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

[Richard Smith]

Sorry, I think we're getting off-track here, my bad. I have a couple of
in-line comments then I'm going to try to pull this discussion together at
the bottom...

On Mon, Mar 31, 2014 at 4:08 PM, Ben Langmuir <blangmuir at apple.com> wrote:

>
> On Mar 31, 2014, at 3:24 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On Mon, Mar 31, 2014 at 11:40 AM, Ben Langmuir <blangmuir at apple.com>
> wrote:
>
>>
>> On Mar 28, 2014, at 4:42 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>>
>> On Fri, Mar 28, 2014 at 3:27 PM, Ben Langmuir <blangmuir at apple.com>
>> wrote:
>>
>>> Hi Richard,
>>>
>>>
>>> On Mar 28, 2014, at 2:45 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>>
>>> On Fri, Mar 28, 2014 at 1:12 PM, Ben Langmuir <blangmuir at apple.com>
>>> wrote:
>>>
>>>> This patch allows multiple modules that have the same name to coexist
>>>> in the module cache.  To differentiate between two modules with the same
>>>> name, we will consider the path the module map file that they are defined
>>>> by* part of the 'key' for looking up the precompiled module (pcm file).
>>>>  Specifically, this patch renames the precompiled module (pcm) files from
>>>>
>>>> cache-path/<module hash>/Foo.pcm
>>>>
>>>> to
>>>>
>>>> cache-path/<module hash>/Foo-<hash of module map path>.pcm
>>>>
>>>
>>> From a high level, I don't really see why we need a second hash here.
>>> Shouldn't the -I options be included in the <module hash>? If I build the
>>> same module with different -I flags, that should resolve to different .pcm
>>> files, regardless of whether it makes the module name resolve to a
>>> different module.map file.
>>>
>>>
>>> If we include the -I options in the module hash, we will explode the
>>> number of module compilations needed.  The following should all be able to
>>> share a module 'A'.
>>>
>>
>> I really don't think they should, if that second -I path is used in any
>> way when building module 'A'.
>>
>>
>> clang -fmodules -I /path/to/A -I /path/to/B some_file.c
>>> clang -fmodules -I /path/to/A -I /path/to/C some_file2.c
>>> clang -fmodules -I /path/to/A -I /path/to/D some_file3.c
>>>
>>
>> I would think a better solution here would be to not have that second -I
>> path in header search when building module A (and thus not include it in
>> the hash for a principled reason).
>>
>> Ultimately, each module should have its own header search path -- the
>> model of one set of include paths for the entire TU (with that include path
>> potentially causing some libraries to find the wrong files) is a broken,
>> antiquated legacy of the non-modules world.
>>
>>
>> I agree with this in principle - having a predictable context for
>> building the modules would solve a lot of problems.  However, that seems to
>> only work for semantic import (@import, not #include), which is the less
>> common case for us at the moment.
>>
>
> Can you explain why? Suppose every module.map file lists the include paths
> for that module -- what problems does that create? (Is this problem
> specific to us generating implicit module.map files for frameworks?)
>
>
> If I write #include, or #import then I expect it to obey the include paths
> I set, since that is how it always worked before (for better or worse). For
> @import, and whatever other new syntax we introduce, I'm fully in favour of
> isolating the module build from the happenstance of the command line and
> using a header search derived from the module map file.
>

I think it's reasonable that adding a module.map file affects the semantics
of a #include uses the module. In the case where someone has written a
module map that specifies an include search path, it seems natural that
that might (and should!) change the behavior of a #include mapped to that
module.

Conversely, I think it'd be surprising for a module build to pick up
headers from some directory in the user of the module's code, whether I
used @import or #include syntax to import the module. This is legacy
brokenness from the pre-modules era that I think we should not bring with
us.

 In the short term, we should probably drop all header search paths that
>> are before the path in which the module map was found.
>>
>>
>> To be clear, this has the same correctness problem as my approach - the
>> header search paths that come before the module map may still change the
>> content of the module, especially if the module depends on other modules.
>>
>
> 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
>
> I don't see how this still leads to collisions (other than the madness
> that results from having multiple module.maps on your header search path
> that define the same module, where the order in which we happen to load
> them affects our semantics).
>
>
> It doesn't introduce collisions, but it may change the actual content of
> the module if the module includes headers that are affected by those
> include paths.
>

I agree, but as noted above, I consider that to be a bug fix. No-one writes
libraries that expect the library user to provide headers for them to
include. If they did, I feel happy telling them that their library is
non-modular and needs to be fixed before it can be used with modules. Can
you give an example of a situation where you imagine this being an issue?

 In general, adding the -I options is brittle when precompiled headers are
>> involved. Since you likely don't have exactly the same -I options when
>> building your PCH as when you use it (you likely have fewer -I paths when
>> building the PCH if it is being widely used) you could potentially have
>> different module hashes in the PCH and in the main file, which leads to
>> trying to load multiple copies of a module from different paths.  I ran
>> into this when I tried to naively add all of the -I options to the module
>> hash.  The heuristic you suggested for dropping the earlier paths might
>> help here, but it would be required for correctness, not merely as an
>> optimization.
>>
>
> I'll need to think about this one some more. My initial reaction is that
> if a PCH loads a module in one configuration and a user of the PCH tries to
> load the same module in a different configuration (because it's being built
> with different include paths), that should be a hard error. It seems to me
> that this is a separate issue from ensuring that we don't have collisions
> in the cache?
>
>
> This patch does make it an error if the user of the PCH does not provide a
> header search context in which  the modules resolve the same way as when
> the pch was built.  I don't think expecting the PCH to be created with
> exactly the same -I options is valuable, because it prevents legitimate
> reuse.
>

OK. I think that's great as far as it goes, but it doesn't solve the whole
problem, because non-modular #includes can still resolve in different ways.

 I am also not sure what effect this would have on the global module index,
>> since the set of modules to load would span multiple hash-directories and
>> we would need to avoid looking at incompatible modules.  I'm not sure if
>> this is a problem or not.
>>
>
> Yes, that's an interesting problem =)
>
>
>> Any thoughts?
>>
>
> I don't think I've got enough information to suggest something concrete
> yet, but this strikes me as a possible avenue to explore:
>
> 1) Teach module.map files to specify the include paths for the module
> 2) When implicitly generating a module.map file, inject the include paths
> from the host build (probably skipping ones prior to the directory
> containing the framework)
> 3) Include a hash of the contents of the implicit module.map file in the
> cache entry for any module that has an implicit module.map
> 4) Don't include the header search paths in the configuration hash, since
> they're not used when determining how to build a module any more.
>
>
> Does this help us with multiple modules having the same name?  I think we
> would still have the problem that when we go to load a pcm file it may have
> been built from a different module.map file than the one we found in this
> compilation (e.g. I have a module MyFoo, and so does another user on the
> system and we happen to share a module cache directory).  According to 4 we
> don't hash the -I paths, so these modules would (potentially) resolve to
> the same location on disk.
>
> This is convincing me more than ever that  regardless of what else we do,
> we need to store the path to the module map file with the pcm so that we
> can verify that the module we are trying to import resolves to the same
> location that it did when it was built. To me, the filename of the pcm is
> the most obvious place for this information, since that allows us to
> restrict ourselves to one module cache directory (ModuleCache/<module
> hash>) just like now, and it allows modules with the same name to live in
> the same cache without trampling each other or the possibility of
> accidentally loading the wrong one.
>

I'd like to take a step back at this point and summarize what I believe the
state to be (sorry that this is rather long). We have three pieces of data
that between them uniquely identify the contents of a module:

Build environment configuration:
--------------------------------

  This covers things like the language options (and, currently,
configuration macros, though those probably belong in the module identity)
  We have one global module index per configuration, and we would like each
compilation action to only touch a single configuration.
  We want to reuse the same build environment configuration as much as
possible, to maximize cache hits.
  A hash of this data is used as the subdirectory in the module cache.

Module identity:
----------------

  This covers things like the name of the module and the module.map file in
which it was defined.
  The proposed patch includes a hash of this data in the name of the .pcm
file.

Module header:
--------------

  This covers things that we discovered when building the module, such as
the module's dependencies and their timestamps.
  The proposed patch adds the list of imported modules to this, such that
header search must find the same module.
  If we have a mismatch here, we rebuild the module.

So far, this division seems entirely reasonable to me. Observations:
 * We only want to use one global index per build
 * We want to minimize the amount of irrelevant stuff in the global index
for efficiency reasons (otherwise we could just have one global index for
all configurations), and
 * We want to minimize the number of times we rebuild modules

>From the above, I think that we should use the following guidelines:
 * For correctness, the (build environment + module identity + module
header) must uniquely identify the module contents (because that's how we
determine whether we have a cache hit).
 * We want to pick the data that goes into the build environment such that
two module builds have the same build environment configuration if they
could conceivably be used in the same TU, and we should try to keep their
build environments different otherwise.
 * We only want to include things in the module header if changes to those
things mean we'll probably never want to use the old .pcm file again.


The main point of contention seems to be how header search paths fit into
the above taxonomy. I think we've discussed a few options here:

1) They are a property of the module, and are independent of the header
search path of the module user. This means they are not part of the above
data.

2) They are a property of the build environment. (This was my original
position.) This seems to mean that the module identity can be just the
module name, if we enforce that the same module name can't be found
multiple times in the include path.

3) They are a property of the module identity.

4) They're not part of anything. The names and resolved paths of imported
modules are included in the module header. We hope that the include path
didn't affect anything else. (This is what the patch we're discussing does.)

Option 4 seems to be only a partial solution to the include path issue,
because we still have cache collisions between modules with different
contents due to differing -I paths. Your suggested flag to make non-modular
#includes inside a module an error would fix this, but I don't think that
is going to work for much real-world code -- I don't think that non-modular
#includes are going away.

I think we can fix option 4 by also including the set of files #included by
non-modular includes in the module header (and anything else that depends
on the include paths), and re-resolving those files too when we import a
module. If the lookup differs, we rebuild the module. This has the
unfortunate effect that we might rebuild a module when the prior .pcm is
still useful, but that seems to be a rare case. I could probably live with
that. Does that sound OK to you?


Another question is what include paths we should use for a module:

1) They inherit all the include paths from the host build. (We keep the
legacy broken model.)
2) They inherit the include paths for themselves and anything later in the
header search path.
3) They get to specify their own include path.

(1) is the status quo. I suggest we allow (3) and fall back to (2) if no
include path is specified.
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