[mono] Work around arm64 MacCatalyst unavailable JITing API (#51669)

* To [JIT on Apple Silicon](https://developer.apple.com/documentation/apple-silicon/porting-just-in-time-compilers-to-apple-silicon) the application must call `pthread_jit_write_protect_np (0)` before writing to a code page and then `pthread_jit_write_protect_np (1)` before executing JITed code.

* Catalyst apps are Mac apps that get to use iOS frameworks (as well as some mac frameworks).  The API access is guarded by `__API_AVAILABLE` and `__API_UNAVAILABLE` macros in Apple framework headers.  If an API is not explicitly marked for catalyst, clang will follow the `ios` availability.

Unfortunately, `pthread_jit_write_protect_np` is marked like this:

```c
__API_AVAILABLE(macos(11.0))
__API_UNAVAILABLE(ios, tvos, watchos)
void pthread_jit_write_protect_np(int enabled);
```

So a Catalyst app running on arm64 cannot call it.  It will get a compile-time error.
```
src/mono/mono/utils/mono-codeman.c:669:3: error: 'pthread_jit_write_protect_np' is unavailable: not available on macCatalyst
                  pthread_jit_write_protect_np (0);
                  ^
  /Applications/Xcode_12.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/usr/include/pthread.h:561:6: note: 'pthread_jit_write_protect_np' has been explicitly marked unavailable here
  void pthread_jit_write_protect_np(int enabled);
       ^
```

---

Turns out the symbol `pthread_jit_write_protect_np` is actually present in the Catalyst framework libraries, and it does the right thing.

This PR works around the unfortunate `pthread.h` declaration by not using it.

We create a new .c file that included our own declaration
```c
void pthread_jit_write_protect_np(int enabled);
```

And then a new `mono_jit_write_protect` function that calls the pthread function.

---

Another option would be to use something like
```
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wpartial-availability" 
...
#pragma clang diagnostic pop
```
around the offending callsites.  But neither ignoring `-Wpartial-availability` nor `-Wunguarded-availability-new` seem to have any effect on the compilation error.

---

Caveat: It's not yet clear whether this is ok to do in a retail (App Store) app.

---

* [mono] define HOST_MACCAT not HOST_MACCATALYST for Mac Catalyst

The C code already has HOST_MACCAT ifdefs.  HOST_MACCATALYST isn't used.

src/mono/CMakeLists.txt

@@ -185,7 +185,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
   set(PTHREAD_POINTER_ID 1)
   set(USE_MACH_SEMA 1)
   if(CMAKE_SYSTEM_VARIANT STREQUAL "MacCatalyst")
-    set(HOST_MACCATALYST 1)
+    set(HOST_MACCAT 1)
   endif()
 elseif(CMAKE_SYSTEM_NAME STREQUAL "iOS" OR CMAKE_SYSTEM_NAME STREQUAL "tvOS")
   # See man cmake-toolchains(7) on which variables
@@ -735,7 +735,7 @@ endif()
 ### End of OS specific checks
 
 add_subdirectory(mono)
-if (ENABLE_MSCORDBI AND NOT TARGET_ARCH STREQUAL "arm64"  AND NOT CMAKE_CROSSCOMPILING AND NOT TARGET_IOS AND NOT TARGET_ANDROID AND NOT TARGET_BROWSER AND NOT HOST_MACCATALYST)
+if (ENABLE_MSCORDBI AND NOT TARGET_ARCH STREQUAL "arm64"  AND NOT CMAKE_CROSSCOMPILING AND NOT TARGET_IOS AND NOT TARGET_ANDROID AND NOT TARGET_BROWSER AND NOT HOST_MACCAT)
   add_subdirectory(dlls/mscordbi)
   add_subdirectory(dlls/dbgshim)
 endif()

src/mono/cmake/config.h.in

@@ -113,6 +113,9 @@
 /* Host Platform is tvOS */
 #cmakedefine HOST_TVOS 1
 
+/* Host Platform is Mac Catalyst */
+#cmakedefine HOST_MACCAT 1
+
 /* Use classic Windows API support */
 #cmakedefine HAVE_CLASSIC_WINAPI_SUPPORT 1
 

src/mono/cmake/configure.cmake

@@ -157,6 +157,6 @@ if(HOST_IOS)
     set(HAVE_EXECVE 0)
     set(HAVE_EXECVP 0)
   endif()
-elseif(HOST_MACCATALYST)
+elseif(HOST_MACCAT)
   set(HAVE_SYSTEM 0)
 endif()

src/mono/mono/utils/CMakeLists.txt

@@ -33,6 +33,8 @@ set(utils_common_sources
     mono-logger.c
     mono-logger-internals.h
     mono-codeman.c
+    write-protect.c
+    write-protect.h
     mono-counters.c
     mono-compiler.h
     mono-dl.c

src/mono/mono/utils/mono-codeman.c

@@ -28,6 +28,7 @@ static void* mono_code_manager_heap;
 
 #include <mono/utils/mono-os-mutex.h>
 #include <mono/utils/mono-tls.h>
+#include <mono/utils/write-protect.h>
 
 static uintptr_t code_memory_used = 0;
 static size_t dynamic_code_alloc_count;
@@ -668,6 +669,11 @@ mono_codeman_enable_write (void)
 		mono_native_tls_set_value (write_level_tls_id, GINT_TO_POINTER (level));
 		pthread_jit_write_protect_np (0);
 	}
+#elif defined(HOST_MACCAT) && defined(__aarch64__)
+        int level = GPOINTER_TO_INT (mono_native_tls_get_value (write_level_tls_id));
+        level ++;
+        mono_native_tls_set_value (write_level_tls_id, GINT_TO_POINTER (level));
+        mono_jit_write_protect (0);
 #endif
 }
 
@@ -689,5 +695,12 @@ mono_codeman_disable_write (void)
 		if (level == 0)
 			pthread_jit_write_protect_np (1);
 	}
+#elif defined(HOST_MACCAT) && defined(__aarch64__)
+		int level = GPOINTER_TO_INT (mono_native_tls_get_value (write_level_tls_id));
+		g_assert (level);
+		level --;
+		mono_native_tls_set_value (write_level_tls_id, GINT_TO_POINTER (level));
+		if (level == 0)
+                        mono_jit_write_protect (1);
 #endif
 }

src/mono/mono/utils/write-protect.c

@@ -0,0 +1,27 @@
+/**
+ * \file
+ */
+
+#include "config.h"
+
+#include "mono-compiler.h"
+#include "write-protect.h"
+
+#if defined (HOST_MACCAT) && defined (__aarch64__)
+
+/* our own declaration of pthread_jit_write_protect_np so that we don't see the __API_UNAVAILABLE__ header */
+void
+pthread_jit_write_protect_np (int enabled);
+
+
+void
+mono_jit_write_protect (int enabled)
+{
+        pthread_jit_write_protect_np (enabled);
+}
+
+#else
+
+MONO_EMPTY_SOURCE_FILE (write_protect);
+
+#endif

src/mono/mono/utils/write-protect.h

@@ -0,0 +1,18 @@
+/**
+ * \file
+ */
+
+#ifndef __MONO_WRITE_PROTECT_H__
+#define __MONO_WRITE_PROTECT_H__
+
+#include <mono/utils/mono-publib.h>
+
+#if defined (HOST_MACCAT) && defined (__aarch64__)
+
+void
+mono_jit_write_protect (int enabled);
+
+#endif /* defined (HOST_MACCAT) && defined (__aarch64__) */
+
+#endif /* __MONO_WRITE_PROTECT_H__ */
+