[System.Xaml] Fix deadlock on exception in XamlBackgroundReader

The ManualResetEvent wouldn't be signaled when an exception happens
in the reader loop so we'd deadlock in Read().

I'm still investigating where the exception comes from but this
should at least help unblocking CI.

https://bugzilla.xamarin.com/show_bug.cgi?id=46683

1 files changed, 14 insertions, 3 deletions

diff --git a/mcs/class/System.Xaml/System.Xaml/XamlBackgroundReader.cs b/mcs/class/System.Xaml/System.Xaml/XamlBackgroundReader.cs
index 2859a14a9d9..d1d54491fd8 100644
--- a/mcs/class/System.Xaml/System.Xaml/XamlBackgroundReader.cs
+++ b/mcs/class/System.Xaml/System.Xaml/XamlBackgroundReader.cs
@@ -23,6 +23,7 @@
 using System;
 using System.Collections.Generic;
 using System.Threading;
+using System.Runtime.ExceptionServices;
 
 namespace System.Xaml
 {
@@ -40,6 +41,7 @@ namespace System.Xaml
 		XamlReader r;
 		XamlNodeQueue q;
 		bool read_all_done, do_work = true;
+		ExceptionDispatchInfo read_exception;
 		ManualResetEvent wait = new ManualResetEvent (true);
 
 		public bool HasLineInfo {
@@ -92,6 +94,10 @@ namespace System.Xaml
 		{
 			if (q.IsEmpty)
 				wait.WaitOne ();
+
+			if (read_exception != null)
+				read_exception.Throw ();
+
 			return q.Reader.Read ();
 		}
 		
@@ -105,11 +111,16 @@ namespace System.Xaml
 			if (thread != null)
 				throw new InvalidOperationException ("Thread has already started");
 			thread = new Thread (new ParameterizedThreadStart (delegate {
-				while (do_work && r.Read ()) {
-					q.Writer.WriteNode (r);
+				try {
+					while (do_work && r.Read ()) {
+						q.Writer.WriteNode (r);
+						wait.Set ();
+					}
+					read_all_done = true;
+				} catch (Exception ex) {
+					read_exception = ExceptionDispatchInfo.Capture (ex);
 					wait.Set ();
 				}
-				read_all_done = true;
 			})) { Name = threadName };
 			thread.Start ();
 		}