Harden _PresubmitResult against varying string types

When presubmit checks fail, we want to print the error message from the
underlying check/tool. We use _PresubmitResult objects to collect these
diagnostics and output them to the console and json files.

This change ensures that the string values passed to the
_PresubmitResult constructor will be converted, as needed, to 'str'
instances. This way, we will avoid encoding errors and make sure to
print the actual messages instead of repr(message).

SHERIFFS: this patch breaks backwards compatibility of presubmit error
handling but we don't expect issues in downstream projects. Please
revert if downstream presubmit checks start failing spuriously.

Bug: 1225052
Change-Id: If7f5c99cb5d730c1bfbd6d108b912f77b5f2455c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3037262
Commit-Queue: Tom McKee <tommckee@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>

presubmit_support.py

@@ -316,7 +316,24 @@ class _PresubmitResult(object):
     """
     self._message = message
     self._items = items or []
-    self._long_text = long_text.rstrip()
+    self._long_text = _PresubmitResult._ensure_str(long_text.rstrip())
+
+  @staticmethod
+  def _ensure_str(val):
+    """
+    val: A "stringish" value. Can be any of str, unicode or bytes.
+    returns: A str after applying encoding/decoding as needed.
+    Assumes/uses UTF-8 for relevant inputs/outputs.
+
+    We'd prefer to use six.ensure_str but our copy of six is old :(
+    """
+    if isinstance(val, str):
+      return val
+    if six.PY2 and isinstance(val, unicode):
+      return val.encode()
+    elif six.PY3 and isinstance(val, bytes):
+      return val.decode()
+    raise ValueError("Unknown string type %s" % type(val))
 
   def handle(self):
     sys.stdout.write(self._message)
@@ -331,7 +348,7 @@ class _PresubmitResult(object):
     if self._long_text:
       sys.stdout.write('\n***************\n')
       # Write separately in case it's unicode.
-      sys.stdout.write(str(self._long_text))
+      sys.stdout.write(self._long_text)
       sys.stdout.write('\n***************\n')
 
   def json_format(self):