Start fleshing out mercurial hook

This also reports errors with hook installation using exceptions and
includes far more exceptions for mercurial since it has more failure
modes.

flake8/exceptions.py

@@ -43,10 +43,52 @@ class InvalidSyntax(Flake8Exception):
         super(InvalidSyntax, self).__init__(*args, **kwargs)
 
 
-class GitHookAlreadyExists(Flake8Exception):
+class HookInstallationError(Flake8Exception):
+    """Parent exception for all hooks errors."""
+    pass
+
+
+class GitHookAlreadyExists(HookInstallationError):
     """Exception raised when the git pre-commit hook file already exists."""
 
     def __init__(self, *args, **kwargs):
         """Initialize the path attribute."""
         self.path = kwargs.pop('path')
         super(GitHookAlreadyExists, self).__init__(*args, **kwargs)
+
+    def __str__(self):
+        """Provide a nice message regarding the exception."""
+        msg = ('The Git pre-commit hook ({0}) already exists. To convince '
+               'Flake8 to install the hook, please remove the existing '
+               'hook.')
+        return msg.format(self.path)
+
+
+class MercurialHookAlreadyExists(HookInstallationError):
+    """Exception raised when a mercurial hook is already configured."""
+
+    hook_name = None
+
+    def __init__(self, *args, **kwargs):
+        """Initialize the relevant attributes."""
+        self.path = kwargs.pop('path')
+        self.value = kwargs.pop('value')
+        super(MercurialHookAlreadyExists, self).__init__(*args, **kwargs)
+
+    def __str__(self):
+        msg = ('The Mercurial {0} hook already exists with "{1}" in {2}. '
+               'To convince Flake8 to install the hook, please remove the '
+               '{0} configuration from the [hooks] section of your hgrc.')
+        return msg.format(self.hook_name, self.value, self.path)
+
+
+class MercurialCommitHookAlreadyExists(MercurialHookAlreadyExists):
+    """Exception raised when the hg commit hook is already configured."""
+
+    hook_name = 'commit'
+
+
+class MercurialQRefreshHookAlreadyExists(MercurialHookAlreadyExists):
+    """Exception raised when the hg commit hook is already configured."""
+
+    hook_name = 'qrefresh'

flake8/main/git.py

@@ -71,7 +71,9 @@ def install():
     if not os.path.exists(hooks_directory):
         os.mkdir(hooks_directory)
 
-    pre_commit_file = os.path.join(hooks_directory, 'hooks', 'pre-commit')
+    pre_commit_file = os.path.abspath(
+        os.path.join(hooks_directory, 'pre-commit')
+    )
     if os.path.exists(pre_commit_file):
         raise exceptions.GitHookAlreadyExists(
             'File already exists',

flake8/main/mercurial.py

@@ -4,13 +4,106 @@
 .. autofunction:: install
 
 """
+import configparser
+import os
+import subprocess
+
+from flake8 import exceptions as exc
 
 __all__ = ('hook', 'install')
 
 
-def hook(lazy=False, strict=False):
-    pass
+def hook(ui, repo, **kwargs):
+    """Execute Flake8 on the repository provided by Mercurial.
+
+    To understand the parameters read more of the Mercurial documentation
+    around Hooks: https://www.mercurial-scm.org/wiki/Hook.
+
+    We avoid using the ``ui`` attribute because it can cause issues with
+    the GPL license tha Mercurial is under. We don't import it, but we
+    avoid using it all the same.
+    """
+    from flake8.main import application
+    hgrc = find_hgrc(create_if_missing=False)
+    if hgrc is None:
+        print('Cannot locate your root mercurial repository.')
+        raise SystemExit(True)
+
+    hgconfig = configparser_for(hgrc)
+    strict = hgconfig.get('flake8', 'strict', fallback=True)
+
+    app = application.Application()
+    app.run()
+
+    if strict:
+        return app.result_count
+    return 0
 
 
 def install():
-    pass
+    """Ensure that the mercurial hooks are installed."""
+    hgrc = find_hgrc(create_if_missing=True)
+    if hgrc is None:
+        print('Could not locate your root mercurial repository.')
+        raise SystemExit(True)
+
+    hgconfig = configparser_for(hgrc)
+
+    if not hgconfig.has_section('hooks'):
+        hgconfig.add_section('hooks')
+
+    if hgconfig.has_option('hooks', 'commit'):
+        raise exc.MercurialCommitHookAlreadyExists(
+            path=hgrc,
+            value=hgconfig.get('hooks', 'commit'),
+        )
+
+    if hgconfig.has_option('hooks', 'qrefresh'):
+        raise exc.MercurialQRefreshHookAlreadyExists(
+            path=hgrc,
+            value=hgconfig.get('hooks', 'qrefresh'),
+        )
+
+    hgconfig.set('hooks', 'commit', 'python:flake8.main.mercurial.hook')
+    hgconfig.set('hooks', 'qrefresh', 'python:flake8.main.mercurial.hook')
+
+    if not hgconfig.has_section('flake8'):
+        hgconfig.add_section('flake8')
+
+    if not hgconfig.has_option('flake8', 'strict'):
+        hgconfig.set('flake8', 'strict', False)
+
+    with open(hgrc, 'w') as fd:
+        hgconfig.write(fd)
+
+
+def find_hgrc(create_if_missing=False):
+    root = subprocess.Popen(
+        ['hg', 'root'],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+    )
+
+    (hg_directory, _) = root.communicate()
+    if callable(getattr(hg_directory, 'decode', None)):
+        hg_directory = hg_directory.decode('utf-8')
+
+    if not os.path.isdir(hg_directory):
+        return None
+
+    hgrc = os.path.abspath(
+        os.path.join(hg_directory, '.hg', 'hgrc')
+    )
+    if not os.path.exists(hgrc):
+        if create_if_missing:
+            open(hgrc, 'w').close()
+        else:
+            return None
+
+    return hgrc
+
+
+def configparser_for(path):
+    parser = configparser.ConfigParser(interpolation=None)
+    parser.read(path)
+    return parser

flake8/main/vcs.py

@@ -1,4 +1,5 @@
 """Module containing some of the logic for our VCS installation logic."""
+from flake8 import exceptions as exc
 from flake8.main import git
 from flake8.main import mercurial
 
@@ -20,8 +21,15 @@ def install(option, option_string, value, parser):
     https://docs.python.org/2/library/optparse.html#optparse-option-callbacks
     """
     installer = _INSTALLERS.get(value)
-    installer()
+    errored = False
+    try:
+        installer()
+    except exc.HookInstallationError as hook_error:
+        print(str(hook_error))
+        errored = True
+    raise SystemExit(errored)
 
 
 def choices():
-    return _INSTALLERS.keys()
+    """Return the list of VCS choices."""
+    return list(_INSTALLERS.keys())