On Thursday 7 November 2024 13:53:43 Pacific Standard Time Jeremy Rifkin via
Std-Proposals wrote:
> Does it have to be prior to the load? What’s wrong with loading it as long
> as you don’t use stdlib constructs across ABI boundary until after
> verifying some match?

By your *strict* answer, that would work. But the devil will be in the
details.

You said "don't use stdlib constructs across ABI boundary". That means the
plugin MUST NOT make any function calls using the stdlib types to any function
that may reside in another DLL/library, before the verification is made. This
includes any and all static initialisation content (global constructors).
Worse, this applies to any library the plugin may have linked to which may
potentially not yet have been loaded into memory by the host application or
library.

The only way to be certain of this is to have a strict requirement of
"absolutely no load-time constructors" and this must apply transitively to all
C++ libraries you link to. The barrier for this is pretty high, considering
you may not control the content of said libraries and you may not even be
distributing them.

It's far easier to query some metadata. Your plugin system can simply install
a file next to the plugin with the information you need. This comes in handy
too to find out the plugin's capabilities and decide at runtime whether you
want it or not. KDE software using Qt 1 to 4 did that with a .desktop file next
to the plugins.

Starting with Qt 5, the Qt plugin mechanism allows you to specify a JSON file
that will be parsed by moc and added to the Qt metadata information in the in
the binary itself. See https://doc.qt.io/qt-6/qpluginloader.html#metaData and
https://doc.qt.io/qt-6/qtplugin.html#Q_PLUGIN_METADATA.

We even have a tool (qtplugininfo) that you can use to query that.

$ qtplugininfo $PWD/plugins/platforms/libq{xcb,wayland-generic}.*
libqxcb.t.so: IID "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.
5.3" Qt 6.9.0 (release)
User Data: {
    "Keys": [
        "xcb"
    ]
}
libqwayland-generic.t.so: IID "org.qt-
project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3" Qt 6.9.0 (release)
User Data: {
    "Keys": [
        "wayland"
    ]
}

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Principal Engineer - Intel DCAI Platform & System Engineering