pex_binary: Support PEXes with no entry points (interpreters)

When using pex without any entry points, it creates a PEX that is an
interpreter that uses the bundled environment. This can be useful to
distribute in order to execute other scripts.

interpreter_only_test.py: Verify that the PEX starts an interpreter.

Fixes the following Skylark exceptions in pex_binary:

File ".../bazel_rules_pex/examples/BUILD", line 24
  pex_binary(name = 'interpreter_only')
File ".../bazel_rules_pex/pex/pex_rules.bzl", line 177, in _pex_binary_impl
  pex_file_types.filter(ctx.files.srcs)[0]
index out of range (index is 0, but sequence has 0 elements)

File ".../bazel_rules_pex/examples/BUILD", line 24
  pex_binary(name = 'interpreter_only')
File ".../bazel_rules_pex/pex/pex_rules.bzl", line 225, in _pex_binary_impl
  main_pkg
name 'main_pkg' is not defined

examples/BUILD

@@ -19,3 +19,14 @@ pex_pytest(
     deps = [":libfoo"],
     size = "small",
 )
+
+# Tests that building a pex without entry points is supported
+pex_binary(
+    name = "interpreter_only",
+)
+sh_test(
+    name = "interpreter_only_test",
+    srcs = ["interpreter_only_test.py"],
+    data = [":interpreter_only"],
+    args = ["$(location :interpreter_only)"],
+)

examples/interpreter_only_test.py

@@ -0,0 +1,17 @@
+#!/usr/bin/env python2.7
+
+import sys
+import subprocess
+
+def main():
+    if len(sys.argv) != 2:
+        sys.stderr.write('Error: Pass path to an interpreter pex\n')
+        sys.exit(1)
+    interpreter_pex_path = sys.argv[1]
+
+    output = subprocess.check_output([interpreter_pex_path], stderr=subprocess.STDOUT)
+    assert 'InteractiveConsole' in output
+    print 'PASS'
+
+if __name__ == '__main__':
+    main()

pex/pex_rules.bzl

@@ -168,13 +168,16 @@ def _pex_binary_impl(ctx):
 
   if ctx.attr.entrypoint and ctx.file.main:
     fail("Please specify either entrypoint or main, not both.")
+  main_file = None
+  main_pkg = ''
   if ctx.attr.entrypoint:
-    main_file = None
     main_pkg = ctx.attr.entrypoint
   elif ctx.file.main:
     main_file = ctx.file.main
   else:
-    main_file = pex_file_types.filter(ctx.files.srcs)[0]
+    filtered_srcs = pex_file_types.filter(ctx.files.srcs)
+    if filtered_srcs:
+      main_file = filtered_srcs[0]
   if main_file:
     # Translate main_file's short path into a python module name
     main_pkg = main_file.short_path.replace('/', '.')[:-3]