llvm dev - Nov 2018 - "devirtualizing" files in the VFS

I'd like to get some more perspectives on the role of the VirtualFileSystem
abstraction in llvm/Support.
(The VFS layer has recently moved from Clang to LLVM, so crossposting to
both lists)

https://reviews.llvm.org/D54277 proposed adding a function to
VirtualFileSystem to get the underlying "real file" path from a VFS
path.
LLDB is starting to use VFS for some filesystem interactions, but
wants/needs to keep using native IO (FILE*, file descriptors) for others.
There's some more context/discussion in the review.

My perspective is coloured by work on clang tooling, clangd etc. There we
rely on VFS to ensure code (typically clang library code) works in a
variety of environments, e.g:
in an IDE the edited file is consistently used rather than the one on disk
clang-tidy checks work on a local codebase, but our code review tool also
runs them as a service
This works because all IO goes through the VFS, so VFSes are substitutable.
We tend to rely on the static type system to ensure this (most people write
lit tests that use the real FS).

Adding facilities to use native IO together with VFS works against this,
e.g. a likely interface is
  // Returns the OS-native path to the specified virtual file.
  // Returns None if Path doesn't describe a native file, or its path is
unknown.
  Optional<string> FileSystem::getNativePath(string Path)
Most potential uses of such a function are going to produce code that
doesn't work well with arbitrary VFSes.
Anecdotally, filesystems are confusing, and most features exposed by VFS
end up getting misused if possible.

So those are my reasons for pushing back on this change, but I'm not sure
how strong they are.
I think broadly the alternatives for LLDB are:
make a change like this to the VFS APIs
migrate to actually doing IO using VFS (likely a lot of work)
know which concrete VFSes they construct, and track the needed info
externally
stop using VFS, and build separate abstractions for tracking remapping of
native files etc
abandon the new features that depend on this file remapping

As a purist, 2 and 4 seem like the cleanest options, but that's easy to say
when it's someone else's work.
What path should we take here?

Cheers, Sam
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I am really not sure if adding real file system functionality strictly into
the VFS is a good approach. This "ExternalFileSystem" thing sounds
weird to
me.

Does LLDB need to *write* the files through the VFS? I'm not sure perhaps a
"WritableVFS" could be implemented, or the necessary
casting/conversion
options.

In case:
 - there is a real path behind the file --- you could spawn an
llvm::RealFileSystem (the fqdn might not be this after the migration
patch!) and use that to obtain the file's buffer.

How can you be sure the file actually exists on the FS? That's what the VFS
should be all about, hiding this abstraction... if you *are* sure it
exists, or want to make sure, you need to pull the appropriate realFS from
the VFS Overlay (most tools have an overlay of a memoryFS above the realFS).

What I am not sure about is extending the general interface in a way that
it caters to a particular (or half of a particular) use case.

For example, in CodeCompass, we used a custom VFS implementation that
"hijacked" the overlay and included itself between the realFS and the
memoryFS. It obtains files from the database!

See:
https://github.com/Ericsson/CodeCompass/blob/a1a7b10e3a9e2e4f493135ea68566cee54adc081/plugins/cpp_reparse/service/src/databasefilesystem.cpp#L191-L224

These files *do not necessarily* (in 99% of the cases, not at all) exist on
the hard drive at the moment of the code wanting to pull the file, hence
why we implemented this to give the file source buffer from DB. The
ClangTool that needs this still gets the memoryFS for its own purposes, and
for the clang libraries, the realFS is still under there.

Perhaps the "Status" type could be extended to carry extra
information?
https://github.com/Ericsson/CodeCompass/blob/a1a7b10e3a9e2e4f493135ea68566cee54adc081/plugins/cpp_reparse/service/src/databasefilesystem.cpp#L85-L87

Sam McCall via cfe-dev <cfe-dev at lists.llvm.org> ezt írta (időpont:
2018.
nov. 15., Cs, 12:02):
> I'd like to get some more perspectives on the role of the
> VirtualFileSystem abstraction in llvm/Support.
> (The VFS layer has recently moved from Clang to LLVM, so crossposting to
> both lists)
>
> https://reviews.llvm.org/D54277 proposed adding a function to
> VirtualFileSystem to get the underlying "real file" path from a
VFS path.
> LLDB is starting to use VFS for some filesystem interactions, but
> wants/needs to keep using native IO (FILE*, file descriptors) for others.
> There's some more context/discussion in the review.
>
> My perspective is coloured by work on clang tooling, clangd etc. There we
> rely on VFS to ensure code (typically clang library code) works in a
> variety of environments, e.g:
> in an IDE the edited file is consistently used rather than the one on disk
> clang-tidy checks work on a local codebase, but our code review tool also
> runs them as a service
> This works because all IO goes through the VFS, so VFSes are
> substitutable. We tend to rely on the static type system to ensure this
> (most people write lit tests that use the real FS).
>
> Adding facilities to use native IO together with VFS works against this,
> e.g. a likely interface is
>   // Returns the OS-native path to the specified virtual file.
>   // Returns None if Path doesn't describe a native file, or its path
is
> unknown.
>   Optional<string> FileSystem::getNativePath(string Path)
> Most potential uses of such a function are going to produce code that
> doesn't work well with arbitrary VFSes.
> Anecdotally, filesystems are confusing, and most features exposed by VFS
> end up getting misused if possible.
>
> So those are my reasons for pushing back on this change, but I'm not
sure
> how strong they are.
> I think broadly the alternatives for LLDB are:
> make a change like this to the VFS APIs
> migrate to actually doing IO using VFS (likely a lot of work)
> know which concrete VFSes they construct, and track the needed info
> externally
> stop using VFS, and build separate abstractions for tracking remapping of
> native files etc
> abandon the new features that depend on this file remapping
>
> As a purist, 2 and 4 seem like the cleanest options, but that's easy to
> say when it's someone else's work.
> What path should we take here?
>
> Cheers, Sam
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
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HI Sam,

Thanks again for taking the time to discuss this. 
> On Nov 15, 2018, at 3:02 AM, Sam McCall <sammccall at google.com>
wrote:
> 
> I'd like to get some more perspectives on the role of the
VirtualFileSystem abstraction in llvm/Support.
> (The VFS layer has recently moved from Clang to LLVM, so crossposting to
both lists)
> 
> https://reviews.llvm.org/D54277 <https://reviews.llvm.org/D54277>
proposed adding a function to VirtualFileSystem to get the underlying "real
file" path from a VFS path. LLDB is starting to use VFS for some filesystem
interactions, but wants/needs to keep using native IO (FILE*, file descriptors)
for others. There's some more context/discussion in the review.
> 
> My perspective is coloured by work on clang tooling, clangd etc. There we
rely on VFS to ensure code (typically clang library code) works in a variety of
environments, e.g:
> in an IDE the edited file is consistently used rather than the one on disk
> clang-tidy checks work on a local codebase, but our code review tool also
runs them as a service
> This works because all IO goes through the VFS, so VFSes are substitutable.
We tend to rely on the static type system to ensure this (most people write lit
tests that use the real FS).
I want to emphasize that I don't have any intention of breaking any of those
or other existing use cases. I opted for the virtual file system because it
provides 95% of the functionality that's needed for reproducers: the real
filesystem and the redirecting file system. It has the yaml mapping writer and
reader, the abstraction level above the two, etc. It feels silly to implement
everything again in LLDB (actually it would be more like copy/pasting
everything) just because we miss that 5%, so I'm really motivated to find a
solution that works for all of us :-)
> Adding facilities to use native IO together with VFS works against this,
e.g. a likely interface is
>   // Returns the OS-native path to the specified virtual file.
>   // Returns None if Path doesn't describe a native file, or its path
is unknown.
>   Optional<string> FileSystem::getNativePath(string Path)
> Most potential uses of such a function are going to produce code that
doesn't work well with arbitrary VFSes.
> Anecdotally, filesystems are confusing, and most features exposed by VFS
end up getting misused if possible.
You're right and this is a problem/limitation for LLDB as well. This was the
motivation for the `ExternalFileSystem` (please forgive me for the terrible
name, just wanted to get the code up in phab) because it had "some"
semantic meaning for both implementations. But I also understand your concerns
there.
> So those are my reasons for pushing back on this change, but I'm not
sure how strong they are.
> I think broadly the alternatives for LLDB are:
> make a change like this to the VFS APIs
> migrate to actually doing IO using VFS (likely a lot of work)
> know which concrete VFSes they construct, and track the needed info
externally
> stop using VFS, and build separate abstractions for tracking remapping of
native files etc
> abandon the new features that depend on this file remapping
Can you elaborate on what you have in mind for (3) and how it differs from (4)?
> As a purist, 2 and 4 seem like the cleanest options, but that's easy to
say when it's someone else's work.
> What path should we take here?
I'll withhold from answering this as I'm one of the stakeholders ;-) 
> 
> Cheers, Sam
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> On Nov 15, 2018, at 3:34 AM, Whisperity <whisperity at gmail.com>
wrote:
> 
> I am really not sure if adding real file system functionality strictly into
the VFS is a good approach. This "ExternalFileSystem" thing sounds
weird to me.
The `ExternalFileSystem` was an attempt to provide a more limited interface
while exposing the "external" path in a way that made sense for the
RedirectingFileSystem. Like Sam said in the review it's not great because it
only does half of the work.
> Does LLDB need to *write* the files through the VFS? I'm not sure
perhaps a "WritableVFS" could be implemented, or the necessary
casting/conversion options.
Most likely yes because of LLDB's design that abstracts over flies without
prior knowledge about whether they'd only get read or written. However
wouldn't it suffer form the exact same problems?
> In case:
>  - there is a real path behind the file --- you could spawn an
llvm::RealFileSystem (the fqdn might not be this after the migration patch!) and
use that to obtain the file's buffer.
I'm not sure I follow what you have in mind here. Can you give a little more
detail?
> How can you be sure the file actually exists on the FS? That's what the
VFS should be all about, hiding this abstraction... if you *are* sure it exists,
or want to make sure, you need to pull the appropriate realFS from the VFS
Overlay (most tools have an overlay of a memoryFS above the realFS).
That makes sense, for LLDB's use case we would be happy having just a real
or redirecting filesystem (with fall through).
> What I am not sure about is extending the general interface in a way that
it caters to a particular (or half of a particular) use case.
I totally understand this sentiment but I don't think that's totally
fair. Finding files in different locations is an important feature of the VFS,
when it was introduced in 2014 this was the only use case. The
"devirtualization" aspect is unfortunate because native IO.
> For example, in CodeCompass, we used a custom VFS implementation that
"hijacked" the overlay and included itself between the realFS and the
memoryFS. It obtains files from the database!
> 
> See:
>
https://github.com/Ericsson/CodeCompass/blob/a1a7b10e3a9e2e4f493135ea68566cee54adc081/plugins/cpp_reparse/service/src/databasefilesystem.cpp#L191-L224
<https://github.com/Ericsson/CodeCompass/blob/a1a7b10e3a9e2e4f493135ea68566cee54adc081/plugins/cpp_reparse/service/src/databasefilesystem.cpp#L191-L224>
> 
> These files *do not necessarily* (in 99% of the cases, not at all) exist on
the hard drive at the moment of the code wanting to pull the file, hence why we
implemented this to give the file source buffer from DB. The ClangTool that
needs this still gets the memoryFS for its own purposes, and for the clang
libraries, the realFS is still under there.
> 
> Perhaps the "Status" type could be extended to carry extra
information?
https://github.com/Ericsson/CodeCompass/blob/a1a7b10e3a9e2e4f493135ea68566cee54adc081/plugins/cpp_reparse/service/src/databasefilesystem.cpp#L85-L87
<https://github.com/Ericsson/CodeCompass/blob/a1a7b10e3a9e2e4f493135ea68566cee54adc081/plugins/cpp_reparse/service/src/databasefilesystem.cpp#L85-L87>
This sounds like an interesting idea. We already have the option to expose the
external name here, would it be reasonable to also expose the external path
here? (of course being an optional)
> 
> Sam McCall via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at
lists.llvm.org>> ezt írta (időpont: 2018. nov. 15., Cs, 12:02):
> I'd like to get some more perspectives on the role of the
VirtualFileSystem abstraction in llvm/Support.
> (The VFS layer has recently moved from Clang to LLVM, so crossposting to
both lists)
> 
> https://reviews.llvm.org/D54277 <https://reviews.llvm.org/D54277>
proposed adding a function to VirtualFileSystem to get the underlying "real
file" path from a VFS path. LLDB is starting to use VFS for some filesystem
interactions, but wants/needs to keep using native IO (FILE*, file descriptors)
for others. There's some more context/discussion in the review.
> 
> My perspective is coloured by work on clang tooling, clangd etc. There we
rely on VFS to ensure code (typically clang library code) works in a variety of
environments, e.g:
> in an IDE the edited file is consistently used rather than the one on disk
> clang-tidy checks work on a local codebase, but our code review tool also
runs them as a service
> This works because all IO goes through the VFS, so VFSes are substitutable.
We tend to rely on the static type system to ensure this (most people write lit
tests that use the real FS).
> 
> Adding facilities to use native IO together with VFS works against this,
e.g. a likely interface is
>   // Returns the OS-native path to the specified virtual file.
>   // Returns None if Path doesn't describe a native file, or its path
is unknown.
>   Optional<string> FileSystem::getNativePath(string Path)
> Most potential uses of such a function are going to produce code that
doesn't work well with arbitrary VFSes.
> Anecdotally, filesystems are confusing, and most features exposed by VFS
end up getting misused if possible.
> 
> So those are my reasons for pushing back on this change, but I'm not
sure how strong they are.
> I think broadly the alternatives for LLDB are:
> make a change like this to the VFS APIs
> migrate to actually doing IO using VFS (likely a lot of work)
> know which concrete VFSes they construct, and track the needed info
externally
> stop using VFS, and build separate abstractions for tracking remapping of
native files etc
> abandon the new features that depend on this file remapping
> 
> As a purist, 2 and 4 seem like the cleanest options, but that's easy to
say when it's someone else's work.
> What path should we take here?
> 
> Cheers, Sam
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
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