Allow passing a preamble file to the CLI (#400)

This PR adds a new CLI option, -p or --preamble-file, that is used to provide a preamble to the pex builder object.

We use pex via its API in the internal build tooling at NerdWallet (👋, we're neighbors!). We've been leveraging it for multi-platform support, but now that #394 / 1.2.9 has landed the only thing missing from the CLI that we leverage is the preamble in the builder, which was pretty easy to hook up. Once this PR has landed we can remove all of the API integration and just rely on the pex CLI!

The CLI option and the PEXBuilder preamble functionality are covered by tests.

Local functional testing:

(pex-foo) 
evanborgstrom@evanborgstrom /tmp/preamble — u:34 j:1 (21:49:54 08.16)
#536 ❯❯❯ echo 'print "foo!"' > preamble
(pex-foo) 
evanborgstrom@evanborgstrom /tmp/preamble — u:34 j:1 (21:50:11 08.16)
#537 ❯❯❯ pex -p preamble 
foo!
Python 2.7.13 (default, Jun  7 2017, 11:02:53) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> 
(pex-foo) 
evanborgstrom@evanborgstrom /tmp/preamble — u:34 j:1 (21:50:18 08.16)
#538 ❯❯❯ pex 
Python 2.7.13 (default, Jun  7 2017, 11:02:53) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>>

pex/bin/pex.py

@@ -355,6 +355,14 @@ def configure_clp():
       help='The name of the generated .pex file: Omiting this will run PEX '
            'immediately and not save it to a file.')
 
+  parser.add_option(
+      '-p', '--preamble-file',
+      dest='preamble_file',
+      metavar='FILE',
+      default=None,
+      type=str,
+      help='The name of a file to be included as the preamble for the generated .pex file')
+
   parser.add_option(
       '-r', '--requirement',
       dest='requirement_files',
@@ -513,8 +521,15 @@ def build_pex(args, options, resolver_option_builder):
   if not interpreters:
     die('Could not find compatible interpreter', CANNOT_SETUP_INTERPRETER)
 
+  try:
+    with open(options.preamble_file) as preamble_fd:
+      preamble = preamble_fd.read()
+  except TypeError:
+    # options.preamble_file is None
+    preamble = None
+
   interpreter = _lowest_version_interpreter(interpreters)
-  pex_builder = PEXBuilder(path=safe_mkdtemp(), interpreter=interpreter)
+  pex_builder = PEXBuilder(path=safe_mkdtemp(), interpreter=interpreter, preamble=preamble)
 
   pex_info = pex_builder.info
   pex_info.zip_safe = options.zip_safe

tests/test_pex_binary.py

@@ -3,8 +3,10 @@
 
 from contextlib import contextmanager
 from optparse import OptionParser
+from tempfile import NamedTemporaryFile
 
-from pex.bin.pex import configure_clp, configure_clp_pex_resolution
+from pex.bin.pex import build_pex, configure_clp, configure_clp_pex_resolution
+from pex.compatibility import to_bytes
 from pex.fetcher import PyPIFetcher
 from pex.package import SourcePackage, WheelPackage
 from pex.resolver_options import ResolverOptionsBuilder
@@ -88,6 +90,19 @@ def test_clp_constraints_txt():
   assert options.constraint_files == ['requirements1.txt']
 
 
+def test_clp_preamble_file():
+  with NamedTemporaryFile() as tmpfile:
+    tmpfile.write(to_bytes('print "foo!"'))
+    tmpfile.flush()
+
+    parser, resolver_options_builder = configure_clp()
+    options, reqs = parser.parse_args(args=['--preamble-file', tmpfile.name])
+    assert options.preamble_file == tmpfile.name
+
+    pex_builder = build_pex(reqs, options, resolver_options_builder)
+    assert pex_builder._preamble == to_bytes('print "foo!"')
+
+
 def test_clp_prereleases():
   with parser_pair() as (builder, parser):
     configure_clp_pex_resolution(parser, builder)

tests/test_pex_builder.py

@@ -101,6 +101,30 @@ def builder(shebang):
           assert fp.read(len(expected_preamble)) == expected_preamble
 
 
+def test_pex_builder_preamble():
+  with temporary_dir() as td:
+    target = os.path.join(td, 'foo.pex')
+    should_create = os.path.join(td, 'foo.1')
+
+    tempfile_preamble = "\n".join([
+      "import sys",
+      "open('{0}', 'w').close()".format(should_create),
+      "sys.exit(3)"
+    ])
+
+    pex_builder = PEXBuilder(preamble=tempfile_preamble)
+    pex_builder.build(target)
+
+    assert not os.path.exists(should_create)
+
+    pex = PEX(target)
+    process = pex.run(blocking=False)
+    process.wait()
+
+    assert process.returncode == 3
+    assert os.path.exists(should_create)
+
+
 def test_pex_builder_compilation():
   with nested(temporary_dir(), temporary_dir(), temporary_dir()) as (td1, td2, td3):
     src = os.path.join(td1, 'src.py')