beam: fix path_sandbox root="/" containment (was requiring "//" prefix)

wizard-work/beam/path_sandbox.py

@@ -59,13 +59,22 @@ def safe_join(root, *parts):
     candidate = os.path.join(rr, *parts) if parts else rr
     # normalize (collapses . and redundant slashes) but does not resolve symlinks
     candidate = os.path.normpath(candidate)
-    # containment check on the normalized (pre-symlink) form
-    if candidate != rr and not candidate.startswith(rr + os.sep):
-        raise PathEscapeError("joined path escapes root: %r" % candidate)
+    # containment check on the normalized (pre-symlink) form.
+    # Special-case root == "/" so we don't require a "//" prefix.
+    if rr == os.sep:
+        if not (candidate == rr or candidate.startswith(rr)):
+            raise PathEscapeError("joined path escapes root: %r" % candidate)
+    else:
+        if candidate != rr and not candidate.startswith(rr + os.sep):
+            raise PathEscapeError("joined path escapes root: %r" % candidate)
     # now resolve symlinks and re-check containment
     resolved = os.path.realpath(candidate)
-    if resolved != rr and not resolved.startswith(rr + os.sep):
-        raise PathEscapeError("symlink escape: %r -> %r" % (candidate, resolved))
+    if rr == os.sep:
+        if not (resolved == rr or resolved.startswith(rr)):
+            raise PathEscapeError("symlink escape: %r -> %r" % (candidate, resolved))
+    else:
+        if resolved != rr and not resolved.startswith(rr + os.sep):
+            raise PathEscapeError("symlink escape: %r -> %r" % (candidate, resolved))
     return resolved
 
 
@@ -86,4 +95,6 @@ def is_within(root, target):
         rt = os.path.realpath(target)
     except (OSError, ValueError):
         return False
+    if rr == os.sep:
+        return rt == rr or rt.startswith(rr)
     return rt == rr or rt.startswith(rr + os.sep)