[BUG] Undefined behavior in nvdsvideotemplate handling custom library properties string

[CristiFati]

nvdsvideotemplate (gst_nvdsvideotemplate_get_property) gets the custom library string properties by calling QueryProperties (returns a char* which is a string pointer rather than a pointer to an (unsigned) integer).

Then at NVIDIA/DeepStream - gstnvdsvideotemplate.cpp:625 it deallocates it: delete str; when it should be delete[] str;. That's Undefined Behavior, although it's known to work.

While we're at it, I think DSCustomLibraryBase::QueryProperties() (or from parent interface) should contain some doc or comment mentioning:

• The returned string is deallocated by videotemplate
• The deallocator used (in order to create the string with the appropriate allocator)

Initially I was returning the contents of a custom library class member, and that triggered gst-inspect-1.0 to segfault. With new[] or strdup (I chose the former) it no longer segfaults, but it's still incorrect.

Steps/Code to reproduce bug

The issue has no observable output (without using debug/profile tools), so using the first (wrong) variant that I had:

1. Create a minimal custom library with e.g.:

   const char CustomLib::m_member[] = "dummy\n";
   
   char* CustomLib::QueryProperties() {
       return const_cast<char*> m_member;  // !nullptr
   }

2. make
3. NVDS_CUSTOMLIB="path/to/libcustomlib.so" gst-inspect-1.0 nvdsvideotemplate

Expected behavior
The output should contain the string, instead of segfaulting.

Environment details (please complete the following information):
N/A. I used a NVidia DS container.

Additional context
N/A

Might also apply to others (audiotemplate?).

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