expose package_metadata_reader to allzparkconfig

[mottosso]

Goal

Enable customisation of how metadata is gathered from a given package.

Motivation

Metadata is currently presumed to exist within a package as a _data dictionary variable.

name = "maya"
version = "2018"
_data = {
  "label": "Autodesk Maya",
  "icon": "maya_{width}_{height}"
}

There are a few issues with that.

1. We can't assume _data is free for us to claim; the user may already be using it for something else.
2. We can't assume metadata is present in the package itself, or even package.py. It may come from a side-car file or database, in which case the package may be be used as merely an indicator of where to find it (e.g. by name/version combination)
3. We can't assume all metadata is equal. For example, project metadata like icon and backdrop may come from a database, whereas the icon for Maya may reside in the package.

Implementation

Expose the current reader to allzparkconfig.py.

def package_metadata_reader(package):
  return {
    "label": package._data["label"],
    "icon": database.query({"key": package.name, "value": "icon"})
  }

To facilitate lengthy queries, make the call asynchronous from within Allzpark.

[mottosso]

Implemented. For example, implementing backwards compatibility can be done like so:

~/marcus/allzparkconfig.py

def read_package_data(package):
    # Augment existing data
    data = allzparkconfig._read_package_data(package)

    # Backwards compatibility with old system
    backwards_icon = getattr(package, "_icons", {}).get("32x32")
    backwards_icon = backwards_icon or data.get("icons", {}).get("32x32")
    data["icon"] = data["icon"] or backwards_icon or ""

    return data

[mottosso]

Icons are currently presumed to be a string template, with 3 keys.

Example

icon = "{root}/any/path/{width}x{height}.png"
icon = "{root}/any/path/{w}x{h}.png"  # Alternatively

It's questionable whether we need to override at this level of granularity, but in case icons are coming from elsewhere or integrating with a third-party system then odds are Allzpark would need to conform to it rather than vice versa. That's open for discussion should that happen. There could alternatively be a specific mechanism for returning a fully qualified QIcon, such that it doesn't need to be a file (e.g. an icon from a database like Shotgun/ftrack).