Assert validity of stream parameters.

recipe_engine/stream.py

Index: recipe_engine/stream.py
diff --git a/recipe_engine/stream.py b/recipe_engine/stream.py
index 831e69b0e9bfab75ca31cbae0cfe30f152bf4f26..5420bdc8091aab12120eb4176a2b89138c4a8606 100644
--- a/recipe_engine/stream.py
+++ b/recipe_engine/stream.py
@@ -46,7 +46,14 @@ def output_iter(stream, it):
       stream.write_line(line)
 
 
+def _check(cond, msg, **kwargs):
+  """Runtime assertion used for parameter correctness."""
+  if not cond:
+    raise ValueError('Failed assertion: %s %r' % (msg, kwargs))
+
+
 class StreamEngine(object):
+
   class Stream(object):
     def write_line(self, line):
       raise NotImplementedError()
@@ -93,6 +100,8 @@ class StreamEngine(object):
 
   def make_step_stream(self, name, **kwargs):
     """Shorthand for creating a step stream from a step configuration dict."""
+    _check('\n' not in name, 'Stream name must not contain a newline.',
+           name=name)
     kwargs['name'] = name
     return self.new_step_stream(recipe_api.StepConfig.create(**kwargs))
 
@@ -314,7 +323,7 @@ class StreamEngineInvariants(StreamEngine):
       self._status = 'SUCCESS'
 
     def write_line(self, line):
-      assert '\n' not in line
+      assert '\n' not in line, 'Individual line must not contain a newline.'
       assert self._open
 
     def close(self):
@@ -328,19 +337,30 @@ class StreamEngineInvariants(StreamEngine):
       assert self._open
       assert log_name not in self._logs, 'Log %s already exists in step %s' % (
           log_name, self._step_name)
+
+      _check('\n' not in log_name, 'Log name must not contain a newline.',
+             log_name=log_name)
       ret = self._engine.LogStream(self, log_name)
       self._logs[log_name] = ret
       return ret
 
     def add_step_text(self, text):
-      pass
+      _check('\n' not in text, 'Step text must not contain a newline.',
+             text=text)
 
     def add_step_summary_text(self, text):
-      pass
+      _check('\n' not in text, 'Step summary text must not contain a newline.',
+             text=text)
 
     def add_step_link(self, name, url):
-      assert isinstance(name, basestring), 'Link name %s is not a string' % name
-      assert isinstance(url, basestring), 'Link url %s is not a string' % url
+      _check(isinstance(name, basestring), 'Link name is not a string',
+             name=name)
+      _check('\n' not in name, 'Link name must not contain a newline.',
+             name=name)
+      _check(isinstance(url, basestring),
+             'Link URL is not a string', url=url)
+      _check('\n' not in url, 'Link URL must not contain a newline.',
+             url=url)
 
     def set_step_status(self, status):
       assert status in ('SUCCESS', 'WARNING', 'FAILURE', 'EXCEPTION')
@@ -351,11 +371,16 @@ class StreamEngineInvariants(StreamEngine):
       self._status = status
 
     def set_build_property(self, key, value):
-      pass
+      _check('\n' not in key, 'Property key must not contain a newline.',
+             key=key)
+      _check('\n' not in value, 'Property value must not contain a newline.',
+             value=value)
+      json.loads(value) # value must be a valid JSON object.
 
     def trigger(self, spec):
-      assert '\n' not in spec # Spec must fit on one line.
-      json.loads(spec) # Spec must be a valid json object.
+      _check('\n' not in spec, 'Spec must not contain a newline.',
+             spec=spec)
+      json.loads(spec) # Spec must be a valid JSON object.
 
   class LogStream(StreamEngine.Stream):
     def __init__(self, step_stream, log_name):
@@ -364,7 +389,7 @@ class StreamEngineInvariants(StreamEngine):
       self._open = True
 
     def write_line(self, line):
-      assert '\n' not in line
+      assert '\n' not in line, 'Individual line must not contain a newline.'
       assert self._step_stream._open
       assert self._open