From a2ec81696de98151bfdf1913042a739ef6d8a36c Mon Sep 17 00:00:00 2001
From: Stanislaw Halik <sthalik@misaki.pl>
Date: Thu, 11 Jan 2018 14:38:06 +0100
Subject: contrib/npclient: fix DataID race

There's a race with how the freetrack protocol operates.

Reported-by: @Len62
cf. https://github.com/opentrack/opentrack/issues/688#issuecomment-356606126
---
 bin/NPClient.dll            | Bin 16384 -> 16384 bytes
 bin/NPClient64.dll          | Bin 17408 -> 17408 bytes
 contrib/npclient/npclient.c |   6 +++---
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/bin/NPClient.dll b/bin/NPClient.dll
index ddfa667e..54a2d0f6 100644
Binary files a/bin/NPClient.dll and b/bin/NPClient.dll differ
diff --git a/bin/NPClient64.dll b/bin/NPClient64.dll
index eb56a256..f8315d50 100644
Binary files a/bin/NPClient64.dll and b/bin/NPClient64.dll differ
diff --git a/contrib/npclient/npclient.c b/contrib/npclient/npclient.c
index 6ffa7f34..71c5ec2a 100644
--- a/contrib/npclient/npclient.c
+++ b/contrib/npclient/npclient.c
@@ -332,19 +332,19 @@ NP_EXPORT(int) NP_GetData(tir_data_t * data)
     frameno++;
     data->frame = frameno;
 
-    int data_id = InterlockedDecrement((LONG volatile*) &pMemData->data.DataID);
-
+    int data_id = InterlockedCompareExchange((LONG volatile*) &pMemData->data.DataID, -1, -1);
     bool running;
 
     if (data_id == 0)
     {
         running = true;
         y = 0, r = 0, p = 0, tx = 0, ty = 0, tz = 0;
-        InterlockedCompareExchange((LONG volatile*) &pMemData->data.DataID, -1, 0);
+        (void)InterlockedCompareExchange((LONG volatile*) &pMemData->data.DataID, -1, 0);
     }
     else if (data_id > 0)
     {
         running = true;
+        (void)InterlockedCompareExchange((LONG volatile*) &pMemData->data.DataID, data_id - 1, data_id);
     }
     else
     {
-- 
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