[context] Split "env" values and prefixes.

recipe_modules/step/api.py

Index: recipe_modules/step/api.py
diff --git a/recipe_modules/step/api.py b/recipe_modules/step/api.py
index 4a250c1b057290e7cebcc247be4c41c866297e16..0c2b9909d683eb8707f1f05a576899233c2c4c18 100644
--- a/recipe_modules/step/api.py
+++ b/recipe_modules/step/api.py
@@ -187,12 +187,24 @@ class StepApi(recipe_api.RecipeApiPlain):
     cwd = self.m.context.cwd
     if cwd is not None and cwd != self.m.path['start_dir']:
       kwargs['cwd'] = cwd
-    kwargs['env'] = self.m.context.env
-    if self._prefix_path:
-      ps = self.m.path.pathsep
-      prefix = ps.join(self._prefix_path)
-      suffix = kwargs['env'].get('PATH', '%(PATH)s')
-      kwargs['env']['PATH'] = '%s%s%s' % (prefix, ps, suffix)
+
+    # Assemble the environment.
+    with self.m.context(env={
+        'PATH': self.m.context.Prefix(*self._prefix_path)}):
+      env_values = self.m.context.env
+      env = {}
+      for k, path_tuple in self.m.context.env_prefixes.iteritems():
+        path_tuple = tuple(str(v) for v in path_tuple)
+        path_tuple +=  (env_values.get(k) or '%%(%s)s' % (k,),)
+        env[k] = self.m.path.pathsep.join(path_tuple)
+
+      for k, v in env_values.iteritems():
+        if k in env or v == '':
+          continue # Already processed, or empty.
+        env[k] = v
+
+      kwargs['env'] = env
+
     kwargs['infra_step'] = self.m.context.infra_step or bool(infra_step)
     kwargs['step_nest_level'] = self.m.context.nest_level
     kwargs['name'] = full_name