MFC r305154: directory(3): Deprecate readdir_r(). Clarify dirent buffers.

In existing implementations including FreeBSD, there is no reason to use
readdir_r() in the common case where potentially multiple threads each list
their own directory. Code using readdir() is simpler.

What's more, although readdir_r() can safely be used on FreeBSD because
NAME_MAX is forced to 255, it cannot be used safely on systems where
{NAME_MAX} is not fixed. As a concrete example, FAT/NTFS filenames can be up
to 255 UTF-16 code units long, which can be up to 765 UTF-8 bytes.

Deprecating readdir_r() in POSIX has been proposed in
http://www.austingroupbugs.net/view.php?id=696
and glibc wants to deprecate it as well.

MFC 282552:
Remove the note about seekdir() removing telldir() cookies. That was
removed back in r269204.

MFH: r282485
Tweak seekdir, telldir and readdir so that when htere are deletes going on,
as seek to teh last location saved will still work. This is needed for Samba
to be able to correctly handle delete requests from windows. This does not
completely fix seekdir when deletes are present but fixes the worst of the
problems. The real solution must involve some changes to the API for eh VFS
and getdirentries(2).

Obtained from: Panzura inc
MFH: r282550 (jhb@)
A few style fixes and expand the comment a bit on what _fixtelldir() is
doing.
MFH: r282560 (jhb@)
Tweak the comment here some more. In particular, the previous opening
sentence was a bit confusing.
Noted by: kib

MFC 268531,269079,269204:
Fix various edge cases with rewinddir(), seekdir(), and telldir():
- In the unionfs case, opendir() and fdopendir() read the directory's full
contents and cache it. This cache is not refreshed when rewinddir() is
called, so rewinddir() will not notice updates to a directory. Fix this
by splitting the code to fetch a directory's contents out of
__opendir_common() into a new _filldir() function and call this from
rewinddir() when operating on a unionfs directory.
- If rewinddir() is called on a directory opened with fdopendir() before
any directory entries are fetched, rewinddir() will not adjust the seek
location of the backing file descriptor. If the file descriptor passed
to fdopendir() had a non-zero offset, the rewinddir() will not rewind to
the beginning. Fix this by always seeking back to 0 in rewinddir().
This means the dd_rewind hack can also be removed.
- Add missing locking to rewinddir()
- POSIX says that passing a location returned by telldir() to seekdir()
after an intervening call to rewinddir() is undefined, so reclaim any
pending telldir() cookies in the directory when rewinddir() is called.
- If telldir() is called immediately after a call to seekdir(), POSIX
requires the return value of telldir() to equal the value passed to
seekdir(). The current seekdir code with SINGLEUSE enabled breaks
this case as each call to telldir() allocates a new cookie. Instead,
remove the SINGLEUSE code and change telldir() to look for an existing
cookie for the directory's current location rather than always creating
a new cookie.

PR: 121656

MFC 282552:
Remove the note about seekdir() removing telldir() cookies. That was
removed back in r269204.

MFH: r282485
Tweak seekdir, telldir and readdir so that when htere are deletes going on,
as seek to teh last location saved will still work. This is needed for Samba
to be able to correctly handle delete requests from windows. This does not
completely fix seekdir when deletes are present but fixes the worst of the
problems. The real solution must involve some changes to the API for eh VFS
and getdirentries(2).

Obtained from: Panzura inc
MFH: r282550 (jhb@)
A few style fixes and expand the comment a bit on what _fixtelldir() is
doing.
MFH: r282560 (jhb@)
Tweak the comment here some more. In particular, the previous opening
sentence was a bit confusing.
Noted by: kib

MFC 268531,269079,269204:
Fix various edge cases with rewinddir(), seekdir(), and telldir():
- In the unionfs case, opendir() and fdopendir() read the directory's full
contents and cache it. This cache is not refreshed when rewinddir() is
called, so rewinddir() will not notice updates to a directory. Fix this
by splitting the code to fetch a directory's contents out of
__opendir_common() into a new _filldir() function and call this from
rewinddir() when operating on a unionfs directory.
- If rewinddir() is called on a directory opened with fdopendir() before
any directory entries are fetched, rewinddir() will not adjust the seek
location of the backing file descriptor. If the file descriptor passed
to fdopendir() had a non-zero offset, the rewinddir() will not rewind to
the beginning. Fix this by always seeking back to 0 in rewinddir().
This means the dd_rewind hack can also be removed.
- Add missing locking to rewinddir()
- POSIX says that passing a location returned by telldir() to seekdir()
after an intervening call to rewinddir() is undefined, so reclaim any
pending telldir() cookies in the directory when rewinddir() is called.
- If telldir() is called immediately after a call to seekdir(), POSIX
requires the return value of telldir() to equal the value passed to
seekdir(). The current seekdir code with SINGLEUSE enabled breaks
this case as each call to telldir() allocates a new cookie. Instead,
remove the SINGLEUSE code and change telldir() to look for an existing
cookie for the directory's current location rather than always creating
a new cookie.

PR: 121656