[clangd-dev] Clangd Path Mappings for Docker Container

William Wagner via clangd-dev clangd-dev at lists.llvm.org
Wed Jun 26 16:47:10 PDT 2019

Hello all,

Sorry for the super long delay here! I’ve created a fork of upstream clangd
that implements this path mapping behavior, and have been using it for the
past couple months. As stated previously, the primary environment I’m using
this in is clangd running inside a docker container, the editor running on
the host, and the source files sync'ed between the two (via a volume

I went with the most straightforward implementation (to me), which is
editing the JSON messages directly, both for inbound and outbound messages.
Basically, I’ve added the following:


   A new --pathMappings string argument, which is of the form

   Logic to recursively loop through all the values in the JSON RPC
   messages, doing path substitution on all values that start with “file://”

   Calling this logic at all inbound locations (i.e.
   MessagesHandler::onNotify/onCall, etc.) and all outbound locations (i.e.
   ReplyOnce::Operator(), ClangdLSPServer::publishDiagnostics, etc)

This is something that works and is pretty useful to my team (and I hope
others too). Does this sound reasonable to try and land in clangd? I’d be
happy to code it up, but would just want some confirmation before going
through the effort to do it properly.

Assuming it’s useful, I have some questions for the ideal implementation:


   Should the path mappings be done on JSON values or the Protocol.h data
   structures? If it’s the latter, I suppose we could create something like
   `template <typename T> T doFilePathMappings(const T& Orig)`, and specialize
   the implementation for the protocol objects that have URIs in them.

   Is there / should there be a centralized place to put this path mapping
   logic? Right now, it seems there's a few different places it could go, but
   polluting the codebase in many different places seems far from ideal.
   Naturally I reached for the ClangdLSPServer::MessagesHandler, but in its
   current implementation it seems to mostly operate on JSON Values, and even
   then some replies bypass the MessagesHandler (e.g.

Happy to hear any feedback!

Thanks for taking the time,


On Wed, Jan 16, 2019 at 8:55 AM Ilya Biryukov <ibiryukov at google.com> wrote:

> The idea that I was playing around with was remapping the paths between
> network mount and a Clangd running on the remote machine.
> I actually wrote an LSP wrapper that would update all the paths used along
> the way. It worked, but using it was a terrible pain. (Offtopic: network
> mounts are a terrible idea, rsyncing the code gives a so much better
> latency!)
> As ugly as this sounds, I actually think that having an option in clangd
> to remap the paths is the preferred option here. It's feels like the only
> way that has a reasonable UX.
> Just to make it clear, your setup is as follows:
> - A development environment (where you run the builds, etc) is configured
> inside a docker image.
> - The source code lives inside a folder on a host OS.
> - The source code folder is mounted into the docker container.
> - The editors run in the host OS, the language server runs inside the
> docker container.
> On Thu, Jan 10, 2019 at 6:01 PM Sam McCall <sammccall at google.com> wrote:
>> Hi William,
>> This is an idea Ilya did some thinking about before. The motivation was
>> slightly different (editor on laptop, clangd on remote workstation) but I
>> believe equivalent.
>> We're not going to get around needing to have all sources on both
>> machines (or mounted from one to the other). Clangd and the editor both
>> need to see them. So I think the path translation is indeed the bit to
>> attack. The scary thing about adding file mapping is doing it consistently
>> everywhere without making mistakes or adding lots of noise to the code.
>> An idea I want to throw out there: clangd performs reads via a VFS. This
>> is an existing abstraction where path translation could be added without
>> any additional complexity to clangd proper.
>> The way this would work: we give clangd a different view of the world and
>> connect it directly to the editor (on the host). This means clangd needs to
>> speak the host's language, so the VFS needs to simulate the FS layout of
>> the host.
>> This leads to the main limitation: we need a compilation database that's
>> consistent with the *host's* file layout. I suspect mechanically
>> transforming compilation databases is hard (relative paths etc), so this
>> may mean running the build system on the host (at least sufficiently to
>> generate the CDB).
>> Not sure if there's a way to modify this idea to avoid the limitation.
>> I'm interested in how else we can get clangd to expose a different view
>> of the world than its VFS sees in a consistent way, too!
>> On Thu, Jan 10, 2019 at 9:12 AM William Wagner via clangd-dev <
>> clangd-dev at lists.llvm.org> wrote:
>>> Hello,
>>> I’ve been using clangd for developing a C++ project at work. Our
>>> development setup requires that we build and run our code inside a docker
>>> container. From a language server perspective this presents a few
>>> challenges:
>>>    1. Communicating with the language server running in the container
>>>    2. Synchronizing the files between the host and the container
>>>    3. Translating file paths between the host lsp client  and docker
>>>    lsp server
>>>    4. Having dependency files that are only in the container available
>>>    to view on the host (for textDocument/definition on dependencies installed
>>>    only in the container)
>>> Fortunately, docker’s facilities make 1) and 2) pretty easy to deal
>>> with.
>>> 1) can be solved by creating a bash script that contains something
>>> similar to:
>>> `docker exec -i <build_container> /path/to/clangd ${@:1}`
>>> and pointing clangd.path to this script
>>> 2) can be solved by using a docker bind mount, which maps the directory
>>> of the project you’re working on into the container
>>> 3) can be solved by mounting the host:/path/to/project to an identical
>>> path in the container. Then there doesn't have to be any translation
>>> between the requests/responses between the file uris that the client and
>>> server send back and forth. However, this may not always be possible and
>>> still leaves the problem of external dependency paths.
>>> 4) has no easy solution yet afaik. A potentially simple one would be to
>>> copy just the dependency headers from the container over to some directory
>>> on the host, and be able to translate clangd response file uris to the
>>> appropriate path on the host. The way I envision that is an extra arg to
>>> the clangd server that supports path substitution, such as:
>>> `clangd
>>> -path-mappings=’/host/home/project/deps:/container/deps;/host/home/project/:container/project`
>>> So if I “Go To Definition” on an external symbol e.g. Foo, which is
>>> defined in /container/deps/foo.hh”, and I have the foo.hh header in
>>> /host/home/project/.deps/foo.hh, then the response from the servers
>>> textDocument/definition would substitute the /container/deps/foo.hh →
>>> /host/home/project/.deps/foo.hh and all would be well.
>>> I believe sourcegraph has implemented an extension to the LSP to handle
>>> missing files on the client/server, but I cannot find the proposed spec.
>>> Also these are some github discussions that touch on the topics above:
>>>    - Ccls supports a path Mappings config option that allows
>>>    substitution, see https://github.com/MaskRay/ccls/issues/75
>>>    - Discussion on remote langauge server proposal
>>>    https://github.com/Microsoft/language-server-protocol/issues/528
>>> I’d be happy to implement this path mapping functionality, but thought
>>> it’d be prudent to see what everyone else thinks before doing so.
>>> Thanks!
>>> _______________________________________________
>>> clangd-dev mailing list
>>> clangd-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/clangd-dev
> --
> Regards,
> Ilya Biryukov
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