[android] pthread_kill cannot be used safely on android

Android's fork() function does not properly update the
pthread_internal_t->threadid variable to point to the tid of the
fork()'d thread, and instead still points to the pid of zygote.
As a result we need to track the android tid's manually since
gettid() is sane, and use an alternate to pthread_kill on android
Additionally, disable sgen support on android until the same
work has been done there

r: kumpera, vargaz

libgc/pthread_stop_world.c

@@ -315,6 +315,7 @@ pthread_t GC_stopping_thread;
 int GC_stopping_pid;
 
 #ifdef PLATFORM_ANDROID
+static
 int android_thread_kill(pid_t tid, int sig)
 {
     int  ret;

mono/metadata/object-internals.h

@@ -401,11 +401,11 @@ struct _MonoInternalThread {
 	MonoThread *root_domain_thread;
 	gpointer interrupt_on_stop;
 	gsize    flags;
+	gpointer android_tid;
 	/* 
 	 * These fields are used to avoid having to increment corlib versions
 	 * when a new field is added to the unmanaged MonoThread structure.
 	 */
-	gpointer unused4;
 	gpointer unused5;
 	gpointer unused6;
 };
@@ -646,6 +646,9 @@ mono_type_initialization_init (void) MONO_INTERNAL;
 void
 mono_type_initialization_cleanup (void) MONO_INTERNAL;
 
+int
+mono_thread_kill           (MonoInternalThread *thread, int signal) MONO_INTERNAL;
+
 guint32
 mono_thread_get_tls_key    (void) MONO_INTERNAL;
 

mono/metadata/sgen-gc.c

@@ -6471,6 +6471,10 @@ mono_gc_base_init (void)
 	char *major_collector_opt = NULL;
 	struct sigaction sinfo;
 
+#ifdef PLATFORM_ANDROID
+	g_assert_not_reached ();
+#endif
+
 	/* the gc_initialized guard seems to imply this method is
 	   idempotent, but LOCK_INIT(gc_mutex) might not be.  It's
 	   defined in sgen-gc.h as nothing, so there's no danger at

mono/metadata/threads.c

@@ -45,6 +45,12 @@
 
 #include <mono/metadata/gc-internal.h>
 
+#ifdef PLATFORM_ANDROID
+#include <errno.h>
+
+extern int tkill (pid_t tid, int signal);
+#endif
+
 /*#define THREAD_DEBUG(a) do { a; } while (0)*/
 #define THREAD_DEBUG(a)
 /*#define THREAD_WAIT_DEBUG(a) do { a; } while (0)*/
@@ -1014,6 +1020,9 @@ mono_thread_attach (MonoDomain *domain)
 
 	thread->handle=thread_handle;
 	thread->tid=tid;
+#ifdef PLATFORM_ANDROID
+	thread->android_tid = (gpointer) gettid ();
+#endif
 	thread->apartment_state=ThreadApartmentState_Unknown;
 	small_id_alloc (thread);
 	thread->stack_ptr = &tid;
@@ -2300,11 +2309,7 @@ static void signal_thread_state_change (MonoInternalThread *thread)
 	QueueUserAPC ((PAPCFUNC)interruption_request_apc, thread->handle, NULL);
 #else
 	/* fixme: store the state somewhere */
-#ifdef PTHREAD_POINTER_ID
-	pthread_kill ((gpointer)(gsize)(thread->tid), mono_thread_get_abort_signal ());
-#else
-	pthread_kill (thread->tid, mono_thread_get_abort_signal ());
-#endif
+	mono_thread_kill (thread, mono_thread_get_abort_signal ());
 
 	/* 
 	 * This will cause waits to be broken.
@@ -4334,3 +4339,30 @@ mono_runtime_has_tls_get (void)
 {
 	return has_tls_get;
 }
+
+int
+mono_thread_kill (MonoInternalThread *thread, int signal)
+{
+#ifdef PTHREAD_POINTER_ID
+	return pthread_kill ((gpointer)(gsize)(thread->tid), mono_thread_get_abort_signal ());
+#else
+#  ifdef PLATFORM_ANDROID
+	if (thread->android_tid != 0) {
+		int  ret;
+		int  old_errno = errno;
+
+		ret = tkill ((pid_t) thread->android_tid, signal);
+		if (ret < 0) {
+			ret = errno;
+			errno = old_errno;
+		}
+
+		return ret;
+	}
+	else
+		return pthread_kill (thread->tid, mono_thread_get_abort_signal ());
+#  else
+	return pthread_kill (thread->tid, mono_thread_get_abort_signal ());
+#  endif
+#endif
+}

mono/mini/debugger-agent.c

@@ -2041,7 +2041,7 @@ notify_thread (gpointer key, gpointer value, gpointer user_data)
 #ifdef HOST_WIN32
 	QueueUserAPC (notify_thread_apc, thread->handle, NULL);
 #else
-	pthread_kill ((pthread_t) tid, mono_thread_get_abort_signal ());
+	mono_thread_kill (thread, mono_thread_get_abort_signal ());
 #endif
 }