[Buildroot] conditional assigment check in check-package [was: Re: [PATCH v2, 2/2] package/knock: add knockd option]

[Yann E. MORIN]

Arnout, All,

On 2021-10-21 20:43 +0200, Arnout Vandecappelle spake thusly:
> On 18/10/2021 22:18, Yann E. MORIN wrote:
> >Thomas, All,
> >On 2021-10-18 22:04 +0200, Thomas Petazzoni spake thusly:
> >>On Sat, 16 Oct 2021 11:32:34 +0200
> >>Fabrice Fontaine <fontaine.fabrice at gmail.com> wrote:
> >>>+ifeq ($(BR2_PACKAGE_KNOCK_KNOCKD),y)
> >>>+KNOCK_DEPENDENCIES = libpcap
> >> From commit 97f3ad7af3dd54a15a10aa35786e7fa08cf5e7b1:
> >>     Repeat after me: "Forcing the value of <pkg>_DEPENDENCIES inside a
> >>     conditional is the root of all evil."
> >>Care to repeat after me ? :-)
> >
> >OK, I missed that in my review of v1...
> >/me hides in shame...
> >
> >But seriously though: why is that not caught by check-package?
> >
> >Because it is not actually an override: the variable is not yet set
> >there, so it is valid to unconditionally set it.
> >
> >check-package should probably recognise that the variable is
> ><FOO>_DEPENDENCIES and consider that conditional assignment should
> >still be forbidden in that case.
> 
>  I had a look at this, and it is not entirely trivial to test for in check-package.
> 
>  Basically, this is allowed:
> 
> ifeq(...)
> FOO_VAR = ...
> endif
> ... $(FOO_VAR) ... # Empty if condition not true
> 
>  And this as well:
> 
> ifeq(...)
> FOO_VAR = ...
> else
> FOO_VAR = ...
> endif
> 
>  But this is not:
> 
> ifeq(...)
> FOO_VAR = ...
> endif
> 
> ifeq(...)
> FOO_VAR = ...
> endif
> 
> 
>  So basically:
> - after a variable is assigned, it can't be assigned to any more in the same
> conditional branch;
> - it can be assigned to again in the else branch (including else ifeq constructs);
> - it can not be assigned to any more after the endif.
> 
>  In addition, conditions can be nested...
> 
>  The only way I can think of implementing this is by keeping a stack of
> conditionally assigned variables. In the else branch, the top of the stack
> is saved en empties. At the endif, the top of the stack is popped, merged
> with the saved one, and merged into the new top of the stack.
> 
>  Wow, by writing it down, I almost implemented it :-) Let's see if I can
> still find the time to actually do it tonight...

Sorry, but I think I got lost somewhere...

Currently, check-package does not whine for:

    ifeq(...)
    FOO_VAR = ...
    endif

but it does whine for:

    FOO_VAR = ...
    ifeq(...)
    FOO_VAR = ...
    endif

And these two behaviours are, I think, correct.

What I am arguing, is that FOO_DEPENDENCIES should be treated specially,
where check-package should whine for:

    ifeq(...)
    FOO_DEPENDENCIES = ...
    endif

as well as for:

    FOO_DEPENDENCIES = ...
    ifeq(...)
    FOO_DEPENDENCIES = ...
    endif

Regards,
Yann E. MORIN.

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