[cfe-dev] RFC: Supporting private module maps for non-framework headers

Richard Smith richard at metafoo.co.uk
Wed Nov 12 14:34:16 PST 2014

On Tue, Nov 11, 2014 at 8:04 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com>

> On Nov 11, 2014, at 7:38 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Tue, Nov 11, 2014 at 6:49 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com>
> wrote:
>> On Nov 11, 2014, at 6:37 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>> On Tue, Nov 11, 2014 at 3:45 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com>
>>  wrote:
>>> On Nov 11, 2014, at 12:34 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>> On Tue, Nov 11, 2014 at 10:00 AM, Argyrios Kyrtzidis <
>>> kyrtzidis at apple.com> wrote:
>>>> On Nov 10, 2014, at 7:48 PM, Richard Smith <richard at metafoo.co.uk>
>>>> wrote:
>>>> On Mon, Nov 10, 2014 at 4:00 PM, Argyrios Kyrtzidis <
>>>> kyrtzidis at apple.com> wrote:
>>>>> Hi all,
>>>>> For frameworks Clang currently supports adding a separate module map
>>>>> file for the private headers of the framework. It looks specifically for
>>>>> the presence of ‘module.private.modulemap’ inside the .framework and parses
>>>>> both the public and the private module maps when it processes its module.
>>>>> We would like to extend support for private module maps for non-framework
>>>>> headers as well.
>>>>> In the Darwin platform, the public SDK headers are located in
>>>>> '/usr/include', while the associated private SDK headers are located in
>>>>> '/usr/local/include’. '/usr/local/include’ comes before '/usr/include’ in
>>>>> the header search paths.
>>>> I worry that this will be fragile. If for any reason we look in
>>>> /usr/include but not in /usr/local/include, we'll not load the private
>>>> extension map and things will probably go quite badly from that point
>>>> onwards. If the presence of the /usr/local/include headers is a fundamental
>>>> part of a /usr/include module, then it seems better to me to specify that
>>>> within the /usr/include module map.
>>>> So here's one possibility: allow 'extern module' declarations to be
>>>> nested within other modules, then write your /usr/include module map as:
>>>> module MyModule {
>>>>   <...>
>>>>   extern module SomethingPrivate
>>>> "/usr/local/include/module.private.map"
>>>> }
>>>> This has drawbacks:
>>>> - Details of the private SDK, “leak out” to the public one. It should
>>>> work similar to frameworks, in that the public SDK remains the same
>>>> irrespective if there is or not a private API, and the private API is a
>>>> straight addition on top of the public one without needing to modify
>>>> something in the public SDK.
>>>> - It is a bit weak as guarantee anyway because the public module map
>>>> must necessarily function even when the extension map is missing, which
>>>> means pointing at the wrong path or missing the private map when you really
>>>> need it will not be detected.
>>>> - Flexibility to extend a module from any path may be valuable for
>>>> testing.
>>> OK, I'm not sure I understand what problem you're solving. If the
>>> /usr/local/include stuff works as a layer on top of /usr/include, why do
>>> you need them to be built as part of the same module? (Do your
>>> /usr/local/include headers override / #include_next some of the
>>> /usr/include headers, perhaps? If so, do you need the #includes in
>>> /usr/include to find the /usr/local/include headers rather than the
>>> /usr/include headers?)
>>> There are some cases of cycles between public/private headers which
>>> would be accommodated by a single module build but the primary motivation
>>> is that we would like the module public/private interface to be under the
>>> same namespace, so you’d do
>>> @import Dispatch;
>>> @import Dispatch.Private;
>>> @import Darwin.POSIX.Foo.Bar;
>>> @import Darwin.POSIX.Foo.Bar.Private;
>>> and generally any kind of private extension:
>>> @import Dispatch.SuperCoolButPrivate;
>> Do you want / need them to be built as a single module file, or not?
>> As I said, cycles may make things difficult for separate module files,
>> but how are we going to get new submodules under the same module name with
>> separate module files ?
> Well, the restriction that module files correspond to top-level module
> names is arbitrary and artificial. (It's also a bad idea for a few reasons.
> It makes incremental refactoring very hard, for instance, because you're
> required to have no cycles at any point between things in different
> top-level modules.)
> Splitting up the description of how to build a module file across various
> module maps seems like a very error-prone strategy, especially if you're
> intending to be able to stop looking before you've read all of the relevant
> module maps.
> I think that the high level parts of my proposal are not dependent on
> whether we build one .pcm file or multiple ones, this is an implementation
> detail.
> To be more specific, if we have
> *module.private.modulemap *(extension):
> extern module Dispatch
> module Dispatch.Private {
>   <headers>
> }
> *module.modulemap:*
> module Dispatch {
>   <headers>
> }
> It is an implementation detail whether we buiild one Dispatch.pcm file or
> a Dispatch.Private.pcm file that depends on another Dispatch.pcm; it should
> make no difference on user code.
> Is this incorrect ?

Whether we build one .pcm or multiple is observable in some circumstances.
1) We concatenate together all the header files built as part of one .pcm
file, and parse them all at once, and that is not always semantically
equivalent to building them in two separate passes. 2) If you have one big
Dispatch.pcm file which also contains the private bits, and by any sequence
of events you end up also pulling in another Dispatch.pcm that contains the
public headers but not the private ones, you may get ambiguity errors. 3)
We do not allow circular references across .pcm files but do allow them
within a .pcm file.

If your Dispatch.Private is simply a layer on top of Dispatch, then
building them as two separate .pcm files seems like the right choice; it
keeps your Dispatch module (for want of a better word) modular. If on the
other hand, you need includes/imports in Dispatch to pull in headers /
submodules from Dispatch.Private, then one big Dispatch .pcm is probably
the right answer, and we'd need something like your proposal so we could
say "here is a Dispatch module that's like the one in /usr/include but
different in the following ways".

(Whichever of these options we pick, we can make the "@import
Dispatch.Private" syntax do the right thing.)
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