[PATCH] Allow multiple modules with the same name to coexist in the module cache

[Richard Smith]

On Mon, Mar 31, 2014 at 5:25 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com>wrote:

>
> On Mar 31, 2014, at 4:44 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On Mon, Mar 31, 2014 at 4:23 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com>wrote:
>
>> >
>> > What I'm suggesting is:
>> >
>> > 1) Drop the -I paths that are earlier than the module in the header
>> search path when building the module
>> > 2) Include the rest of the header search paths in the configuration
>> hash for the module
>>
>> This will prevent us from sharing system modules across projects, and, as
>> Ben already mentioned, will result in an explosion of module files, even
>> within the same project.
>>
>
> Why? System modules would only have system include paths, and these would
> usually be the same across projects, right?
>
>
> I see what you mean, but we also need to support the case where the system
> framework developer can point to the framework in his local directory to be
> used, instead of the system one, how do you suggest we allow this ?
>

I'd like for a module to be able to specify its own include paths. If a
module does so, the include paths of the host build should not be included
in its hash.

 Even if we include the search paths in the module hash we will still need
>> to re-lookup the module dependencies before loading the module, because a
>> new module.map may have showed up somewhere in the search paths since the
>> time we built the module; unless I'm missing something, I don't see any
>> benefit in including the header search paths in the hash.
>
>
> The hash should include everything that affects the way the module is
> built. If we're transferring include paths from the user of the module to
> the module build, the hash should include those paths, or two modules with
> different search paths could collide in the cache.
>
>
> The collision is avoided by using the path to the module.map as Ben
> proposes; if different search paths resolve to different module
> dependencies, this is a case where we need to rebuild, but the same is true
> if the same paths are used but a different module.map shows up as
> dependency.
>

I don't see how that's sufficient -- the include paths can also affect how
non-modular #includes within the module behave.

As a somewhat-contrived example, suppose I have a library

  blah/
    module.map
    foo.h
    mode1/
      foo-impl.h
    mode2/
      foo-impl.h

... where foo-impl.h is textually-included into foo.h, and this is supposed
to be used in two modes: one with a -I path pointing to mode1, and one with
a -I path pointing to mode2. We should ensure those two modules don't
collide in the cache. (Maybe mode1 and mode2 provide inline assembly for
different CPU variants, or such.)
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