[iOS][Android] Fix crash in Exception.CaptureDispatchState (dotnet#70970

)

There is a crash in `Exception.CaptureDispatchState` when called from one thread at the same time another calls into `Exception.RestoreDispatchState`. The reason for the crash is due to the way we do not update `foreignExceptionFrames` in a thread-safe way.  `foreignExceptionFrames` is used in both methods and can crash when the size changes before the array is copied.

The fix copies `foreignExceptionFrame` into a local variable in `CaptureDispatchState`

Fixes dotnet#70081

src/mono/System.Private.CoreLib/src/System/Exception.Mono.cs

@@ -48,6 +48,8 @@ public DispatchState(MonoStackFrame[]? stackFrames)
 
         private bool HasBeenThrown => _traceIPs != null;
 
+        private readonly object frameLock = new object();
+
         public MethodBase? TargetSite
         {
             get
@@ -70,11 +72,17 @@ internal DispatchState CaptureDispatchState()
                 if (stackFrames.Length > 0)
                     stackFrames[stackFrames.Length - 1].isLastFrameFromForeignException = true;
 
-                if (foreignExceptionsFrames != null)
+                // Make sure foreignExceptionsFrames does not change at this point.
+                // Otherwise, the Array.Copy into combinedStackFrames can fail due to size differences
+                //
+                // See https://github.com/dotnet/runtime/issues/70081
+                MonoStackFrame[]? feFrames = foreignExceptionsFrames;
+
+                if (feFrames != null)
                 {
-                    var combinedStackFrames = new MonoStackFrame[stackFrames.Length + foreignExceptionsFrames.Length];
-                    Array.Copy(foreignExceptionsFrames, 0, combinedStackFrames, 0, foreignExceptionsFrames.Length);
-                    Array.Copy(stackFrames, 0, combinedStackFrames, foreignExceptionsFrames.Length, stackFrames.Length);
+                    var combinedStackFrames = new MonoStackFrame[stackFrames.Length + feFrames.Length];
+                    Array.Copy(feFrames, 0, combinedStackFrames, 0, feFrames.Length);
+                    Array.Copy(stackFrames, 0, combinedStackFrames, feFrames.Length, stackFrames.Length);
 
                     stackFrames = combinedStackFrames;
                 }
@@ -90,7 +98,6 @@ internal DispatchState CaptureDispatchState()
         internal void RestoreDispatchState(in DispatchState state)
         {
             foreignExceptionsFrames = state.StackFrames;
-
             _stackTraceString = null;
         }