Custom Fonts on Windows broken if ExitDll called but DLL not actually unloaded

[nickdowell]

Steps to reproduce:

• Load an instance of a plug-in that uses custom fonts in VST3PluginTestHost
• Remove the instance
• Add another instance. The custom fonts will no longer display correctly.

D2DFont::terminate () (called from ExitDll ()) resets customFonts but since customFontsOnceFlag is not and cannot be reset, customFonts will never be recreated by subsequent calls to getCustomFonts () unless the plug-in's DLL is removed from memory, which may not always happen.

static std::unique_ptr<CustomFonts> customFonts;
static std::once_flag customFontsOnceFlag;

static CustomFonts* getCustomFonts ()
{
    std::call_once (customFontsOnceFlag, [] () { customFonts = std::make_unique<CustomFonts> (); });
    return customFonts.get ();
}

void D2DFont::terminate () { D2DFontPrivate::customFonts = nullptr; }

One work-around is to comment out the body of D2DFont::terminate ()

---

Update: I was seeing this behaviour due to a long-running thread in my plug-in that was expecting to be torn down in a static destructor. This increased the DLL's reference count thereby preventing unloading. Using a static Steinberg::ModuleTerminator to terminate the thread has resolved the issue, but I still think this use of a static std::once_flag is problematic due to the uncertainty around library unloading.