Adding a New Linter to the Tree¶
Linter Requirements¶
For a linter to be integrated into the mozilla-central tree, it needs to have:
Any required dependencies should be installed as part of
./mach bootstrap
A
./mach lint
interfaceRunning
./mach lint
command must pass (note, linters can be disabled for individual directories)Taskcluster/Treeherder integration
In tree documentation (under
docs/code-quality/lint
) to give a basic summary, links and any other useful informationUnit tests (under
tools/lint/test
) to make sure that the linter works as expected and we don’t regress.
The review group in Phabricator is #linter-reviewers
.
Linter Basics¶
A linter is a yaml file with a .yml
extension. Depending on how the type of linter, there may
be python code alongside the definition, pointed to by the ‘payload’ attribute.
Here’s a trivial example:
no-eval.yml
EvalLinter:
description: Ensures the string eval doesn't show up.
extensions: ['js']
type: string
payload: eval
Now no-eval.yml
gets passed into LintRoller.read()
.
Linter Types¶
There are four types of linters, though more may be added in the future.
string - fails if substring is found
regex - fails if regex matches
external - fails if a python function returns a non-empty result list
structured_log - fails if a mozlog logger emits any lint_error or lint_warning log messages
As seen from the example above, string and regex linters are very easy to create, but they should be avoided if possible. It is much better to use a context aware linter for the language you are trying to lint. For example, use eslint to lint JavaScript files, use ruff to lint Python files, etc.
Which brings us to the third and most interesting type of linter,
external. External linters call an arbitrary python function which is
responsible for not only running the linter, but ensuring the results
are structured properly. For example, an external type could shell out
to a 3rd party linter, collect the output and format it into a list of
Issue
objects. The signature for this python
function is lint(files, config, **kwargs)
, where files
is a list of
files to lint and config
is the linter definition defined in the .yml
file.
Structured log linters are much like external linters, but suitable
for cases where the linter code is using mozlog and emits
lint_error
or lint_warning
logging messages when the lint
fails. This is recommended for writing novel gecko-specific lints. In
this case the signature for lint functions is lint(files, config, logger,
**kwargs)
.
Linter Definition¶
Each .yml
file must have at least one linter defined in it. Here are the supported keys:
description - A brief description of the linter’s purpose (required)
type - One of ‘string’, ‘regex’ or ‘external’ (required)
payload - The actual linting logic, depends on the type (required)
include - A list of file paths that will be considered (optional)
exclude - A list of file paths or glob patterns that must not be matched (optional)
extensions - A list of file extensions to be considered (optional)
exclude_extensions - A list of file extensions to be excluded (optional)
setup - A function that sets up external dependencies (optional)
support-files - A list of glob patterns matching configuration files (optional)
find-dotfiles - If set to
true
, run on dot files (.*) (optional)ignore-case - If set to
true
andtype
is regex, ignore the case (optional)
Note that a linter may not have both extensions
and exclude_extensions
specified at the
same time.
In addition to the above, some .yml
files correspond to a single lint rule. For these, the
following additional keys may be specified:
message - A string to print on infraction (optional)
hint - A string with a clue on how to fix the infraction (optional)
rule - An id string for the lint rule (optional)
level - The severity of the infraction, either ‘error’ or ‘warning’ (optional)
For structured_log lints the following additional keys apply:
logger - A StructuredLog object to use for logging. If not supplied one will be created (optional)
Example¶
Here is an example of an external linter that shells out to the Python ruff linter,
let’s call the file ruff_lint.py
(in-tree version):
import json
import os
import subprocess
from collections import defaultdict
from shutil import which
from mozlint import result
RUFF_NOT_FOUND = """
Could not find ruff! Install ruff and try again.
""".strip()
def lint(paths, config, **lintargs):
binary = which('ruff')
if not binary:
print(RUFF_NOT_FOUND)
return 1
cmd = ["ruff", "check", "--force-exclude", "--format=json"] + paths
output = subprocess.run(cmd, stdout=subprocess.PIPE, env=os.environ).output
# all passed
if not output:
return []
try:
issues = json.loads(output)
except json.JSONDecodeError:
log.error(f"Could not parse output: {output}")
results = []
for issue in issues:
# convert ruff's format to mozlint's format
res = {
"path": issue["filename"],
"lineno": issue["location"]["row"],
"column": issue["location"]["column"],
"lineoffset": issue["end_location"]["row"] - issue["location"]["row"],
"message": issue["message"],
"rule": issue["code"],
"level": "error",
}
if issue["fix"]:
res["hint"] = issue["fix"]["message"]
results.append(result.from_config(config, **res))
return {"results": results, "fixed": fixed}
Now here is the linter definition that would call it:
ruff:
description: Python Linter
include: ["."]
extensions: ["py"]
support-files:
- "**/.ruff.toml"
- "**/ruff.toml"
- "**/pyproject.toml"
type: external
payload: py.ruff:lint
Notice the payload has two parts, delimited by ‘:’. The first is the module
path, which mozlint
will attempt to import. The second is the object path
within that module (e.g, the name of a function to call). It is up to consumers
of mozlint
to ensure the module is in sys.path
. Structured log linters
use the same import mechanism.
The support-files
key is used to list configuration files or files related
to the running of the linter itself. If using --outgoing
or --workdir
and one of these files was modified, the entire tree will be linted instead of
just the modified files.
Result definition¶
When generating the list of results, the following values are available.
Name |
Description |
Optional |
---|---|---|
linter |
Name of the linter that flagged this error |
|
path |
Path to the file containing the error |
|
message |
Text describing the error |
|
lineno |
Line number that contains the error |
|
column |
Column containing the error |
|
level |
Severity of the error, either ‘warning’ or ‘error’ (default ‘error’) |
Yes |
hint |
Suggestion for fixing the error |
Yes |
source |
Source code context of the error |
Yes |
rule |
Name of the rule that was violated |
Yes |
lineoffset |
Denotes an error spans multiple lines, of the form (<lineno offset>, <num lines>) |
Yes |
diff |
A diff describing the changes that need to be made to the code |
Yes |
Automated testing¶
Every new checker must have associated tests. If your linter is mylinter
then the
test file should be named tools/lint/test/test_mylinter.py
and any example files
named like tools/lint/test/files/mylinter/my-example-file
. Be sure that your test
has been added as a section ["test_mylinter.py"]
in the manifest tools/lint/test/python.toml
.
They should be pretty easy to write as most of the work is managed by the Mozlint
framework. The key declaration is the LINTER
variable which must match
the linker declaration.
As an example, the ruff test looks like the following snippet:
import mozunit
LINTER = 'ruff'
def test_lint_ruff(lint, paths):
results = lint(paths('bad.py'))
assert len(results) == 2
assert results[0].rule == 'F401'
assert results[1].rule == 'E501'
assert results[1].lineno == 5
if __name__ == '__main__':
mozunit.main()
As always with tests, please make sure that enough positive and negative cases are covered.
To run the tests:
$ ./mach python-test --subsuite mozlint
To run a specific test:
./mach python-test --subsuite mozlint tools/lint/test/test_black.py
More tests can be found in-tree.
Tracking fixed issues¶
All the linters that provide fix support
returns a dictionary instead of a list.
{"results":result,"fixed":fixed}
results - All the linting errors it was not able to fix
fixed - Count of fixed errors (for
fix=False
this is 0)
Some linters (example: codespell) might require two passes to count the number of fixed issues. Others might just need some tuning.
For adding tests to check your fixed count, add a global variable fixed = 0
and write a function to add your test as mentioned under Automated testing
section.
Here’s an example
fixed = 0
def test_lint_codespell_fix(lint, create_temp_file):
# Typo has been fixed in the contents to avoid triggering warning
# 'informations' ----> 'information'
contents = """This is a file with some typos and information.
But also testing false positive like optin (because this isn't always option)
or stuff related to our coding style like:
aparent (aParent).
but detects mistakes like mozilla
""".lstrip()
path = create_temp_file(contents, "ignore.rst")
lint([path], fix=True)
assert fixed == 2
Bootstrapping Dependencies¶
Many linters, especially 3rd party ones, will require a set of dependencies. It could be as simple as installing a binary from a package manager, or as complicated as pulling a whole graph of tools, plugins and their dependencies.
Either way, to reduce the burden on users, linters should strive to provide
automated bootstrapping of all their dependencies. To help with this,
mozlint
allows linters to define a setup
config, which has the same
path object format as an external payload. For example (in-tree version):
ruff:
description: Python linter
include: ['.']
extensions: ['py']
type: external
payload: py.ruff:lint
setup: py.ruff:setup
The setup function takes a single argument, the root of the repository being
linted. In the case of ruff
, it might look like:
import subprocess
from shutil import which
def setup(root, **lintargs):
# This is a simple example. Please look at the actual source for better examples.
if not which("ruff"):
subprocess.call(["pip", "install", "ruff"])
The setup function will be called implicitly before running the linter. This means it should return fast and not produce any output if there is no setup to be performed.
The setup functions can also be called explicitly by running mach lint
--setup
. This will only perform setup and not perform any linting. It is
mainly useful for other tools like mach bootstrap
to call into.
Adding the linter to the CI¶
First, the job will have to be declared in Taskcluster.
This should be done in the mozlint Taskcluster configuration. You will need to define a symbol, how it is executed and on what kind of change.
For example, for ruff, the configuration is the following:
py-ruff:
description: run ruff over the gecko codebase
treeherder:
symbol: py(ruff)
run:
mach: lint -l ruff -f treeherder -f json:/builds/worker/mozlint.json .
when:
files-changed:
- '**/*.py'
- '**/.ruff.toml'
If the linter requires an external program, you will have to install it in the setup script and maybe install the necessary files in the Docker configuration.
Note
If the defect found by the linter is minor, make sure that it is logged as
a warning by setting {“level”: “warning”} in the
Issue
. This means the defect will not cause a
backout if landed, but will still be surfaced by reviewbot at review time,
or when using -W/–warnings locally.