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, 25 insertions, 14 deletions

diff --git a/spawn.py b/spawn.py
index 53876880..88832727 100644
--- a/spawn.py
+++ b/spawn.py
@@ -172,21 +172,32 @@ def find_executable(executable, path=None):
     A string listing directories separated by 'os.pathsep'; defaults to
     os.environ['PATH'].  Returns the complete filename or None if not found.
     """
-    if path is None:
-        path = os.environ.get('PATH', os.defpath)
-
-    paths = path.split(os.pathsep)
-    base, ext = os.path.splitext(executable)
-
+    _, ext = os.path.splitext(executable)
     if (sys.platform == 'win32') and (ext != '.exe'):
         executable = executable + '.exe'
 
-    if not os.path.isfile(executable):
-        for p in paths:
-            f = os.path.join(p, executable)
-            if os.path.isfile(f):
-                # the file exists, we have a shot at spawn working
-                return f
-        return None
-    else:
+    if os.path.isfile(executable):
         return executable
+
+    if path is None:
+        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
+
+    paths = path.split(os.pathsep)
+    for p in paths:
+        f = os.path.join(p, executable)
+        if os.path.isfile(f):
+            # the file exists, we have a shot at spawn working
+            return f
+    return None