Bug 1436792 - [py-compat] Improve error messaging when python is missing, r=jmaher

This patch makes a few changes around error handling:

1) Prints the name of the linter that produced non-json output
2) Changes the 'python not found' error to a warning (as this is not fatal)
3) Makes sure said warning only gets printed once (by moving it to the setup function)

MozReview-Commit-ID: Dkq7CulTs91

--HG--
extra : rebase_source : 5d4bd32a62264a88520c09420f5acd90edcdc740

tools/lint/py2.yml

@@ -61,3 +61,4 @@ py2:
     extensions: ['py']
     type: external
     payload: python.compat:lintpy2
+    setup: python.compat:setuppy2

tools/lint/py3.yml

@@ -47,3 +47,4 @@ py3:
     extensions: ['py']
     type: external
     payload: python.compat:lintpy3
+    setup: python.compat:setuppy3

tools/lint/python/compat.py

@@ -30,18 +30,28 @@ class PyCompatProcess(ProcessHandlerMixin):
         try:
             res = json.loads(line)
         except ValueError:
-            print('Non JSON output from linter, will not be processed: {}'.format(line))
+            print('Non JSON output from {} linter: {}'.format(self.config['name'], line))
             return
 
         res['level'] = 'error'
         results.append(result.from_config(self.config, **res))
 
 
+def setup(python):
+    """Setup doesn't currently do any bootstrapping. For now, this function
+    is only used to print the warning message.
+    """
+    binary = find_executable(python)
+    if not binary:
+        # TODO Bootstrap python2/python3 if not available
+        print('warning: {} not detected, skipping py-compat check'.format(python))
+
+
 def run_linter(python, paths, config, **lintargs):
     binary = find_executable(python)
     if not binary:
-        # TODO bootstrap python3 if not available
-        print('error: {} not detected, aborting py-compat check'.format(python))
+        # If we're in automation, this is fatal. Otherwise, the warning in the
+        # setup method was already printed.
         if 'MOZ_AUTOMATION' in os.environ:
             return 1
         return []
@@ -77,9 +87,17 @@ def run_linter(python, paths, config, **lintargs):
     return results
 
 
+def setuppy2(root):
+    return setup('python2')
+
+
 def lintpy2(*args, **kwargs):
     return run_linter('python2', *args, **kwargs)
 
 
+def setuppy3(root):
+    return setup('python3')
+
+
 def lintpy3(*args, **kwargs):
     return run_linter('python3', *args, **kwargs)