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 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