Fix named mutex workaround for .NET Mono runtime

src/Compilers/Core/Portable/InternalUtilities/PlatformInformation.cs

@@ -31,5 +31,24 @@ public static bool IsRunningOnMono
                 }
             }
         }
+        /// <summary>
+        /// Are we running on .NET 5 or later using the Mono runtime?
+        /// Will also return true when running on Mono itself; if necessary
+        /// we can use IsRunningOnMono to distinguish.
+        /// </summary>
+        public static bool IsUsingMonoRuntime
+        {
+            get
+            {
+                try
+                {
+                    return !(Type.GetType("Mono.RuntimeStructs", throwOnError: false) is null);
+                }
+                catch
+                {
+                    return false;
+                }
+            }
+        }
     }
 }

src/Compilers/Server/VBCSCompilerTests/BuildClientTests.cs

@@ -79,7 +79,7 @@ public void ConnectToServerFails()
                 // to connect. When it fails it should fall back to in-proc
                 // compilation.
                 bool holdsMutex;
-                using (var serverMutex = new Mutex(initiallyOwned: true,
+                using (var serverMutex = BuildServerConnection.OpenOrCreateMutex(
                                                    name: BuildServerConnection.GetServerMutexName(_pipeName),
                                                    createdNew: out holdsMutex))
                 {

src/Compilers/Server/VBCSCompilerTests/CompilerServerApiTest.cs

@@ -103,7 +103,7 @@ public void MutexStopsServerStarting()
             var mutexName = BuildServerConnection.GetServerMutexName(pipeName);
 
             bool holdsMutex;
-            using (var mutex = new Mutex(initiallyOwned: true,
+            using (var mutex = BuildServerConnection.OpenOrCreateMutex(
                                          name: mutexName,
                                          createdNew: out holdsMutex))
             {
@@ -119,7 +119,7 @@ public void MutexStopsServerStarting()
                 }
                 finally
                 {
-                    mutex.ReleaseMutex();
+                    mutex.Dispose();
                 }
             }
         }

src/Compilers/Server/VBCSCompilerTests/CompilerServerTests.cs

@@ -304,7 +304,7 @@ public async Task ServerFailsWithLongTempPathUnix()
             var newTempDir = _tempDirectory.CreateDirectory(new string('a', 100 - _tempDirectory.Path.Length));
             await ApplyEnvironmentVariables(
                 new[] { new KeyValuePair<string, string>("TMPDIR", newTempDir.Path) },
-                async () =>
+                async () => await Task.Run(async () =>
             {
                 using var serverData = await ServerUtil.CreateServer(_logger);
                 var result = RunCommandLineCompiler(
@@ -317,7 +317,7 @@ await ApplyEnvironmentVariables(
 
                 var listener = await serverData.Complete();
                 Assert.Equal(CompletionData.RequestCompleted, listener.CompletionDataList.Single());
-            });
+            }));
         }
 
         [Fact]

src/Compilers/Server/VBCSCompilerTests/VBCSCompilerServerTests.cs

@@ -101,7 +101,7 @@ public async Task NoServerConnection()
 
                     var thread = new Thread(() =>
                     {
-                        using (var mutex = new Mutex(initiallyOwned: true, name: mutexName, createdNew: out created))
+                        using (var mutex = BuildServerConnection.OpenOrCreateMutex(name: mutexName, createdNew: out created))
                         using (var stream = NamedPipeUtil.CreateServer(pipeName))
                         {
                             readyMre.Set();
@@ -112,7 +112,7 @@ public async Task NoServerConnection()
                             stream.Close();
 
                             doneMre.WaitOne();
-                            mutex.ReleaseMutex();
+                            mutex.Dispose();
                         }
                     });
 
@@ -153,15 +153,14 @@ public async Task ServerShutdownsDuringProcessing()
                     {
                         using (var stream = NamedPipeUtil.CreateServer(pipeName))
                         {
-                            var mutex = new Mutex(initiallyOwned: true, name: mutexName, createdNew: out created);
+                            var mutex = BuildServerConnection.OpenOrCreateMutex(name: mutexName, createdNew: out created);
                             readyMre.Set();
 
                             stream.WaitForConnection();
                             connected = true;
 
                             // Client is waiting for a response.  Close the mutex now.  Then close the connection 
                             // so the client gets an error.
-                            mutex.ReleaseMutex();
                             mutex.Dispose();
                             stream.Close();