[context] Split "env" values and prefixes.

recipe_engine/recipe_api.py

Index: recipe_engine/recipe_api.py
diff --git a/recipe_engine/recipe_api.py b/recipe_engine/recipe_api.py
index 6ec91c3137bcfc74aa533b73cbc9083ce8df6773..b2e231f342cd42cf78d79841ff310e23783904cf 100644
--- a/recipe_engine/recipe_api.py
+++ b/recipe_engine/recipe_api.py
@@ -257,9 +257,9 @@ class TriggerSpec(_TriggerSpec):
 
 
 _StepConfig = collections.namedtuple('_StepConfig',
-    ('name', 'base_name', 'cmd', 'cwd', 'env', 'allow_subannotations',
-     'trigger_specs', 'timeout', 'infra_step', 'stdout', 'stderr', 'stdin',
-     'ok_ret', 'step_test_data', 'nest_level'))
+    ('name', 'base_name', 'cmd', 'cwd', 'env_prefixes', 'env',
+     'allow_subannotations', 'trigger_specs', 'timeout', 'infra_step', 'stdout',
+     'stderr', 'stdin', 'ok_ret', 'step_test_data', 'nest_level'))
 
 class StepConfig(_StepConfig):
   """
@@ -277,6 +277,16 @@ class StepConfig(_StepConfig):
         "delete_this": None,
         "static_value": "something",
     }
+
+  The optional "env_prefix" parameter contains values that, if specified, will
+  transform an environment variable into a "pathsep"-delimited sequence of
+  items:
+    - If an environment variable is also specified for this key, it will be
+      appended as the last element: <prefix0>:...:<prefixN>:ENV
+    - If no enviornment variable is specified, the current environment's value
+      will be appended, unless it's empty: <prefix0>:...:<prefixN>[:ENV]?
+    - If an enviornment variable with a value of None (delete) is specified,
+      nothing will be appeneded: <prefix0>:...:<prefixN>
   """
 
   _RENDER_WHITELIST=frozenset((
@@ -291,10 +301,11 @@ class StepConfig(_StepConfig):
   ))
 
   @classmethod
-  def create(cls, name, base_name=None, cmd=None, cwd=None, env=None,
-             allow_subannotations=None, trigger_specs=None, timeout=None,
-             infra_step=None, stdout=None, stderr=None, stdin=None,
-             ok_ret=None, step_test_data=None, step_nest_level=None):
+  def create(cls, name, base_name=None, cmd=None, cwd=None, env_prefixes=None,
+             env=None, allow_subannotations=None, trigger_specs=None,
+             timeout=None, infra_step=None, stdout=None, stderr=None,
+             stdin=None, ok_ret=None, step_test_data=None,
+             step_nest_level=None):
     """
     Initializes a new StepConfig step API dictionary.
 
@@ -305,6 +316,7 @@ class StepConfig(_StepConfig):
           name component of just this step. If None, this will be set to "name".
       cmd: command to run. Acceptable types: str, Path, Placeholder, or None.
       cwd (str or None): absolute path to working directory for the command
+      env_prefixes (dict): environment prefix variables, described above.
       env (dict): overrides for environment variables, described above.
       allow_subannotations (bool): if True, lets the step emit its own
           annotations. NOTE: Enabling this can cause some buggy behavior. Please
@@ -333,7 +345,8 @@ class StepConfig(_StepConfig):
         base_name=(base_name or name),
         cmd=cmd,
         cwd=cwd,
-        env=env,
+        env_prefixes={k: tuple(v) for k, v in (env_prefixes or {}).iteritems()},
+        env=env or {},
         allow_subannotations=bool(allow_subannotations),
         trigger_specs=[TriggerSpec._create(**trig)
                        for trig in (trigger_specs or ())],
@@ -349,6 +362,7 @@ class StepConfig(_StepConfig):
 
   def render_to_dict(self):
     self = self._replace(
+        env_prefixes={k: list(v) for k, v in self.env_prefixes.iteritems()},
         trigger_specs=[trig._render_to_dict()
                        for trig in (self.trigger_specs or ())],
     )