RFR: 8306882: (fs) Path.toRealPath(LinkOption.NOFOLLOW_LINKS) fails when "../../" follows a link

[Brian Burkhalter]

On Wed, 23 Aug 2023 01:09:16 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

> Modify `Path.toRealPath` such that it does not collapse links such as "link/.." or "link/../.." when `LinkOption.NOFOLLOW_LINKS` is specified or, in some cases, not.

> I think that [the Unix implementation] needs a summary in the first place to understand why that change is needed.

I can add a comment in the source, but here's some illustrative behavior using `jshell`. It uses some files as:

./dir/subdir
./aaa@ -> dir/subdir
./out.txt

and attempts to convert `aaa/../../out.txt` to a real path.

**master**

jshell> Path p = Path.of("aaa/../../out.txt")
p ==> aaa/../../out.txt

jshell> p.toRealPath()
$2 ==> /Users/bpb/dev/bugs/jdk/Path-toRealPath-8306882/out.txt                                        

jshell> p.toRealPath(LinkOption.NOFOLLOW_LINKS)
|  Exception java.nio.file.NoSuchFileException: /Users/bpb/dev/bugs/jdk/Path-toRealPath-8306882/aaa/out.txt
|        at UnixException.translateToIOException (UnixException.java:92)
|        at UnixException.rethrowAsIOException (UnixException.java:106)
|        at UnixException.rethrowAsIOException (UnixException.java:111)
|        at UnixPath.toRealPath (UnixPath.java:927)
|        at (#3:1)

As can be seen, when attempting to resolve the path without following links, the result is a non-existent file.

**patch**

jshell> Path p = Path.of("aaa/../../out.txt")
p ==> aaa/../../out.txt

jshell> p.toRealPath()
$2 ==> /Users/bpb/dev/bugs/jdk/Path-toRealPath-8306882/out.txt

jshell> p.toRealPath(LinkOption.NOFOLLOW_LINKS)
$3 ==> /Users/bpb/dev/bugs/jdk/Path-toRealPath-8306882/aaa/../../out.txt

Here the correct result is obtained.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/15397#issuecomment-1698040599