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Constructor CairoOutputDev::CairoOutputDev is called whenever is opened a PDF file. Opening PDF files from multiple threads can cause multiple initialisation of ft_lib.

FT_Library CairoOutputDev::ft_lib;
bool CairoOutputDev::ft_lib_initialized = false;

CairoOutputDev::CairoOutputDev() {
  doc = nullptr;

  if (!ft_lib_initialized) {
    FT_Init_FreeType(&ft_lib);
    ft_lib_initialized = true;
  }

This could cause memory leak or even something worse depending on what is doing FT_Init_FreeType. And seems it is accessing and dereferencing its parameter multiple times instead of simply outputing FT_Library into pointer so something really bad can happen:

 FT_Init_FreeType( FT_Library  *alibrary )
  {
...
    error = FT_New_Library( memory, alibrary );
    if ( error )
      FT_Done_Memory( memory );
    else
      FT_Add_Default_Modules( *alibrary );

    FT_Set_Default_Properties( *alibrary );

There can be either used singleton with static local variable or std::call_once:

FT_Library CairoOutputDev::ft_lib;
std::once_flag CairoOutputDev::ft_lib_once_flag;

CairoOutputDev::CairoOutputDev() {
  doc = nullptr;

  std::call_once(ft_lib_once_flag, []()
  {
    FT_Init_FreeType(&ft_lib);
  });

Or:

...
  std::call_once(ft_lib_once_flag, FT_Init_FreeType, &ft_lib);