Make cmdline decoding more robust

At least don't fail with unrelated inputs. As long as we can reasonably
get the shell, it's good enough. We can deal with these edge cases if
the proc parsing code is to be extracted somewhere else one day.

Fix #10.

Author: uranusjr

news/10.bugfix.rst

@@ -0,0 +1 @@
+Improve ``/proc`` content parsing robustness to not fail with non-decodable command line arguments.

src/shellingham/posix/proc.py

@@ -1,5 +1,7 @@
+import io
 import os
 import re
+import sys
 
 from ._core import Process
 
@@ -27,14 +29,22 @@ def detect_proc():
 
 
 def _get_stat(pid, name):
-    with open(os.path.join('/proc', str(pid), name)) as f:
+    path = os.path.join('/proc', str(pid), name)
+    with io.open(path, encoding='ascii', errors='replace') as f:
+        # We only care about TTY and PPID -- all numbers.
         parts = STAT_PATTERN.findall(f.read())
         return parts[STAT_TTY], parts[STAT_PPID]
 
 
 def _get_cmdline(pid):
-    with open(os.path.join('/proc', str(pid), 'cmdline')) as f:
-        return tuple(f.read().split('\0')[:-1])
+    path = os.path.join('/proc', str(pid), 'cmdline')
+    encoding = sys.getfilesystemencoding() or 'utf-8'
+    with io.open(path, encoding=encoding, errors='replace') as f:
+        # XXX: Command line arguments can be arbitrary byte sequences, not
+        # necessarily decodable. For Shellingham's purpose, however, we don't
+        # care. (pypa/pipenv#2820)
+        # cmdline appends an extra NULL at the end, hence the [:-1].
+        return tuple(f.read().split(b'\0')[:-1])
 
 
 class ProcFormatError(EnvironmentError):