New submission from Serhiy Storchaka:
For now posixpath.realpath() don't raise an exception if encounter broken link.
Instead it just lefts broken link name and following path components
unresolved. This is dangerous since broken link name can be collapsed with
following ".." and resulting valid path can point at wrong location. May be
this is even security issue.
On other hand, Path.resolve() raises an exception when encounters broken link.
This is not always desirable, there is a wish to make it more lenient. See
issue19717 for more information.
The readlink utility from GNU coreutils has three mode for resolving file path:
-f, --canonicalize
canonicalize by following every symlink in every component of the
given name recursively; all but the last component must exist
-e, --canonicalize-existing
canonicalize by following every symlink in every component of the
given name recursively, all components must exist
-m, --canonicalize-missing
canonicalize by following every symlink in every component of the
given name recursively, without requirements on components existence
Current behavior of posixpath.realpath() is matches (besides one minor detail)
to `readlink -m`. The behavior of Path.resolve() matches `readlink -e`.
Proposed preliminary patch implements the support of all three modes in
posixpath.realpath(): CAN_MISSING, CAN_ALL_BUT_LAST and CAN_EXISTING. It
exactly matches the behavior of readlink. The default mode is CAN_MISSING.
There is minor behavior difference in the default mode. If there is a file
"file", a link "link" that points to "file" and a broken link "broken", then
"broken/../link" was resolved to "link" and now it is resolved to "file".
The patch lacks the documentation. Ternary flag looks as not the best API.
Binary flag would be better. But I don't know what can be dropped. CAN_MISSING
is needed for compatibility, but it looks less useful and may be insecure (not
more than normpath()). CAN_EXISTING and CAN_ALL_BUT_LAST is needed in different
cases. I think that in many cases CAN_ALL_BUT_LAST is actually needed instead
of CAN_MISSING.
After resolving this issue the solution will be adopted for Path.resolve().
----------
components: Library (Lib)
files: realpath_mode.patch
keywords: patch
messages: 265335
nosy: serhiy.storchaka
priority: normal
severity: normal
status: open
title: Support different modes in posixpath.realpath()
type: enhancement
versions: Python 3.6
Added file: http://bugs.python.org/file42821/realpath_mode.patch
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