[gentoo-dev] [RFC] Restricting allowed nesting of REQUIRED_USE

["Michał Górny" <mgorny@gentoo.org>]

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Hi, everyone.

As you may or may not know, PMS says rather little about REQUIRED_USE
[1,2]. The largest past of the definition is shared with other
dependency-like specifications [3].

Similarly to regular dependency specifications, PMS is rather lax in
nesting things. While this isn't a major problem for dependencies where
the syntax is limited to any-of, all-of and USE-conditional groups
(though it already may cause some confusion there), it allows quite
a bit of a mayhem with the full set of REQUIRED_USE clauses.

We have five different kinds of clauses there: any-of, at-most-one-of,
exactly-one-of, all-of and USE-conditional. Furthermore, unlike
in dependency specifications, the last type is circular with flags
enforced by REQUIRED_USE constraints.

While nesting all of those clauses is technically valid (and can be
logically verified), it has no proven usability. As a result, it is
either not used at all or has a few use cases which suffer from poor
readability and can be easily replaced with *much simpler* constraints.
In fact, allowing them is not solving any issues but only introducing
more when developers fail at using them.

I would therefore like to discuss restricting nesting of REQUIRED_USE
clauses.


What's my take in this? As you have probably noticed (and stopped
reading) I am working with Alexis on solving REQUIRED_USE constraints
automatically. We're working towards a few goals: keeping things simple,
giving predictable solutions, and being able to automatically validate
whether the constraints are solvable.

While we're near solving almost everything, the complex clauses add
unnecessary complexity (both to the spec and to the code) which does not
really benefit anyone, and bring solutions that can not be predictable
because the clauses are ambiguous by design.

To avoid adding this complexity, it would be reasonable to ban at least
some of the non-useful combinations. This means either banning them
completely (in a future EAPI + possibly repoman) so that developers do
not even try to use them, or disabling autosolving when they are being
used).


Below I have listed the clauses I'd like to ban in a few logical groups,
along with explanations and examples.


1. Nested ||, ?? and ^^ groups
------------------------------

Technically, any level of ||, ?? and ^^ nesting is valid. Practically,
any nesting is hardly readable, and could be replaced by something
simpler. For a few examples:

  || ( a || ( b c ) )  <->  || ( a b c )

  || ( a ?? ( b c ) )  <->  b? ( c? ( a ) )

  ?? ( ?? ( a b ) c )  <->  !a? ( !c ) !b ( !c )

  ?? ( a || ( b c ) )  <->  a? ( !b !c )

The 'simpler versions' of those constraints may seem weird but that's
only because the constraints themselves are weird as hell and it's hard
to tell what the original intent might be. I've skipped ^^ as it is
equivalent to the conjunction of || and ??.

FWICS, we have only two cases of this kind of nesting in ::gentoo:

A. app-backup/bacula:

  || ( ^^ ( mysql postgres sqlite ) bacula-clientonly )

which could be written equivalently as:

  !bacula-clientonly? ( ^^ ( mysql postgres sqlite ) )

B. dev-games/ogre:

  ?? ( gl3plus ( || ( gles2 gles3 ) ) ) gles3? ( gles2 )

which is completely insane. Per the above examples, it could be replaced
e.g. by more predictable:

  gl3plus? ( !gles2 !gles3 ) gles3? ( gles2 )

To summarize, I don't think we really need or want this kind of nesting.
I would therefore want to disallow nesting any of ||, ??, ^^ inside any
other of ||, ??, ^^ (including as subexpressions).


2. All-of groups inside ??, ^^
------------------------------

This one is technically valid and not even hard to solve. However,
I haven't found any use case for it and it's impossible to solve it
in a completely predictable way.

Let's take a simple case here:

  ?? ( A ( B C ) )

The meaning is rather simple: you can't enable both A and (B and C).
If we put the preference on A, then this constraint can be solved
by either disabling B or C, or both. And there's no definitive answer
on what would be the preferred action here.

So it'd really be better to be clearer on the desired result, e.g.:

  A? ( !B !C )
  A? ( !B )

etc. I'm aware that it would become more complex with more clauses;
however, nobody has been able to come up with even one so far.

The only use cases of all-of groups inside ??/^^ in ::gentoo are:

A. sci-chemistry/icm:

  ^^ ( ( !32bit 64bit ) ( 32bit !64bit ) ( 32bit 64bit ) )

which is much more readable as:

  || ( 64bit 32bit )

B. media-sound/snd:

  ^^ ( ( !ruby !s7 ) ( ruby !s7 ) ( !ruby s7 ) )

which, once again, is much less confusing as:

  ?? ( ruby s7 )

All that considered, I think this has no real use case and only
encourages people to do stupid things. Since it's ambiguous
and unreadable, I would like to ban it.



3. USE-conditionals inside ||, ??, ^^ groups
--------------------------------------------

This one is not as horrible as the others mentioned but it seems to have
barely any use, unnecessarily complexifies the AST and damages
readability. I'm talking about cases like:

  || ( a foo? ( b ) c )

which is roughly equivalent to:

  foo? ( || ( a b c ) )
  !foo? ( || ( a c ) )

The only use I've seen is media-video/mpv:

  || ( cli libmpv ) [...]
  opengl? ( || ( aqua egl X raspberry-pi !cli? ( libmpv ) ) )

which is probably completely meaningless since you have to either enable
cli or libmpv in the first place.

I don't have *that* very strong opinion on this but I'd rather ban it
and expect people to use more straightforward (= readable
and predictable expressions).


Resulting AST
=============

If all three classes I've mentioned were banned, the AST would look
like:

REQUIRED_USE := [<top-expr>...]

top-expr := <flag> | <use-cond> | <any-of> | <most-one-of> | <exactly-one-of>

flag := ['!']<flag-name>

use-cond := ['!']<flag-name>'? (' <top-expr>... ')'

any-of := '|| (' <any-of-expr>... ')'

any-of-expr := <flag> | <all-of>

most-one-of := '??' ( <flag>... )

exactly-one-of := '^^' ( <flag>... )

all-of := '(' <flag>... ')'


Note that only USE conditionals are deeply nested now. Of all other
groups, || can contain pure flags and/or all-of blocks (which contain
only pure flags), and ?? and ^^ contain pure flags only.

Your thoughts?


[1]:https://projects.gentoo.org/pms/6/pms.html#x1-690007.3
[2]:https://projects.gentoo.org/pms/6/pms.html#x1-910008.2.7
[3]:https://projects.gentoo.org/pms/6/pms.html#x1-780008.2

-- 
Best regards,
Michał Górny

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