@title Arcanist User Guide: Script and Regex Linter
@group userguide

Explains how to use the Script and Regex linter to invoke an existing
lint engine that is not integrated with Arcanist.

The Script and Regex linter is a simple glue linter which runs some
script on each path, and then uses a regex to parse lint messages from
the script's output. (This linter uses a script and a regex to
interpret the results of some real linter, it does not itself lint
both scripts and regexes).

Configure this linter by setting these keys in your configuration:

  - `script-and-regex.script` Script command to run. This can be
    the path to a linter script, but may also include flags or use shell
    features (see below for examples).
  - `script-and-regex.regex` The regex to process output with. This
    regex uses named capturing groups (detailed below) to interpret output.

The script will be invoked from the project root, so you can specify a
relative path like `scripts/lint.sh` or an absolute path like
`/opt/lint/lint.sh`.

This linter is necessarily more limited in its capabilities than a normal
linter which can perform custom processing, but may be somewhat simpler to
configure.

== Script... ==

The script will be invoked once for each file that is to be linted, with
the file passed as the first argument. The file may begin with a "-"; ensure
your script will not interpret such files as flags (perhaps by ending your
script configuration with "--", if its argument parser supports that).

Note that when run via `arc diff`, the list of files to be linted includes
deleted files and files that were moved away by the change. The linter should
not assume the path it is given exists, and it is not an error for the
linter to be invoked with paths which are no longer there. (Every affected
path is subject to lint because some linters may raise errors in other files
when a file is removed, or raise an error about its removal.)

The script should emit lint messages to stdout, which will be parsed with
the provided regex.

For example, you might use a configuration like this:

  "script-and-regex.script": "/opt/lint/lint.sh --flag value --other-flag --"

stderr is ignored. If you have a script which writes messages to stderr,
you can redirect stderr to stdout by using a configuration like this:

  "script-and-regex.script": "sh -c '/opt/lint/lint.sh \"$0\" 2>&1'"

The return code of the script must be 0, or an exception will be raised
reporting that the linter failed. If you have a script which exits nonzero
under normal circumstances, you can force it to always exit 0 by using a
configuration like this:

  "script-and-regex.script": "sh -c '/opt/lint/lint.sh \"$0\" || true'"

Multiple instances of the script will be run in parallel if there are
multiple files to be linted, so they should not use any unique resources.
For instance, this configuration would not work properly, because several
processes may attempt to write to the file at the same time:

  COUNTEREXAMPLE
  "script-and-regex.script": "sh -c '/opt/lint/lint.sh --output /tmp/lint.out \"$0\" && cat /tmp/lint.out'"

There are necessary limits to how gracefully this linter can deal with
edge cases, because it is just a script and a regex. If you need to do
things that this linter can't handle, you can write a phutil linter and move
the logic to handle those cases into PHP. PHP is a better general-purpose
programming language than regular expressions are, if only by a small margin.

== ...and Regex ==

The regex must be a valid PHP PCRE regex, including delimiters and flags.

The regex will be matched against the entire output of the script, so it
should generally be in this form if messages are one-per-line:

  /^...$/m

The regex should capture these named patterns with `(?P<name>...)`:

  - `message` (required) Text describing the lint message. For example,
    "This is a syntax error.".
  - `name` (optional) Text summarizing the lint message. For example,
    "Syntax Error".
  - `severity` (optional) The word "error", "warning", "autofix", "advice",
    or "disabled", in any combination of upper and lower case. Instead, you
    may match groups called `error`, `warning`, `advice`, `autofix`, or
    `disabled`. These allow you to match output formats like "E123" and
    "W123" to indicate errors and warnings, even though the word "error" is
    not present in the output. If no severity capturing group is present,
    messages are raised with "error" severity. If multiple severity capturing
    groups are present, messages are raised with the highest captured
    severity. Capturing groups like `error` supersede the `severity`
    capturing group.
  - `error` (optional) Match some nonempty substring to indicate that this
    message has "error" severity.
  - `warning` (optional) Match some nonempty substring to indicate that this
    message has "warning" severity.
  - `advice` (optional) Match some nonempty substring to indicate that this
    message has "advice" severity.
  - `autofix` (optional) Match some nonempty substring to indicate that this
    message has "autofix" severity.
  - `disabled` (optional) Match some nonempty substring to indicate that this
    message has "disabled" severity.
  - `file` (optional) The name of the file to raise the lint message in. If
    not specified, defaults to the linted file. It is generally not necessary
    to capture this unless the linter can raise messages in files other than
    the one it is linting.
  - `line` (optional) The line number of the message.
  - `char` (optional) The character offset of the message.
  - `offset` (optional) The byte offset of the message. If captured, this
    supersedes `line` and `char`.
  - `original` (optional) The text the message affects.
  - `replacement` (optional) The text that the range captured by `original`
    should be automatically replaced by to resolve the message.
  - `code` (optional) A short error type identifier which can be used
    elsewhere to configure handling of specific types of messages. For
    example, "EXAMPLE1", "EXAMPLE2", etc., where each code identifies a
    class of message like "syntax error", "missing whitespace", etc. This
    allows configuration to later change the severity of all whitespace
    messages, for example.
  - `ignore` (optional) Match some nonempty substring to ignore the match.
    You can use this if your linter sometimes emits text like "No lint
    errors".
  - `stop` (optional) Match some nonempty substring to stop processing input.
    Remaining matches for this file will be discarded, but linting will
    continue with other linters and other files.
  - `halt` (optional) Match some nonempty substring to halt all linting of
    this file by any linter. Linting will continue with other files.
  - `throw` (optional) Match some nonempty substring to throw an error, which
    will stop `arc` completely. You can use this to fail abruptly if you
    encounter unexpected output. All processing will abort.

Numbered capturing groups are ignored.

For example, if your lint script's output looks like this:

  error:13 Too many goats!
  warning:22 Not enough boats.

...you could use this regex to parse it:

  /^(?P<severity>warning|error):(?P<line>\d+) (?P<message>.*)$/m

The simplest valid regex for line-oriented output is something like this:

  /^(?P<message>.*)$/m
