmokeTestProject)" />
     <SmokeTestProject Include="$(MonoProjectRoot)sample\wasm\browser-threads\Wasm.Browser.Threads.Sample.csproj" />
     <SmokeTestProject Include="$(MSBuildThisFileDirectory)Common/tests/Common.Tests.csproj" />
+    <SmokeTestProject Include="$(MSBuildThisFileDirectory)System.Threading.Thread\tests\System.Threading.Thread.Tests.csproj" />
     <SmokeTestProject Include="$(MSBuildThisFileDirectory)System.Runtime.InteropServices.JavaScript\tests\System.Runtime.InteropServices.JavaScript.UnitTests\System.Runtime.InteropServices.JavaScript.Tests.csproj" />
     <SmokeTestProject Include="$(MSBuildThisFileDirectory)System.Net.WebSockets.Client\tests\System.Net.WebSockets.Client.Tests.csproj" />
     <SmokeTestProject Include="$(MSBuildThisFileDirectory)System.Net.Http\tests\FunctionalTests\System.Net.Http.Functional.Tests.csproj" />

src/native/libs/CMakeLists.txt

@@ -9,6 +9,11 @@ if ("${CMAKE_CURRENT_SOURCE_DIR}" STREQUAL "${CMAKE_SOURCE_DIR}")
     include(../../../eng/native/configurepaths.cmake)
     include(${CLR_ENG_NATIVE_DIR}/configurecompiler.cmake)
 
+    if (CLR_CMAKE_TARGET_BROWSER AND CLR_CMAKE_WASM_ENABLE_THREADS)
+        add_compile_options(-pthread)
+        add_link_options(-pthread)
+    endif ()
+
     add_subdirectory(${CLR_SRC_NATIVE_DIR}/minipal minipal)
 endif()