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:
- When using this script to re/start the server with
cros_sdk -- clangd
after the sudo password timeout, you must prime sudo to be passwordless with asudo
command. - The
cd
destination will vary depending on the location of the CrOS SDK checkout. - Anecdotally
clangd
may be more stable ifcros_sdk
is invoked with--no-ns-pid
, e.g.cros_sdk --no-ns-pid -- clangd ...
.
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.