Handle an arc-autofix-script in lintconfig.yaml.

Summary:
This is a continuing part of our effort to move logic from khan-linter
into the repos.  In this case the logic is the data we append to some
lint lines to tell `arc lint` how to fix the lint error.  If the
repo's lintconfig.yaml says it has such a script, we use it for
autofixing.

This will allow us to move linters with autofixing, such as our eslint
linter, into other repos, which was not possible before.

Test Plan:
First, I made sure webapp's lintconfig.yaml did //not// have an
`arc-autofix-script`, to test legacy behavior.  I added this "bad"
line to the end of webapp's pubsub.yaml:
    - t: "4"%
I then ran
   ka-lint pubsub.yaml
and it said:
    Error  (S&RX) E=yaml=
    bad indentation of a sequence entry at line 345, column 9:
so it properly ran when no autofix-script was specified.

I then added this line to lintconfig.yaml:
   arc-autofix-script: ["sed", "s/q/QQQ/"]
and re-ran `ka-lint pubsub.yaml`.  This time it emitted
      Error  (S&RX) E=yaml=
      bad indentation of a seQQQuence entry at line 345, column 9:
showing that the pipe works!

Reviewers: benkraft

Reviewed By: benkraft

Subscribers: kevinb

Differential Revision: https://phabricator.khanacademy.org/D63014

linters.py

@@ -122,11 +122,12 @@ def report(self, lint):
 
 class DelegatingLinter(Linter):
     """A linter that just calls out to a binary to do its actual work."""
-    def __init__(self, argv, cwd, logger):
+    def __init__(self, argv, cwd, arc_autofix_script, logger):
         """argv is the command to run (as a lint).  cwd is where to run it."""
         super(DelegatingLinter, self).__init__(logger)
         self._argv = argv
         self._cwd = cwd
+        self._arc_autofix_script = arc_autofix_script
 
     def process_files(self, files):
         # We need to refer to all filenames relative to self._cwd.
@@ -142,6 +143,23 @@ def process_files(self, files):
 
         if stderr:
             raise RuntimeError("Unexpected stderr from linter:\n%s" % stderr)
+
+        if stdout.strip() and self._arc_autofix_script:
+            # Run through the autofix-script filter to modify each
+            # lint-line to include arc fixing information.
+            autofix_pipe = subprocess.Popen(
+                self._arc_autofix_script,
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE,
+                cwd=self._cwd)
+            autofix_stdout, autofix_stderr = autofix_pipe.communicate(
+                input=stdout)
+            if autofix_stderr:
+                raise RuntimeError("Unexpected stderr from autofixer:\n%s"
+                                   % autofix_stderr)
+            stdout = autofix_stdout
+
         lines = stdout.rstrip('\n').splitlines()
         for line in lines:
             self.report(line)

runlint.py

@@ -635,12 +635,12 @@ def _find_repo_config(file_to_lint):
 _DELEGATING_LINTER_CACHE = {}
 
 
-def _get_delegating_linter(cmd, repo_config_dir):
+def _get_delegating_linter(cmd, repo_config_dir, arc_autofix_script):
     """Return a (hopefully cached) linter running this cmd from this dir."""
     key = (tuple(cmd), repo_config_dir)
     if key not in _DELEGATING_LINTER_CACHE:
         _DELEGATING_LINTER_CACHE[key] = linters.DelegatingLinter(
-            cmd, repo_config_dir, _get_logger())
+            cmd, repo_config_dir, arc_autofix_script, _get_logger())
     return _DELEGATING_LINTER_CACHE[key]
 
 
@@ -704,9 +704,14 @@ def _get_linters_for_file(file_to_lint, lang, propose_arc_fixes):
     # language.  See if our repository does so.
     (repo_config_dir, repo_config) = _find_repo_config(file_to_lint)
     if repo_config:
+        if propose_arc_fixes:
+            arc_autofix_script = repo_config.get('arc-autofix-script')
+        else:
+            arc_autofix_script = None
         cmds = repo_config.get('khan-linter-overrides', {}).get(file_lang)
         if cmds is not None:
-            return [_get_delegating_linter(cmd, repo_config_dir)
+            return [_get_delegating_linter(cmd, repo_config_dir,
+                                           arc_autofix_script)
                     for cmd in cmds]
 
     # We support multiple eslint configuration files as well as eslint