remove/deprecate unsafe XLinkConnection::isClosed/checkClosed

- partial fix luxonis#520

include/depthai/xlink/XLinkConnection.hpp

@@ -104,8 +104,10 @@ class XLinkConnection {
 
     /**
      * Is the connection already closed (or disconnected)
+     *
+     * @warning This function is thread-unsafe and may return incorrect values.
      */
-    bool isClosed() const;
+    [[deprecated("thread-unsafe and may return incorrect values")]] bool isClosed() const;
 
    private:
     friend struct XLinkReadError;
@@ -116,7 +118,6 @@ class XLinkConnection {
     static std::string convertErrorCodeToString(XLinkError_t errorCode);
 
     void initDevice(const DeviceInfo& deviceToInit, XLinkDeviceState_t expectedState = X_LINK_BOOTED);
-    void checkClosed() const;
 
     bool bootDevice = true;
     bool bootWithPath = true;

src/xlink/XLinkConnection.cpp

@@ -307,17 +307,16 @@ XLinkConnection::XLinkConnection(const DeviceInfo& deviceDesc, XLinkDeviceState_
     initDevice(deviceDesc, expectedState);
 }
 
+// BUGBUG this function is thread-unsafe. The `closed` value is only known and valid
+// within the context of the lock_guard. The value is immediately invalid and outdated
+// when it is returned by value to the caller
 bool XLinkConnection::isClosed() const {
-    std::unique_lock<std::mutex> lock(closedMtx);
+    std::lock_guard<std::mutex> lock(closedMtx);
     return closed;
 }
 
-void XLinkConnection::checkClosed() const {
-    if(isClosed()) throw std::invalid_argument("XLinkConnection already closed or disconnected");
-}
-
 void XLinkConnection::close() {
-    std::unique_lock<std::mutex> lock(closedMtx);
+    std::lock_guard<std::mutex> lock(closedMtx);
     if(closed) return;
 
     using namespace std::chrono;