Issue #23605: Fix os.walk(topdown=True), don't cache entry.is_symlink() because

the caller can replace the directory with a different file kind.

The bottom-up way, os.walk(topdown=False), still uses entry.is_symlink(), and
so can be faster than Python 3.4.

Lib/os.py

@@ -354,7 +354,6 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
 
     dirs = []
     nondirs = []
-    symlinks = set()
 
     # We may not have read permission for top, in which case we can't
     # get a list of the files the directory contains.  os.walk
@@ -364,7 +363,23 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
     try:
         # Note that scandir is global in this module due
         # to earlier import-*.
-        for entry in scandir(top):
+        scandir_it = scandir(top)
+    except OSError as error:
+        if onerror is not None:
+            onerror(error)
+        return
+
+    while True:
+        try:
+            try:
+                entry = next(scandir_it)
+            except StopIteration:
+                break
+        except OSError as error:
+            if onerror is not None:
+                onerror(error)
+            return
+
         try:
             is_dir = entry.is_dir()
         except OSError:
@@ -377,33 +392,40 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
         else:
             nondirs.append(entry.name)
 
-            if is_dir and not followlinks:
+        if not topdown and is_dir:
+            # Bottom-up: recurse into sub-directory, but exclude symlinks to
+            # directories if followlinks is False
+            if followlinks:
+                walk_into = True
+            else:
                 try:
-                    if entry.is_symlink():
-                        symlinks.add(entry.name)
+                    is_symlink = entry.is_symlink()
                 except OSError:
                     # If is_symlink() raises an OSError, consider that the
                     # entry is not a symbolik link, same behaviour than
                     # os.path.islink().
-                    pass
-    except OSError as error:
-        # scandir() or iterating into scandir() iterator raised an OSError
-        if onerror is not None:
-            onerror(error)
-        return
+                    is_symlink = False
+                walk_into = not is_symlink
+
+            if walk_into:
+                yield from walk(entry.path, topdown, onerror, followlinks)
 
     # Yield before recursion if going top down
     if topdown:
         yield top, dirs, nondirs
 
         # Recurse into sub-directories
+        islink, join = path.islink, path.join
         for name in dirs:
-        if followlinks or name not in symlinks:
-            new_path = path.join(top, name)
+            new_path = join(top, name)
+            # Issue #23605: os.path.islink() is used instead of caching
+            # entry.is_symlink() result during the loop on os.scandir() because
+            # the caller can replace the directory entry during the "yield"
+            # above.
+            if followlinks or not islink(new_path):
                 yield from walk(new_path, topdown, onerror, followlinks)
-
+    else:
         # Yield after recursion if going bottom up
-    if not topdown:
         yield top, dirs, nondirs
 
 __all__.append("walk")