Fixed double-free during multi-threaded hidapi access

The error string is not protected by a mutex, and can be set from multiple threads at the same time. Without this change, it can be double-freed. It can still be double-allocated, leading to a memory leak, but at least it won't crash now.

Signed-off-by: Sam Lantinga <slouken@libsdl.org>

src/hidapi/SDL_hidapi.c

@@ -529,6 +529,7 @@ static void HIDAPI_ShutdownDiscovery(void)
 /* Platform HIDAPI Implementation */
 
 #define HIDAPI_IGNORE_DEVICE(VID, PID)  SDL_HIDAPI_ShouldIgnoreDevice(VID, PID)
+#define HIDAPI_ATOMIC_SET_POINTER(a, v) SDL_AtomicSetPtr((void **)a, v)
 
 struct PLATFORM_hid_device_;
 typedef struct PLATFORM_hid_device_ PLATFORM_hid_device;

src/hidapi/linux/hid.c

@@ -130,8 +130,19 @@ static wchar_t *utf8_to_wchar_t(const char *utf8)
  * Use register_error_str(NULL) to free the error message completely. */
 static void register_error_str(wchar_t **error_str, const char *msg)
 {
-	free(*error_str);
-	*error_str = utf8_to_wchar_t(msg);
+	wchar_t *old_string;
+#ifdef HIDAPI_ATOMIC_SET_POINTER
+	old_string = HIDAPI_ATOMIC_SET_POINTER(error_str, NULL);
+#else
+	old_string = *error_str; *error_str = NULL;
+#endif
+	if (old_string) {
+		free(old_string);
+	}
+
+	if (msg) {
+		*error_str = utf8_to_wchar_t(msg);
+	}
 }
 
 /* Semilar to register_error_str, but allows passing a format string with va_list args into this function. */

src/hidapi/mac/hid.c

@@ -243,8 +243,19 @@ static wchar_t *utf8_to_wchar_t(const char *utf8)
  * Use register_error_str(NULL) to free the error message completely. */
 static void register_error_str(wchar_t **error_str, const char *msg)
 {
-	free(*error_str);
-	*error_str = utf8_to_wchar_t(msg);
+	wchar_t *old_string;
+#ifdef HIDAPI_ATOMIC_SET_POINTER
+	old_string = HIDAPI_ATOMIC_SET_POINTER(error_str, NULL);
+#else
+	old_string = *error_str; *error_str = NULL;
+#endif
+	if (old_string) {
+		free(old_string);
+	}
+
+	if (msg) {
+		*error_str = utf8_to_wchar_t(msg);
+	}
 }
 
 /* Similar to register_error_str, but allows passing a format string with va_list args into this function. */

src/hidapi/windows/hid.c

@@ -255,8 +255,15 @@ static void free_hid_device(hid_device *dev)
 
 static void register_winapi_error_to_buffer(wchar_t **error_buffer, const WCHAR *op)
 {
-	free(*error_buffer);
-	*error_buffer = NULL;
+	wchar_t *old_string;
+#ifdef HIDAPI_ATOMIC_SET_POINTER
+	old_string = HIDAPI_ATOMIC_SET_POINTER(error_buffer, NULL);
+#else
+	old_string = *error_buffer; *error_buffer = NULL;
+#endif
+	if (old_string) {
+		free(old_string);
+	}
 
 	/* Only clear out error messages if NULL is passed into op */
 	if (!op) {
@@ -320,8 +327,15 @@ static void register_winapi_error_to_buffer(wchar_t **error_buffer, const WCHAR
 
 static void register_string_error_to_buffer(wchar_t **error_buffer, const WCHAR *string_error)
 {
-	free(*error_buffer);
-	*error_buffer = NULL;
+	wchar_t *old_string;
+#ifdef HIDAPI_ATOMIC_SET_POINTER
+	old_string = HIDAPI_ATOMIC_SET_POINTER(error_buffer, NULL);
+#else
+	old_string = *error_buffer; *error_buffer = NULL;
+#endif
+	if (old_string) {
+		free(old_string);
+	}
 
 	if (string_error) {
 		*error_buffer = _wcsdup(string_error);