Add support for LUCI_CONTEXT.

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 144801481e209f12b0b66e6b6e263586e7ebb489..bde79b189e12fa90aaf5197648a3a5f3e189b0df 100644
--- a/recipe_engine/recipe_api.py
+++ b/recipe_engine/recipe_api.py
@@ -175,7 +175,7 @@ class StepClient(object):
 
 
   class StepConfig(collections.namedtuple('_StepConfig', (
-      'name', 'base_name', 'cmd', 'cwd', 'env', 'env_prefixes',
+      'name', 'base_name', 'cmd', 'cwd', 'env', 'env_prefixes', 'luci_context',
       'allow_subannotations', 'trigger_specs', 'timeout', 'infra_step',
       'stdout', 'stderr', 'stdin', 'ok_ret', 'step_test_data', 'nest_level'))):
 
@@ -196,6 +196,7 @@ class StepClient(object):
       env (dict): overrides for environment variables, described above.
       env_prefixes (dict): environment prefix variables, mapping environment
         variable names to EnvPrefix values.
+      luci_context (dict): overrides for the LUCI_CONTEXT dict.
       allow_subannotations (bool): if True, lets the step emit its own
           annotations. NOTE: Enabling this can cause some buggy behavior. Please
           strongly consider using step_result.presentation instead. If you have
@@ -266,6 +267,7 @@ class StepClient(object):
 
     _RENDER_BLACKLIST=frozenset((
       'base_name',
+      'luci_context',  # it's ambient and used only for advanced stuff

[iannucci, 2017/08/04 18:55:50]

This blacklist is used for excluding the entry from the expectations; I don't
think we want that.

Additionally, you may want to tweak
https://github.com/luci/recipes-py/blob/master/recipe_engine/step_runner.py#L290
to prevent it from dumping this to stdout (e.g. see the little preamble thingy
here:
https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.linux...
).

Can you also add a test (e.g. to
https://github.com/luci/recipes-py/blob/master/recipe_engine/unittests/run_te...)
which runs some luci_context-using recipe for real and asserts that it doesn't
show up in stdout? E.g. put some value like "super_secret_token" in it and
assert that it's not in stdout. Regressing this would be bad.

       'nest_level',
       'ok_ret',
       'step_test_data',
@@ -282,6 +284,7 @@ class StepClient(object):
           cwd=(str(sc.cwd) if sc.cwd else (None)),
           env=sc.env or {},
           env_prefixes=sc.env_prefixes or cls.EnvPrefix.empty(),
+          luci_context=sc.luci_context or {},
           base_name=sc.base_name or sc.name,
           allow_subannotations=bool(sc.allow_subannotations),
           trigger_specs=sc.trigger_specs or (),