What's new:
-----------

- Revamped pseudo console support.  Conditionally activating it only when
  a non-cygwin application is run.

- New C11 threads API: call_once, cnd_broadcast, cnd_destroy, cnd_init,
  cnd_signal, cnd_timedwait, cnd_wait, mtx_destroy, mtx_init, mtx_lock,
  mtx_timedlock, mtx_trylock, mtx_unlock, thrd_create, thrd_current,
  thrd_detach, thrd_equal, thrd_exit, thrd_join, thrd_sleep, thrd_yield,
  tss_create, tss_delete, tss_get, tss_set.

- In cygwin console, new thread which handles special keys/signals such
  as Ctrl-Z (VSUSP), Ctrl-\ (VQUIT), Ctrl-S (VSTOP), Ctrl-Q (VSTART) and
  SIGWINCH has been introduced. There have been a long standing issue
  that these keys/signals are handled only when app calls read() or
  select(). Now, these work even if app does not call read() or select().

- fchmodat(2) now has limited support for the AT_SYMLINK_NOFOLLOW flag.

- Cygwin now recognizes native Windows AF_UNIX sockets (as regular
  files, not as socket files).  This allows tools like 'ls' and 'rm'
  to work.

What changed:
-------------

- Allow ~5000 child processes per process on 64 bit, ~1200 child processes
  per process on 32 bit.  So far, only 256 child processes per process were
  supported.

- A few FAQ updates.

- Have tmpfile(3) make use of Win32 FILE_ATTRIBUTE_TEMPORARY via open(2)
  flag O_TMPFILE.
  Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247304.html

- Utilize Windows 10 1809 FILE_DISPOSITION_IGNORE_READONLY_ATTRIBUTE
  flag to allow simpler unlink of files with DOS readonly flags set.

- getdtablesize(3), sysconf(_SC_OPEN_MAX), and
  getrlimit(RLIMIT_NOFILE) now return the true limit on the number of
  open descriptors, 3200.  Previously they returned the current size
  of Cygwin's internal file descriptor table, which can grow
  dynamically.

- facl(2) now fails with EBADF on a file opened with O_PATH.


Bug Fixes
---------

- Iterate at least 4 times over pthread_key_t destructors per POSIX.

- The pthread_yield declaration in pthread is now visible by default
  or when defining _BSD_SOURCE, too.

- Fix SEGV in modfl call.
  Addresses: https://cygwin.com/pipermail/cygwin/2020-August/246056.html

- Fix a collision of offical and internally used file flags.
  Addresses: https://cygwin.com/pipermail/cygwin/2020-September/246174.html

- Fix assertion failure on an invalid path under /proc/<pid>/fd/.
  Addresses: https://cygwin.com/pipermail/cygwin/2020-September/246160.html

- Fix crash on stat(2)'ing /dev/ptmx on 32 bit.
  Addresses: https://cygwin.com/pipermail/cygwin/2020-September/246218.html

- Fix return value of sqrtl on negative infinity.
  Addresses: https://cygwin.com/pipermail/cygwin/2020-October/246606.html

- Fix a path handling problem if there is a WSL symlink in PATH.
  Addresses: https://cygwin.com/pipermail/cygwin/2020-December/246938.html

- Fix a bug in fstatat(2) on 32 bit that could cause it to return garbage.
  Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247399.html

- Fix the errno when a path contains .. and the prefix exists but is
  not a directory.
  Addresses: https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00214.html

- Fix the return value when ptsname_r(3) is called with a bad file descriptor
  Addresses: https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00245.html

- Fix path handling in case the Cygwin installation dir is accessed via
  a Windows junction point.
  Addresses: https://cygwin.com/pipermail/cygwin-developers/2021-February/012054.html

- Fix potential handle leaks when dup'ing descriptors
  Addresses: https://cygwin.com/pipermail/cygwin-developers/2021-February/012041.html

- Fix a bug that could cause fstat(2) to return incorrect results on a FIFO.

- Fix some system calls on AF_LOCAL sockets that are not socket files.
  Addresses: https://cygwin.com/pipermail/cygwin-developers/2021-February/012066.html

- Fix access to block devices under /proc/sys.
  Addresses: https://sourceware.org/pipermail/cygwin-patches/2020q4/010843.html