Bug 1436639 - [lint] Make sure flake8 is run with same python as |mach lint| was, r=jmaher

This fixes a bug which can happen when the default version of python differs from
the version of python used with mach.

For example, mach explicitly looks for python2.7. This means running |mach lint -l flake8|
should also run flake8 with version 2.7. But if the default is python3, and flake8 is also
installed there, the subprocess call that invokes flake8 will run under python3. This can
lead to errors like "undefined name 'basestring'" and other 2to3 gotchas.

This patch ensures that we run the flake8 binary (and the pip for installing flake8) from
the same interpreter as mach, no matter the system default.

MozReview-Commit-ID: HSuMzDsAvsW

tools/lint/python/flake8.py

@@ -4,11 +4,12 @@
 
 import json
 import os
+import platform
 import signal
 import subprocess
+import sys
 from collections import defaultdict
 
-import which
 from mozprocess import ProcessHandlerMixin
 
 from mozlint import result
@@ -55,6 +56,13 @@ The offset is of the form (lineno_offset, num_lines) and is passed
 to the lineoffset property of `ResultContainer`.
 """
 
+# We use sys.prefix to find executables as that gets modified with
+# virtualenv's activate_this.py, whereas sys.executable doesn't.
+if platform.system() == 'Windows':
+    bindir = os.path.join(sys.prefix, 'Scripts')
+else:
+    bindir = os.path.join(sys.prefix, 'bin')
+
 results = []
 
 
@@ -90,23 +98,12 @@ class Flake8Process(ProcessHandlerMixin):
         signal.signal(signal.SIGINT, orig)
 
 
-def get_flake8_binary():
-    """
-    Returns the path of the first flake8 binary available
-    if not found returns None
-    """
-    try:
-        return which.which('flake8')
-    except which.WhichError:
-        return None
-
-
 def _run_pip(*args):
     """
     Helper function that runs pip with subprocess
     """
     try:
-        subprocess.check_output(['pip'] + list(args),
+        subprocess.check_output([os.path.join(bindir, 'pip')] + list(args),
                                 stderr=subprocess.STDOUT)
         return True
     except subprocess.CalledProcessError as e:
@@ -143,9 +140,8 @@ def setup(root):
 
 
 def lint(paths, config, **lintargs):
-    binary = get_flake8_binary()
     cmdargs = [
-        binary,
+        os.path.join(bindir, 'flake8'),
         '--format', '{"path":"%(path)s","lineno":%(row)s,'
                     '"column":%(col)s,"rule":"%(code)s","message":"%(text)s"}',
     ]