[2.7] bpo-32903: Fix a memory leak in os.chdir() on Windows (GH-5801). (#5947)

(cherry picked from commit 3e197c7)

Co-authored-by: Alexey Izbyshev <[email protected]>

Author: izbyshev

Misc/NEWS.d/next/Windows/2018-02-28-11-03-24.bpo-32903.1SXY4t.rst

@@ -0,0 +1,2 @@
+Fix a memory leak in os.chdir() on Windows if the current directory is set
+to a UNC path.

Modules/posixmodule.c

@@ -984,6 +984,7 @@ win32_wchdir(LPCWSTR path)
     wchar_t _new_path[MAX_PATH+1], *new_path = _new_path;
     int result;
     wchar_t env[4] = L"=x:";
+    int is_unc_like_path;
 
     if(!SetCurrentDirectoryW(path))
         return FALSE;
@@ -1002,15 +1003,15 @@ win32_wchdir(LPCWSTR path)
             return FALSE;
         }
     }
-    if (wcsncmp(new_path, L"\\\\", 2) == 0 ||
-        wcsncmp(new_path, L"//", 2) == 0)
-        /* UNC path, nothing to do. */
-        return TRUE;
-    env[1] = new_path[0];
-    result = SetEnvironmentVariableW(env, new_path);
+    is_unc_like_path = (wcsncmp(new_path, L"\\\\", 2) == 0 ||
+                        wcsncmp(new_path, L"//", 2) == 0);
+    if (!is_unc_like_path) {
+        env[1] = new_path[0];
+        result = SetEnvironmentVariableW(env, new_path);
+    }
     if (new_path != _new_path)
         free(new_path);
-    return result;
+    return result ? TRUE : FALSE;
 }
 #endif