Don't change os.sep with an empty MSYSTEM env var, not just a missing one

Up until now this didn't really happen when calling from cygwin
because empty env vars were removed before Python would run.

But msys2/msys2-runtime#101 changed that.

To avoid breaking users that did something like

MSYSTEM= python ...

not only check that MSYSTEM isn't set but also that it isn't empty
when deciding if os.sep/os.altsep should be switched.

Also, guard the msystem env check to execute only on MINGW

Co-authored-by: Naveen M K <[email protected]>

Author: lazka

Lib/importlib/_bootstrap_external.py

@@ -43,7 +43,7 @@
 else:
     path_separators = ['/']
 
-if 'MSYSTEM' in _os.environ:
+if _os.environ.get('MSYSTEM', ''):
     path_separators = path_separators[::-1]
 
 # Assumption made in _path_join()

Lib/ntpath.py

@@ -21,7 +21,7 @@
 import genericpath
 from genericpath import *
 
-if sys.platform == "win32" and "MSYSTEM" in os.environ:
+if sys.platform == "win32" and os.environ.get("MSYSTEM", ""):
     sep = '/'
     altsep = '\\'
 else:

Lib/pathlib.py

@@ -115,7 +115,7 @@ class _WindowsFlavour(_Flavour):
 
     sep = '\\'
     altsep = '/'
-    if 'MSYSTEM' in os.environ:
+    if os.environ.get('MSYSTEM', ''):
         sep, altsep = altsep, sep
     has_drv = True
     pathmod = ntpath

Python/pathconfig.c

@@ -2,7 +2,7 @@
 
 #include "Python.h"
 #include "marshal.h"              // PyMarshal_ReadObjectFromString
-#include "osdefs.h"               // DELIM
+#include "osdefs.h"               // DELIM, SEP
 #include "pycore_initconfig.h"
 #include "pycore_fileutils.h"
 #include "pycore_pathconfig.h"
@@ -50,7 +50,6 @@ Py_StartsWithW(const wchar_t * str, const wchar_t * prefix)
 char
 Py_GetSepA(const char *name)
 {
-    char* msystem = (char*)2; /* So that non Windows use / as sep */
     static char sep = '\0';
 #ifdef _WIN32
     /* https://msdn.microsoft.com/en-gb/library/windows/desktop/aa365247%28v=vs.85%29.aspx
@@ -65,12 +64,14 @@ Py_GetSepA(const char *name)
     if (sep != '\0')
         return sep;
 #if defined(__MINGW32__)
-    msystem = Py_GETENV("MSYSTEM");
-#endif
-    if (msystem != NULL)
+    char* msystem = getenv("MSYSTEM");
+    if (msystem != NULL && strcmp(msystem, "") != 0)
         sep = '/';
     else
         sep = '\\';
+#else
+    sep = SEP;
+#endif
     return sep;
 }
 
@@ -103,7 +104,6 @@ Py_NormalizeSepsA(char *name)
 wchar_t
 Py_GetSepW(const wchar_t *name)
 {
-    char* msystem = (char*)2; /* So that non Windows use / as sep */
     static wchar_t sep = L'\0';
 #ifdef _WIN32
     /* https://msdn.microsoft.com/en-gb/library/windows/desktop/aa365247%28v=vs.85%29.aspx
@@ -118,12 +118,14 @@ Py_GetSepW(const wchar_t *name)
     if (sep != L'\0')
         return sep;
 #if defined(__MINGW32__)
-    msystem = Py_GETENV("MSYSTEM");
-#endif
-    if (msystem != NULL)
+    char* msystem = getenv("MSYSTEM");
+    if (msystem != NULL && strcmp(msystem, "") != 0)
         sep = L'/';
     else
         sep = L'\\';
+#else
+    sep = SEP;
+#endif
     return sep;
 }
 

mingw_smoketests.py

@@ -29,7 +29,7 @@
 import unittest
 import sysconfig
 
-if "MSYSTEM" in os.environ:
+if os.environ.get("MSYSTEM", ""):
     SEP = "/"
 else:
     SEP = "\\"

setup.py

@@ -81,7 +81,7 @@ def get_platform():
 # as otherwise all the io redirection will fail.
 # Arguably, this could happen inside the real os.system
 # rather than this monkey patch.
-if sys.platform == "win32" and "MSYSTEM" in os.environ:
+if sys.platform == "win32" and os.environ.get("MSYSTEM", ""):
     os_system = os.system
     def msys_system(command):
         command_in_sh = 'sh.exe -c "%s"' % command.replace("\\", "\\\\")