[3.6] bpo-31178: Avoid concatenating bytes with str in subprocess error (GH-3066) (#3388)

Avoid concatenating bytes with str in the typically rare subprocess error path (exec failed). Includes a mock based unittest to exercise the codepath.
(cherry picked from commit 3fc499bca18454b9f432b9b0106cab662bfeb549)

1 files changed, 6 insertions, 3 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index d013234..575413d 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -1314,15 +1314,18 @@ class Popen(object):
                 try:
                     exception_name, hex_errno, err_msg = (
                             errpipe_data.split(b':', 2))
+                    # The encoding here should match the encoding
+                    # written in by the subprocess implementations
+                    # like _posixsubprocess
+                    err_msg = err_msg.decode()
                 except ValueError:
                     exception_name = b'SubprocessError'
                     hex_errno = b'0'
-                    err_msg = (b'Bad exception data from child: ' +
-                               repr(errpipe_data))
+                    err_msg = 'Bad exception data from child: {!r}'.format(
+                                  bytes(errpipe_data))
                 child_exception_type = getattr(
                         builtins, exception_name.decode('ascii'),
                         SubprocessError)
-                err_msg = err_msg.decode(errors="surrogatepass")
                 if issubclass(child_exception_type, OSError) and hex_errno:
                     errno_num = int(hex_errno, 16)
                     child_exec_never_called = (err_msg == "noexec")