bpo-35755: shutil.which() uses os.confstr("CS_PATH") (GH-12858)

shutil.which() and distutils.spawn.find_executable() now use
os.confstr("CS_PATH") if available instead of os.defpath, if the PATH
environment variable is not set.

Don't use os.confstr("CS_PATH") nor os.defpath if the PATH
environment variable is set to an empty string to mimick Unix 'which'
command behavior.

Changes:

* find_executable() now starts by checking for the executable in the
  current working directly case. Add an explicit
  "if not path: return None".
* Add tests for PATH='' (empty string), PATH=':' and for PATHEXT.

1 files changed, 12 insertions, 1 deletions

diff --git a/Lib/shutil.py b/Lib/shutil.py
index 7dd470dfab..34df9cc474 100644
--- a/Lib/shutil.py
+++ b/Lib/shutil.py
@@ -1309,9 +1309,20 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
     use_bytes = isinstance(cmd, bytes)
 
     if path is None:
-        path = os.environ.get("PATH", os.defpath)
+        path = os.environ.get("PATH", None)
+        if path is None:
+            try:
+                path = os.confstr("CS_PATH")
+            except (AttributeError, ValueError):
+                # os.confstr() or CS_PATH is not available
+                path = os.defpath
+        # bpo-35755: Don't use os.defpath if the PATH environment variable is
+        # set to an empty string to mimick Unix which command behavior
+
+    # PATH='' doesn't match, whereas PATH=':' looks in the current directory
     if not path:
         return None
+
     if use_bytes:
         path = os.fsencode(path)
         path = path.split(os.fsencode(os.pathsep))