[runtime] Don't raise ArgumentNullException in mono_monitor_enter_fast (#3158)

Just return that you couldn't enter the monitor and let the slow path handle it.

Fixes [#41705](https://bugzilla.xamarin.com/show_bug.cgi?id=41705)

diff --git a/mono/metadata/monitor.c b/mono/metadata/monitor.c
index b9a305121641975286467b84392ceba8856a4111..3dfd3c4e08f720871d564e55926e9009fa568e7a 100644 (file)
--- a/mono/metadata/monitor.c
+++ b/mono/metadata/monitor.c
@@ -951,11 +951,6 @@ mono_monitor_try_enter_internal (MonoObject *obj, guint32 ms, gboolean allow_int
 
        LOCK_DEBUG (g_message("%s: (%d) Trying to lock object %p (%d ms)", __func__, id, obj, ms));
 
-       if (G_UNLIKELY (!obj)) {
-               mono_set_pending_exception (mono_get_exception_argument_null ("obj"));
-               return FALSE;
-       }
-
        lw.sync = obj->synchronisation;
 
        if (G_LIKELY (lock_word_is_free (lw))) {
@@ -1001,18 +996,31 @@ mono_monitor_try_enter_internal (MonoObject *obj, guint32 ms, gboolean allow_int
 gboolean 
 mono_monitor_enter (MonoObject *obj)
 {
+       if (G_UNLIKELY (!obj)) {
+               mono_set_pending_exception (mono_get_exception_argument_null ("obj"));
+               return FALSE;
+       }
        return mono_monitor_try_enter_internal (obj, INFINITE, FALSE) == 1;
 }
 
 gboolean 
 mono_monitor_enter_fast (MonoObject *obj)
 {
+       if (G_UNLIKELY (!obj)) {
+               /* don't set pending exn on the fast path, just return
+                * FALSE and let the slow path take care of it. */
+               return FALSE;
+       }
        return mono_monitor_try_enter_internal (obj, 0, FALSE) == 1;
 }
 
 gboolean
 mono_monitor_try_enter (MonoObject *obj, guint32 ms)
 {
+       if (G_UNLIKELY (!obj)) {
+               mono_set_pending_exception (mono_get_exception_argument_null ("obj"));
+               return FALSE;
+       }
        return mono_monitor_try_enter_internal (obj, ms, FALSE) == 1;
 }
 
@@ -1075,6 +1083,10 @@ void
 ves_icall_System_Threading_Monitor_Monitor_try_enter_with_atomic_var (MonoObject *obj, guint32 ms, char *lockTaken)
 {
        gint32 res;
+       if (G_UNLIKELY (!obj)) {
+               mono_set_pending_exception (mono_get_exception_argument_null ("obj"));
+               return;
+       }
        do {
                res = mono_monitor_try_enter_internal (obj, ms, TRUE);
                /*This means we got interrupted during the wait and didn't got the monitor.*/