kernelbase: Don't allow converting thread to fiber more than once.

dlls/kernel32/tests/fiber.c

@@ -128,10 +128,17 @@ static VOID WINAPI FiberMainProc(LPVOID lpFiberParameter)
 
 static void test_ConvertThreadToFiber(void)
 {
+    void *ret;
+
     if (pConvertThreadToFiber)
     {
         fibers[0] = pConvertThreadToFiber(&testparam);
         ok(fibers[0] != NULL, "ConvertThreadToFiber failed with error %u\n", GetLastError());
+
+        SetLastError(0xdeadbeef);
+        ret = pConvertThreadToFiber(&testparam);
+        ok(!ret, "Got non NULL ret.\n");
+        ok(GetLastError() == ERROR_ALREADY_FIBER, "Got unexpected error %u.\n", GetLastError());
     }
     else
     {
@@ -141,10 +148,17 @@ static void test_ConvertThreadToFiber(void)
 
 static void test_ConvertThreadToFiberEx(void)
 {
+    void *ret;
+
     if (pConvertThreadToFiberEx)
     {
         fibers[0] = pConvertThreadToFiberEx(&testparam, 0);
         ok(fibers[0] != NULL, "ConvertThreadToFiberEx failed with error %u\n", GetLastError());
+
+        SetLastError(0xdeadbeef);
+        ret = pConvertThreadToFiberEx(&testparam, 0);
+        ok(!ret, "Got non NULL ret.\n");
+        ok(GetLastError() == ERROR_ALREADY_FIBER, "Got unexpected error %u.\n", GetLastError());
     }
     else
     {

dlls/kernelbase/thread.c

@@ -990,6 +990,12 @@ LPVOID WINAPI DECLSPEC_HOTPATCH ConvertThreadToFiberEx( LPVOID param, DWORD flag
 {
     struct fiber_data *fiber;
 
+    if (NtCurrentTeb()->Tib.u.FiberData)
+    {
+        SetLastError( ERROR_ALREADY_FIBER );
+        return NULL;
+    }
+
     if (!(fiber = HeapAlloc( GetProcessHeap(), 0, sizeof(*fiber) )))
     {
         SetLastError( ERROR_NOT_ENOUGH_MEMORY );