do not print infer.py usage message on wrong arguments

Summary:
On wrong arguments (or on no arguments at all), `infer` would spew the error
message of `infer.py`, which makes no sense. Make the python code swallow error
messages and exit with a special code on errors coming from command line
parsing so that the OCaml side is in charge of printing usage messages.

Reviewed By: cristianoc

Differential Revision: D3731594

fbshipit-source-id: fe49cda

infer/lib/python/infer.py

@@ -78,8 +78,23 @@ def split_args_to_parse():
     return (sys_argv[1:dd_index], cmd_raw)
 
 
+class FailSilentlyArgumentParser(argparse.ArgumentParser):
+    '''We want to leave the handling of printing usage messages to the
+    OCaml code. To do so, swallow error messages from ArgumentParser
+    and exit with a special error code (101) that infer.ml looks for.
+    '''
+
+    def error(self, message):
+        utils.stderr(message)
+        utils.stderr('')
+        exit(22)  # in sync with infer.ml
+
+    def print_help(self, file=None):
+        exit(22)  # in sync with infer.ml
+
+
 def create_argparser(parents=[]):
-    parser = argparse.ArgumentParser(
+    parser = FailSilentlyArgumentParser(
         parents=[analyze.infer_parser] + parents,
         add_help=False,
         formatter_class=argparse.RawDescriptionHelpFormatter,

infer/src/backend/infer.ml

@@ -104,6 +104,9 @@ let () =
     ) in
   let pid = Unix.create_process args_py.(0) args_py Unix.stdin Unix.stdout Unix.stderr in
   let _, status = Unix.waitpid [] pid in
+  if status = Unix.WEXITED 22 then
+    (* swallow infer.py argument parsing error *)
+    Config.print_usage_exit ();
   (* collect crashcontext summaries *)
   let analysis_is_crashcontext = match Config.analyzer with
     | Some Crashcontext -> true