[debugger] NRE when 2 threads try to call GetThreads at same time (#19122)

As analysed by @jstedfast and me the only way to get this exception with this callstack below is 2 threads call GetThreads at same time, so while one of them is filling the items in the array, the other one is already trying to use the array with null items.

```
System.NullReferenceException: Object reference not set to an instance of an object
  at Mono.Debugging.Soft.SoftDebuggerSession.GetId (Mono.Debugger.Soft.ThreadMirror thread) [0x00000] in /Users/runner/runners/2.164.6/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging.Soft/SoftDebuggerSession.cs:3227
  at Mono.Debugging.Soft.SoftDebuggerSession.OnGetThreads (System.Int64 processId) [0x0002c] in /Users/runner/runners/2.164.6/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging.Soft/SoftDebuggerSession.cs:879
  at Mono.Debugging.Soft.SoftDebuggerSession.GetThread (Mono.Debugging.Client.ProcessInfo process, Mono.Debugger.Soft.ThreadMirror thread) [0x00008] in /Users/runner/runners/2.164.6/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging.Soft/SoftDebuggerSession.cs:902
  at Mono.Debugging.Soft.SoftDebuggerSession.HandleBreakEventSet (Mono.Debugger.Soft.Event[] es, System.Boolean dequeuing) [0x00501] in /Users/runner/runners/2.164.6/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging.Soft/SoftDebuggerSession.cs:2038
  at Mono.Debugging.Soft.SoftDebuggerSession.HandleEventSet (Mono.Debugger.Soft.EventSet es) [0x0005b] in /Users/runner/runners/2.164.6/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging.Soft/SoftDebuggerSession.cs:1687
  at Mono.Debugging.Soft.SoftDebuggerSession.EventHandler () [0x0005d] in /Users/runner/runners/2.164.6/work/1/s/monodevelop/main/external/debugger-libs/Mono.Debugging.Soft/SoftDebuggerSession.cs:1625

```
 PR on mono/debugger-libs -> https://github.com/mono/debugger-libs/pull/300

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1070384/

1 files changed, 4 insertions, 3 deletions

diff --git a/mcs/class/Mono.Debugger.Soft/Mono.Debugger.Soft/VirtualMachine.cs b/mcs/class/Mono.Debugger.Soft/Mono.Debugger.Soft/VirtualMachine.cs
index c9026a02682..95ed7e2d3b5 100644
--- a/mcs/class/Mono.Debugger.Soft/Mono.Debugger.Soft/VirtualMachine.cs
+++ b/mcs/class/Mono.Debugger.Soft/Mono.Debugger.Soft/VirtualMachine.cs
@@ -171,8 +171,7 @@ namespace Mono.Debugger.Soft
 
 		HashSet<ThreadMirror> threadsToInvalidate = new HashSet<ThreadMirror> ();
 		ThreadMirror[] threadCache;
-		object threadCacheLocker = new object ();
-
+		
 		void InvalidateThreadAndFrameCaches () {
 			lock (threadsToInvalidate) {
 				foreach (var thread in threadsToInvalidate)
@@ -200,7 +199,7 @@ namespace Mono.Debugger.Soft
 				var fetchingEvent = new ManualResetEvent (false);
 				vm.conn.VM_GetThreads ((threadsIds) => {
 					ids = threadsIds;
-					threadCache = threads = new ThreadMirror [threadsIds.Length];
+					threads = new ThreadMirror [threadsIds.Length];
 					fetchingEvent.Set ();
 				});
 				if (WaitHandle.WaitAny (new []{ vm.conn.DisconnectedEvent, fetchingEvent }) == 0) {
@@ -215,6 +214,8 @@ namespace Mono.Debugger.Soft
 				//if (threadCache != threads) {//While fetching threads threadCache was invalidated(thread was created/destoyed)
 				//	return GetThreads ();
 				//}
+				Thread.MemoryBarrier ();
+				threadCache = threads;
 				return threads;
 			} else {
 				return threads;