RFR: 8251329: (zipfs) Files.walkFileTree walks infinitely if zip has dir named "." inside [v3]

[Jaikiran Pai]

> Can I please get a review of this proposed fix for https://bugs.openjdk.java.net/browse/JDK-8251329?
> 
> As noted in that issue, if a zip filesystem created on top of a jar containing a "./" entry is "walked" using the `Files.walkFileTree`, it leads to a infinite never ending iteration (which ultimately fails with Java heap space OOM).
> 
> Alan notes in that issue that:
> 
>> This is more likely an issue with the zipfs DirectoryStream implementation. A DirectoryStream is specified to not include elements that for the special links to the current or parent directory. It should be rare. 
> 
> This indeed turned out to be an issue in the `jdk.nio.zipfs.ZipDirectoryStream#iterator()` implementation where it calls the `jdk.nio.zipfs.ZipFileSystem#iteratorOf(...)` implementation.  The implementation, unlike what the javadoc of `java.nio.file.DirectoryStream` states, was including the "." and ".." paths in its returned iterator:
> 
>> The elements returned by the iterator are in no specific order. Some file
>  systems maintain special links to the directory itself and the directory's
>   parent directory. Entries representing these links are not returned by the
>   iterator.
> 
> 
> The proposed fix in this patch checks the paths for "." and "..", similar to what the `sun.nio.fs.UnixDirectoryStream` does in its `isSelfOrParent` and skips those paths from being added into the returned iterator. The `jdk.nio.zipfs.ZipFileSystem#iteratorOf(...)` (where this change has been done) is currently only used by `jdk.nio.zipfs.ZipDirectoryStream#iterator()` and has package-private visibility, so this change shouldn't impact any other usage/expectations.
> 
> A new jtreg test has been added to reproduce this issue and verify the fix. Local testing of the `test/jdk/jdk/nio/` (including this new test) went fine without any issues after this change. I will be triggering a `tier1` test locally in a while.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  implement review suggestions:
   - reduce the toString() calls by creating a helper
   - fs.getPath("/") instead of fs.getRootDirectories().iterator().next()

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4604/files
  - new: https://git.openjdk.java.net/jdk/pull/4604/files/3971ad6f..8e77d289

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4604&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4604&range=01-02

  Stats: 18 lines in 1 file changed: 6 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4604.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4604/head:pull/4604

PR: https://git.openjdk.java.net/jdk/pull/4604