[cfe-dev] Dangling pointer to Module in CompilerInstance::KnownModules

[Nemanja Ivanovic via cfe-dev]

I have spent more time than I would like to admit diagnosing a super weird
and super hard to reproduce problem with clang that turned out to be yet
another instance of the old "dangling pointer" problem.

This email is really meant just to outline the problem, not offer a
solution - I am hoping that the clang experts on this list will offer the
best solution.

So here's the problem...

When a Preprocessor object is created by calling
CompilerInstance::createPreprocessor(), the existing shared pointer may
already be pointing to an existing object. That existing object will own a
HeaderSearch which will own a ModuleMap which will own a number of Module
objects. Pointers to those Module objects may have been cached in the
CompilerInstance::KnownModules map by CompilerInstance::loadModule().
However, creating the new Preprocessor will make the shared pointer point
to that new one and since there are no further references to the old one,
it will be destroyed. Of course, everything the old Preprocessor owns will
be destroyed as well so we now have dangling pointers in the
CompilerInstance::KnownModules map.

As it turns out, it appears to be very difficult to end up in a situation
where this is actually a problem. It seems to be related to the path of
whatever this function is looking for. So with a very specific path for the
build tree, we come across the problem with the last RUN command in
test/Modules/builtins.m. Changing the build tree path by a single character
seems to make the problem go away for some reason.

Now obviously, this problem can be fixed by just clearing
CompilerInstance::KnownModules if the shared pointer PP isn't null in
CompilerInstance::createPreprocessor(). However, I am not sure if that is
the desired fix.

The attached patch isn't the fix I mentioned but a patch that can show the
problem I have described. Applying that patch and running that test should
cause the expected assert.

I would really appreciate feedback with regard to how best to fix this
problem.
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