Introduction

If there is no C or C++ binding for a package manager, PackageKit can run the backend as a separate helper program written in any language. The daemon starts the helper once, keeps it running between transactions, and exchanges JSON Lines messages with it over a dedicated socket. The helper’s standard output and standard error are captured as log output.

A spawned backend consists of a directory $libdir/packagekit/backends/<name>/ holding a keyfile manifest, backend.conf, and the helper program it names:

[Backend]
Exec=portageBackend.py
Description=Portage
Author=Jane Example <jane@example.org>
AllowSigkill=true

When DefaultBackend=<name> names a backend for which no libpk_backend_<name>.so exists but such a manifest does, the daemon loads its generic spawn module for it. Everything else about the backend, including the roles, filters and groups it supports, is reported by the helper itself in the handshake.

Writing a helper

Python helpers use the packagekit_backend library that PackageKit installs privately for them: subclass Backend, implement the roles you support as methods named after them, and call main(). The library speaks the protocol, exports the transaction context to the environment, turns exceptions into errors, and handles cancellation.

Helpers in other languages read the protocol descriptor number from the PK_BACKEND_PROTOCOL_FD environment variable and implement the protocol directly. It is specified in full in the spawned backend protocol.