fix a python 2.7 vs 3 compat issue with exec

Summary:
This is a bit of a tricky issue; here's the bug in python 2.7
that prevents using the function-like syntax:
https://bugs.python.org/issue21591

The nature of this bug is that scoped exec only works with nested functions
when using the `exec` statement in 2.7.   That is a syntax error in python 3,
which only allows using the `exec` function.

To make the function syntax work in 2.7 we therefore need to avoid using
lambdas or nested functions and have to resort to using a global variable
to act as the "closure".

Refs: https://github.com/facebook/watchman/pull/639

Reviewed By: snarkmaster

Differential Revision: D9552413

fbshipit-source-id: a2cb9d325e18fb6fb06b9e284a84f0a0c80cfe7e

build/fbcode_builder/README.md

@@ -55,4 +55,6 @@ internal continuous-integration platform using the same build-step DSL.
 
 Please follow the ambient style (or PEP-8), and keep the code Python 2.6
 compatible -- since `fbcode_builder`'s only dependency is Docker, we want to
-allow building projects on even fairly ancient base systems.
+allow building projects on even fairly ancient base systems.   We also wish
+to be compatible with Python 3, and would appreciate it if you kept that
+in mind while making changes also.

build/fbcode_builder/utils.py

@@ -38,21 +38,31 @@ def make_temp_dir(d):
         shutil.rmtree(d, ignore_errors=True)
 
 
+def _inner_read_config(path):
+    '''
+    Helper to read a named config file.
+    The grossness with the global is a workaround for this python bug:
+    https://bugs.python.org/issue21591
+    The bug prevents us from defining either a local function or a lambda
+    in the scope of read_fbcode_builder_config below.
+    '''
+    global _project_dir
+    full_path = os.path.join(_project_dir, path)
+    return read_fbcode_builder_config(full_path)
+
+
 def read_fbcode_builder_config(filename):
     # Allow one spec to read another
     # When doing so, treat paths as relative to the config's project directory.
-    project_dir = os.path.dirname(filename)
-
-    def inner_read_config(path):
-        full_path = os.path.join(project_dir, path)
-        return read_fbcode_builder_config(full_path)
+    # _project_dir is a "local" for _inner_read_config; see the comments
+    # in that function for an explanation of the use of global.
+    global _project_dir
+    _project_dir = os.path.dirname(filename)
 
-    scope = {'read_fbcode_builder_config': inner_read_config}
+    scope = {'read_fbcode_builder_config': _inner_read_config}
     with open(filename) as config_file:
-        # Note that this will need to be changed to an exec() function call for
-        # python 3 compatibility.  Unfortunately python 2.7 does not seem to
-        # treat the scope correctly when using exec() function syntax here.
-        exec config_file.read() in scope
+        code = compile(config_file.read(), filename, mode='exec')
+    exec(code, scope)
     return scope['config']