extending sections using <= fails transitively

[Dylan Jay]

A buildout such as the following will fail

parts = C B
[A]
recipe = blah

[B]
<=A

[C]
<=B

This will cause C to inadvertently remove the <=A from [B]. B will then have no recipe.

[Vincent Pelletier (vincent-nexedi)]

Hi.

I experienced this bug too. I attach an almost stand-alone test case (based on the "<=" section in buildout doc on pypi). The bug triggers when filling myfiles2 section (copy/paste of myfiles section, with different path).
Also, I developped a slightly different (one-word-er) fix for this bug (I didn't look in the bug tracker first...), I attach it as well.

Regards,
Vincent Pelletier

[Vincent Pelletier (vincent-nexedi)]

Patch in #3 is wrong: extended section properties take precedence over overriding's properties. This case is not demonstrated by the documentation, but I'm convinced the current behaviour is intentional.

[Vincent Pelletier (vincent-nexedi)]

New version of patch #3, this time preserving different-name section extension (aka "<=", as opposed to same-name aka "[buildout]\nextends = ...") precedence.

[Vincent Pelletier (vincent-nexedi)]

I found a case which is better served by patch in comment #1: if some section alters an extended section during initialisation, my fix would hide that alteration, while patch #1 would make it available (because extentions are always walked through entirely, while my patch caches the result of first walkthrough).

I cannot think of a valid case for such alteration, but nevertheless patch #1 pushes less assumptions to the code than mine, hence it is better.

[Marius Gedminas (mgedmin)]

There's now a suggested fix at https://github.com/buildout/buildout/pull/37