[Bug 198819] rewinddir(3) after fdopendir(3) results in double entries

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=198819

            Bug ID: 198819
           Summary: rewinddir(3) after fdopendir(3) results in double
                    entries
           Product: Base System
           Version: 10.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: markrwilliams at gmail.com

Created attachment 154687
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=154687&action=edit
example program that produces incorrect output on UFS

The attached program attempts to list the current working directory.  It
obtains a pointer to the directory stream via fdopendir(3) and then calls
rewinddir(3) before a standard loop over readdir(3).

On two out of four machines I tested, the program lists the contents of the
current directory twice, like so:

$ ls -1
.
..
readdir_duplicate_entries
readdir_duplicate_entries.c
$ ./readdir_duplicate_entries 
.
..
readdir_duplicate_entries.c
readdir_duplicate_entries
.
..
readdir_duplicate_entries.c
readdir_duplicate_entries
$

The program produces the correct output if rewinddir(3) is not called or if
opendir(3) is called instead of open(2)/fdopendir(3).

uname -a for the affected machines:

FreeBSD lithos.ferv.org 10.1-RELEASE-p5 FreeBSD 10.1-RELEASE-p5 #0: Tue Jan 27
08:55:07 UTC 2015    
root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64
FreeBSD ip-172-31-19-241 10.1-RELEASE-p6 FreeBSD 10.1-RELEASE-p6 #0: Tue Feb 24
19:00:21 UTC 2015    
root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64

All mounts on both of the affected machines are UFS.

All mounts on the the third and fourth machines, on which the test program
produced *correct* output, are ZFS.

uname information for those:

FreeBSD glob 10.0-RELEASE-p9 FreeBSD 10.0-RELEASE-p9 #0: Mon Sep 15 14:35:52
UTC 2014     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC 
amd64
FreeBSD 10.1-RELEASE-p6 (GENERIC) #0: Tue Feb 24 19:00:21 UTC 2015

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