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Using Clangd LSP Server in the Chroot

Motivation

IDE features like macro-expansion, auto-complete, find-references, and go-to-declaration are indisuputably useful for programmers.

Language Servers provide these features without committing to configuring a full IDE like CLion. So if you work in Emacs, Vim, Sublime, VSCode or similar but don't have all these features, this guide is for you (assuming you like work being easier).

For Googlers that work in CLion, checkout go/clion-for-chromeos.

Disclaimer

This guide has only been tested with a Zephyr RTOS Project development workflow.

Background on Language Servers

Language Servers do the work of an IDE and give that information to clients. The LSP (Language Server Protocol) attempts to abstract away much of the work done by editors to provide standard IDE features from a server. An editor only must have an LSP client.

See the official page for more information.

Setup Steps

Chroot Setup

If you want to use an editor outside the chroot, then you must be able to invoke cros_sdk and in following invocations not require a password due to a longer sudo password timeout. If you followed CROS Developer Guide then you should have this setup; specifically if you followed how to make sudo a little more permissive.

Create a Compilation Database

So that ClangD can understand how your project is built, you need to generate a special JSON called a compilation database, on a per-project-basis; typically named compile_commands.json. See Clangd JSON Compilation Database Spec for more information on compilation databases. There are several methods for generating the database, and the best method for a particular project may depend on its build system.

Bear

Bear is a tool available in package managers inside and outside the chroot (dev-util/bear in Portage, bear in Debian repositories). Note that Bear runs the build command as a side effect of generating the database. Using Bear is as simple as invoking bear -- <YOUR BUILD COMMAND>. For example: bear -- zmake testall.

compiledb

compiledb is a tool for generating databases from make builds. It does this by running a dry-run build. Despite this, compiledb is somewhat slower than Bear. It is available via pip inside and outside the chroot. Use it like this: compiledb make <target>.

gn

Users of gn can generate compile_commands.json with ninja by invoking ninja -t compdb.

Other tools

See the Sarcasm Notebook on Generating CompilationDatabases for other build systems and tools.

Updating the database

You should not have to regenerate this unless the structure of your build changes. If Clangd was working great but then started to have issues, regenerating the compilation database can be a quick fix. For minor build changes, tools should be able to quickly, incrementally update the database.

In theory, the build could have changed any time you check out a new commit in Git. To keep up with this, you can use Git hooks to update the database each time HEAD changes. With Bear (for instance), that would look something like this:

$HOME/bin/compiledb_hook.sh:

#!/bin/sh

bear make -j 72 <target> >/dev/null 2>&1 &

$HOME/bin/post-rewrite_hook.sh:

#!/bin/sh
# Only execute for rebase; the other case for post-rewrite (commit --amend) is
# covered by post-commit.
case "$1" in
    rebase) exec .git/hooks/post-merge ;;
esac

$ ls -l $PROJECT_DIRECTORY/.git/hooks/
total 60K
lrwxrwxrwx 1 $USER primarygroup   35 Oct 27 12:50 post-checkout -> /home/$USER/bin/compiledb_hook.sh
lrwxrwxrwx 1 $USER primarygroup   35 Oct 27 12:50 post-commit -> /home/$USER/bin/compiledb_hook.sh
lrwxrwxrwx 1 $USER primarygroup   35 Oct 27 12:50 post-merge -> /home/$USER/bin/compiledb_hook.sh
lrwxrwxrwx 1 $USER primarygroup   38 Oct 27 12:50 post-rewrite -> /home/$USER/bin/post-rewrite_hook.sh
...

Install an LSP Clangd Client

Your editor needs a client that speaks LSP. The LSP project page provides a comprehensive list of LSP clients.

Connecting to Chroot Clangd

The build that generates compile_commands.json runs inside the chroot, and the paths in compile_commands.json are chroot paths. To understand these paths and access the codebase, clangd must also run inside the chroot.

Launching your editor from inside chroot

If you launch your editor inside the chroot then you can just point your editor's LSP client at the chroot's clangd binary and you're done!

Launching your editor from outside chroot

For launching your editor outside of the chroot we need to write a short simple script that essentially wraps the chroot clangd.

It's going to look something like below:

#!/bin/bash

cd ${HOME}/chromiumos

# This will run until killed by the LSP client.
# Pass through any arguments passed in by the LSP client.
cros_sdk -- clangd $@

NOTES:

If you use other language servers in the chroot with an editor outside, a similar wrapper script is likely needed.

Make sure your wrapper script is executable!

Client configuration

The client must know how to invoke clangd. Here is an illustrative snippet for YouCompleteMe, a Vim plugin that acts as an LSP client:

" Set to the path of the above wrapper script.
let g:ycm_clangd_binary_path = "~/bin/cros_ec_clangd.sh"
let g:ycm_clangd_args = [
			\'--completion-style=detailed',
			\'--background-index',
			\'--clang-tidy',
			\'--header-insertion=never',
			\'--path-mappings=<HOME>/chromiumos=/mnt/host/source'
			\]

If the LSP client is running outside the chroot, the binary path must be the path of the above wrapper script, and --path-mappings is required. The left side of the = is the path to the SDK outside the chroot, as seen by the client; the developer must replace it with their actual SDK path. The path on the right side is the corresponding path inside the chroot, as seen by clangd, the build, and compile_commands.json. Each path must be absolute.

Other arguments to clangd are less critical and may vary according to taste.

See also