Support pom-model fragments in the pomless build

[laeubi]

Currently tycho pomless is very powerful, but sometimes it is required to still configure things in the xml style (e.g. profiles).

It would therefore be good if one can deine a pom-model fragment that is then merged with the automatic generated one. If this works out well, we can even contribute this upstream to the polyglot project, this is a bit similar to https://github.com/repaint-io/maven-tiles but much more simpler.

[Frettman]

I've been looking for this as well. Right now it's all or nothing: Have it conveniently generated or write the complete POM yourself. When all you want to do is e.g. add a single plug-in to your module. And since the generated POM is deleted right after, you can't even use those as a starting template. Unless I'm missing something?

[laeubi]

You have different options:

1. define a plugin in the configurator pom and define a property (default to true) that skips the execution, then in your pomless module you can enable it with https://github.com/eclipse-tycho/tycho/wiki/Tycho-Pomless#define-properties this requires some support from the mojo but many have support for that.
2. You can use a profile that is enabled when there is a special file present and configures the plugin, this is useful if you already have a file (e.g. for code generation) but of course also a dedicated one works

Beside that, you can let Tycho keep the generated pom.xml: https://github.com/eclipse-tycho/tycho/wiki/Tycho-Pomless#useful-tips

[Frettman]

Thank you, @laeubi, very useful information.
I needed to add some Maven plug-ins to all Eclipse plug-ins. As the common parent, I was looking to maybe merge those into the generated POM of my plugins aggregation folder. But a profile in my root POM with <activation><file><exists>plugin.xml</exists></file></activation> is actually even better.

Beside that, you can let Tycho keep the generated pom.xml: https://github.com/eclipse-tycho/tycho/wiki/Tycho-Pomless#useful-tips

I've been explicitly looking for this, but this little nugget has eluded me until now. Thank you :)