Support drop-in configuration files for systemd

[twiggler]

Introduction

systemd supports "drop-in" configuration directories (see https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html)

Suppose we have the following network configuration file in /etc/systemd/network/wlan0.network

[Match]
Name=wlan0

[Network]
Address=REDACTED IP/24
DHCPServer=true
IPMasquerade=ipv4

Without modifying the original configuration file, we can drop in a directory /etc/systemd/network/wlan0.network.d/ with additional configuration files, such as new-address.network:

[Network]
Address=REDACTED IP/24

These drop in configuration files are merged with the original configuration. Note that a drop-in directory can contain multiple configuration files, which are processed in alphabetical order. The drop-in directory can also reside in a different place: see the documentation for details.

Proposal

Add support for drop-in configuration as an extension to the systemd configuration parser, or as a new parser.

Open problems:

• It is a bit unclear how values are merged exactly. Some sections and keys are allowed to occur multiple times, in which case the additional configuration extends the original, whereas other sections / keys only occur once. When keys occur only once, does the drop-in file overwrite the original value?

Answer, from the doc: Yes. When multiple files specify the same option, for options which accept just a single value, the entry in the file sorted last takes precedence, and for options which accept a list of values, entries are collected as they occur in the sorted files.

This would imply that the parser needs to know which keys / sections are singletons, and which ones can occur multiple times.

Additionally, some keys can be cleared by assigning it an empty string:

Address=