[lldb-dev] About lldbHost

[Pavel Labath via lldb-dev]

I agree that the next module that needs figuring on is the host one.
However I am not sure if the decision on what to put in what module
should be motivated by the FileSpec class, as I think it's current
interface has a some issues, and our choice on how to resolve them can
greatly affect what things it depends on.

The main thing that bugs me with FileSpec is that it is used for a two
distinct purposes:
- manipulations on abstract path names (e.g. PrependPathComponent)
- manipulations of actual files present on the host (e.g. MemoryMapFileContents)
For the first one you don't need the files to exist on the host (in
fact they may not even use the same path syntax as the host). For the
other one, they *must* exist and *must* use the same path syntax. This
is currently not very well separated and enforced, and I think it
should be. I believe this is the reason the FileSystem pseudo-class
was created.

So, my counter-proposal would be to finish moving the host-specific
things out of the FileSpec class (basically just replace
file_spec.Foo(...) with FileSystem::Foo(file_spec, ...). At that point
FileSpec should only depend on string manipulation functions, and we
should be able to move it without much hassle. After that, we can take
another look and decide what to do next.

The thing I really like about this idea is that we will end up with
two classes that very closely mirror llvm functionality (FileSpec is a
version of Support/Path.h that does not assume host path syntax,
FileSystem is similar to Support/FileSystem.h, but it supports some
more fancy operations like mmap()). Then we could proceed to merge
this functionality with llvm pretty much independently of any other
refactoring we will be doing.

What do you think?



On 15 February 2017 at 01:48, Zachary Turner via lldb-dev
<lldb-dev at lists.llvm.org> wrote:
> After https://reviews.llvm.org/D29964, we finally have a starting point at
> which we can begin unravelling the cross-project cyclic dependencies in
> LLDB.  lldb/Utility now is very similar in spirit to llvm/Support.
>
> But llvmSupport goes one step further and includes what lldb would normally
> put under Host.  I think this makes some sense.  Practically all parts of a
> codebase have need of a single OS abstraction layer.  So, I think that a lot
> of the functionality currently in lldbHost is in fact needed by the rest of
> LLDB.
>
> So, I wonder if it makes sense to follow the path that LLVM has taken, and
> start moving some of this code from Host down to Utility.  Doing so would
> allow us to break one of the biggest links in the dependency cycle in the
> entire codebase, which is that Host depends on everything, and everything
> depends on Host.
>
> Of course, it can't just be a straight move.  Some things in Host interact
> with Target, with CommandInterpreter, and with many other things.  And stuff
> going into Utility can't take a dependency.
>
> So there will be some splitting, some moving, some refactoring, etc.  But to
> me tackling Host seems like the logical next step, in large part because
> Host is where FileSpec is, and it's going to be hard to break any
> dependencies without first addressing FileSpec.
>
> The way LLVM handles cross-platform differences in Support is that you
> include a single header and it does conditional includes into a platform
> specific subdirectory for the parts that differ.
>
> I'm thinking to follow the same pattern here in lldb/Utility, and begin
> looking for ways to get pieces of Host down into Utility this way, until
> ultimately I can get FileSpec down there.
>
> Thoughts?
>
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