Use SafeHandle to avoid freeing native resources while in use

Thread 1 is reading from the stream
Thread 2 closes the zstream
caos ensues

mcs/class/System/System.IO.Compression/DeflateStream.cs

@@ -34,6 +34,7 @@
 using System.IO;
 using System.Runtime.InteropServices;
 using System.Runtime.Remoting.Messaging;
+using Microsoft.Win32.SafeHandles;
 
 #if MONOTOUCH
 using MonoTouch;
@@ -316,8 +317,7 @@ class DeflateStreamNative
 		UnmanagedReadOrWrite feeder; // This will be passed to unmanaged code and used there
 
 		Stream base_stream;
-		IntPtr z_stream;
-		GCHandle data;
+		SafeNativeZStreamHandle z_stream;
 		bool disposed;
 		byte [] io_buffer;
 
@@ -328,10 +328,9 @@ private DeflateStreamNative ()
 		public static DeflateStreamNative Create (Stream compressedStream, CompressionMode mode, bool gzip)
 		{
 			var dsn = new DeflateStreamNative ();
-			dsn.data = GCHandle.Alloc (dsn);
 			dsn.feeder = mode == CompressionMode.Compress ? new UnmanagedReadOrWrite (UnmanagedWrite) : new UnmanagedReadOrWrite (UnmanagedRead);
-			dsn.z_stream = CreateZStream (mode, gzip, dsn.feeder, GCHandle.ToIntPtr (dsn.data));
-			if (dsn.z_stream == IntPtr.Zero) {
+			dsn.z_stream = SafeNativeZStreamHandle.Create (mode, gzip, dsn, dsn.feeder);
+			if (dsn.z_stream == null) {
 				dsn.Dispose (true);
 				return null;
 			}
@@ -353,14 +352,10 @@ public void Dispose (bool disposing)
 
 				io_buffer = null;
 
-				IntPtr zz = z_stream;
-				z_stream = IntPtr.Zero;
-				if (zz != IntPtr.Zero)
-					CloseZStream (zz); // This will Flush() the remaining output if any
-			}
-
-			if (data.IsAllocated) {
-				data.Free ();
+				SafeNativeZStreamHandle zz = z_stream;
+				z_stream = null;
+				if (zz != null)
+					zz.Dispose();
 			}
 		}
 
@@ -476,26 +471,65 @@ static void CheckResult (int result, string where)
 			throw new IOException (error + " " + where);
 		}
 
+		class SafeNativeZStreamHandle : SafeHandleZeroOrMinusOneIsInvalid
+		{
+			public static SafeNativeZStreamHandle Create (CompressionMode mode, bool gzip, DeflateStreamNative deflateStreamNative, UnmanagedReadOrWrite feeder)
+			{
+				GCHandle deflateStreamNativeHandle = GCHandle.Alloc (deflateStreamNative);
+				SafeNativeZStreamHandle nativeSafeHandle = CreateZStream (mode, gzip, feeder, GCHandle.ToIntPtr (deflateStreamNativeHandle));
+				if (nativeSafeHandle == null || nativeSafeHandle.IsInvalid) {
+					if (deflateStreamNativeHandle.IsAllocated)
+						deflateStreamNativeHandle.Free();
+
+					return null;
+				}
+
+				nativeSafeHandle.deflateStreamNativeHandle = deflateStreamNativeHandle;
+
+				return nativeSafeHandle;
+			}
+
+			private SafeNativeZStreamHandle () :
+				base (true)
+			{
+			}
+
+			protected override bool ReleaseHandle()
+			{
+				if (handle != IntPtr.Zero) {
+					CloseZStream (handle); // This will Flush() the remaining output if any
+				}
+
+				if (deflateStreamNativeHandle.IsAllocated) {
+					deflateStreamNativeHandle.Free ();
+				}
+
+				return true;
+			}
+
+			[DllImport (LIBNAME, CallingConvention=CallingConvention.Cdecl)]
+			static extern SafeNativeZStreamHandle CreateZStream (CompressionMode compress, bool gzip, UnmanagedReadOrWrite feeder, IntPtr data);
+
+			[DllImport (LIBNAME, CallingConvention=CallingConvention.Cdecl)]
+			static extern int CloseZStream (IntPtr stream);
+
+			GCHandle deflateStreamNativeHandle;
+		}
+
 #if MONOTOUCH || MONODROID
 		const string LIBNAME = "__Internal";
 #else
 		const string LIBNAME = "MonoPosixHelper";
 #endif
 
 		[DllImport (LIBNAME, CallingConvention=CallingConvention.Cdecl)]
-		static extern IntPtr CreateZStream (CompressionMode compress, bool gzip, UnmanagedReadOrWrite feeder, IntPtr data);
-
-		[DllImport (LIBNAME, CallingConvention=CallingConvention.Cdecl)]
-		static extern int CloseZStream (IntPtr stream);
-
-		[DllImport (LIBNAME, CallingConvention=CallingConvention.Cdecl)]
-		static extern int Flush (IntPtr stream);
+		static extern int Flush (SafeNativeZStreamHandle stream);
 
 		[DllImport (LIBNAME, CallingConvention=CallingConvention.Cdecl)]
-		static extern int ReadZStream (IntPtr stream, IntPtr buffer, int length);
+		static extern int ReadZStream (SafeNativeZStreamHandle stream, IntPtr buffer, int length);
 
 		[DllImport (LIBNAME, CallingConvention=CallingConvention.Cdecl)]
-		static extern int WriteZStream (IntPtr stream, IntPtr buffer, int length);
+		static extern int WriteZStream (SafeNativeZStreamHandle stream, IntPtr buffer, int length);
 	}
 }