Adding a New Linter to the Tree
===============================

A linter is a python file with a ``.lint`` extension and a global dict called LINTER. Depending on how
complex it is, there may or may not be any actual python code alongside the LINTER definition.

Here's a trivial example:

no-eval.lint

.. code-block:: python

    LINTER = {
        'name': 'EvalLinter',
        'description': "Ensures the string 'eval' doesn't show up."
        'include': "**/*.js",
        'type': 'string',
        'payload': 'eval',
    }

Now ``no-eval.lint`` gets passed into :func:`LintRoller.read`.


Linter Types
------------

There are three types of linters, though more may be added in the future.

1. string - fails if substring is found
2. regex - fails if regex matches
3. external - fails if a python function returns a non-empty result list
4. 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 flake8 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
:class:`ResultContainer` objects. The signature for this python
function is ``lint(files, **kwargs)``, where ``files`` is a list of
files to lint.

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, logger,
**kwargs)``.

LINTER Definition
-----------------

Each ``.lint`` file must have a variable called LINTER which is a dict containing metadata about the
linter. Here are the supported keys:

* name - The name of the linter (required)
* 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 glob patterns that must be matched (optional)
* exclude - A list of glob patterns that must not be matched (optional)
* extensions - A list of file extensions to be considered (optional)
* setup - A function that sets up external dependencies (optional)

In addition to the above, some ``.lint`` 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 flake8 linter:

.. code-block:: python

    import json
    import os
    import subprocess
    from collections import defaultdict

    from mozlint import result


    FLAKE8_NOT_FOUND = """
    Could not find flake8! Install flake8 and try again.
    """.strip()


    def lint(files, **lintargs):
        import which

        binary = os.environ.get('FLAKE8')
        if not binary:
            try:
                binary = which.which('flake8')
            except which.WhichError:
                print(FLAKE8_NOT_FOUND)
                return 1

        # Flake8 allows passing in a custom format string. We use
        # this to help mold the default flake8 format into what
        # mozlint's ResultContainer object expects.
        cmdargs = [
            binary,
            '--format',
            '{"path":"%(path)s","lineno":%(row)s,"column":%(col)s,"rule":"%(code)s","message":"%(text)s"}',
        ] + files

        proc = subprocess.Popen(cmdargs, stdout=subprocess.PIPE, env=os.environ)
        output = proc.communicate()[0]

        # all passed
        if not output:
            return []

        results = []
        for line in output.splitlines():
            # res is a dict of the form specified by --format above
            res = json.loads(line)

            # parse level out of the id string
            if 'code' in res and res['code'].startswith('W'):
                res['level'] = 'warning'

            # result.from_linter is a convenience method that
            # creates a ResultContainer using a LINTER definition
            # to populate some defaults.
            results.append(result.from_linter(LINTER, **res))

        return results


    LINTER = {
        'name': "flake8",
        'description': "Python linter",
        'include': ['**/*.py'],
        'type': 'external',
        'payload': lint,
    }