Make sure that shared memory object name meets the length requirements

src/libraries/System.IO.MemoryMappedFiles/src/System/IO/MemoryMappedFiles/MemoryMappedFile.Unix.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
 using Microsoft.Win32.SafeHandles;
 
 namespace System.IO.MemoryMappedFiles
@@ -170,11 +171,17 @@ private static FileStream CreateSharedBackingObject(Interop.Sys.MemoryMappedProt
                 ?? CreateSharedBackingObjectUsingFile(protections, capacity, inheritability);
         }
 
-        private static FileStream? CreateSharedBackingObjectUsingMemory(
+        private static unsafe FileStream? CreateSharedBackingObjectUsingMemory(
            Interop.Sys.MemoryMappedProtections protections, long capacity, HandleInheritability inheritability)
         {
-            // The POSIX shared memory object name must begin with '/'.  After that we just want something short and unique.
-            string mapName = string.Create(null, stackalloc char[128], $"/corefx_map_{Guid.NewGuid():N}");
+            const int MaxSharedMemoryObjectNameLength = 32; // SHM_NAME_MAX on OSX ARM64, on other systems it's equal PATH_MAX (250)
+            // The POSIX shared memory object name must begin with '/'.  After that we just want something short (32) and unique.
+            Span<char> characters = stackalloc char[MaxSharedMemoryObjectNameLength];
+            char format = 'N';
+            bool success = Guid.NewGuid().TryFormat(characters, out int charsWritten, new ReadOnlySpan<char>(&format, 1));
+            Debug.Assert(success && charsWritten == MaxSharedMemoryObjectNameLength);
+            characters[0] = '/';
+            string mapName = new string(characters);
 
             // Determine the flags to use when creating the shared memory object
             Interop.Sys.OpenFlags flags = (protections & Interop.Sys.MemoryMappedProtections.PROT_WRITE) != 0 ?

src/libraries/System.IO.MemoryMappedFiles/tests/MemoryMappedFile.CreateNew.Tests.cs

@@ -11,7 +11,6 @@ namespace System.IO.MemoryMappedFiles.Tests
     /// <summary>
     /// Tests for MemoryMappedFile.CreateNew.
     /// </summary>
-    [ActiveIssue("https://github.com/dotnet/runtime/issues/63240", typeof(PlatformDetection), nameof(PlatformDetection.IsOSX), nameof(PlatformDetection.IsArm64Process), nameof(PlatformDetection.IsNotMonoRuntime))]
     public class MemoryMappedFileTests_CreateNew : MemoryMappedFilesTestBase
     {
         /// <summary>

src/libraries/System.IO.MemoryMappedFiles/tests/MemoryMappedViewAccessor.Tests.cs

@@ -11,7 +11,6 @@ namespace System.IO.MemoryMappedFiles.Tests
     /// <summary>
     /// Tests for MemoryMappedViewAccessor.
     /// </summary>
-    [ActiveIssue("https://github.com/dotnet/runtime/issues/63240", typeof(PlatformDetection), nameof(PlatformDetection.IsOSX), nameof(PlatformDetection.IsArm64Process), nameof(PlatformDetection.IsNotMonoRuntime))]
     public class MemoryMappedViewAccessorTests : MemoryMappedFilesTestBase
     {
         /// <summary>

src/libraries/System.IO.MemoryMappedFiles/tests/MemoryMappedViewStream.Tests.cs

@@ -11,7 +11,6 @@ namespace System.IO.MemoryMappedFiles.Tests
     /// <summary>
     /// Tests for MemoryMappedViewStream.
     /// </summary>
-    [ActiveIssue("https://github.com/dotnet/runtime/issues/63240", typeof(PlatformDetection), nameof(PlatformDetection.IsOSX), nameof(PlatformDetection.IsArm64Process), nameof(PlatformDetection.IsNotMonoRuntime))]
     public class MemoryMappedViewStreamTests : MemoryMappedFilesTestBase
     {
         /// <summary>

src/libraries/System.IO.MemoryMappedFiles/tests/MemoryMappedViewStreamConformanceTests.cs

@@ -45,7 +45,6 @@ private Task<Stream> CreateStream(byte[] initialData, FileAccess access)
         }
     }
 
-    [ActiveIssue("https://github.com/dotnet/runtime/issues/63240", typeof(PlatformDetection), nameof(PlatformDetection.IsOSX), nameof(PlatformDetection.IsArm64Process), nameof(PlatformDetection.IsNotMonoRuntime))]
     public class AnonymousMemoryMappedViewStreamConformanceTests : MemoryMappedViewStreamConformanceTests
     {
         protected override MemoryMappedFile CreateFile(int length) =>

src/native/libs/System.Native/pal_io.c

@@ -332,6 +332,12 @@ int32_t SystemNative_Unlink(const char* path)
 
 intptr_t SystemNative_ShmOpen(const char* name, int32_t flags, int32_t mode)
 {
+#if defined(SHM_NAME_MAX) // macOS
+    assert(strlen(name) <= SHM_NAME_MAX);
+#elif defined(PATH_MAX) // other Unixes
+    assert(strlen(name) <= PATH_MAX);
+#endif
+
 #if HAVE_SHM_OPEN_THAT_WORKS_WELL_ENOUGH_WITH_MMAP
     flags = ConvertOpenFlags(flags);
     if (flags == -1)