[context] Add path prefix/suffix manipulation.

recipe_modules/context/examples/full.py

 # Copyright 2017 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 from recipe_engine import recipe_api, config
 
 DEPS = [
   'context',
   'path',
   'step',
 ]
 
 
 def RunSteps(api):
   api.step('default step', ['bash', '-c', 'echo default!'])
 
   noop_context = {}
   with api.context(**noop_context):
     # nothing happens! this is exactly the same as above, but this optimization
     # is helpful when recipes need to calculate contextual values.
     api.step('default step', ['bash', '-c', 'echo default!'])
 
   # can change cwd
-  api.step('mk subdir', ['mkdir', 'subdir'])
+  api.step('mk subdir', ['mkdir', '-p', 'subdir'])
   with api.context(cwd=api.path['start_dir'].join('subdir')):
     api.step('subdir step', ['bash', '-c', 'pwd'])
     api.step('other subdir step', ['bash', '-c', 'echo hi again!'])
 
-  # can set envvars
-  with api.context(env={'HELLO': 'WORLD', 'HOME': None}):
-    api.step('env step', ['bash', '-c', 'echo $HELLO; echo $HOME'])
+  # can set envvars, and path prefix.
+  with api.context(env={'FOO': 'bar'}):
+    api.step('env step', ['bash', '-c', 'echo $FOO'])
+
+    pants = api.path['start_dir'].join('pants')
+    shirt = api.path['start_dir'].join('shirt')
+    with api.context(env={'FOO': api.context.Prefix(pants, shirt)}):
+      api.step('env step with prefix',
+               ['bash', '-c', 'echo $FOO'])
 
   # %-formats are errors (for now). Double-% escape them.
   bad_examples = ['%format', '%s']
   for example in bad_examples:
     try:
       with api.context(env={'BAD': example}):
         assert False  # pragma: no cover
     except ValueError:
       pass
 
   # this is fine though:
   with api.context(env={'FINE': '%%format'}):
     pass
 
   # can increment nest level... note that this is a low level api, prefer
   # api.step.nest instead:
   # YES:
   with api.step.nest('nested'):
     api.step('properly indented', ['bash', '-c', 'echo yay!'])
   # AVOID:
   with api.context(increment_nest_level=True):
     api.step('indented with wrong name', ['bash', '-c', 'echo indent?'])
 
 
 def GenTests(api):
   yield api.test('basic')