This is probably the last feature release for the 1.x line for NNG.
This fixes quite a few bugs, the main new features here are around TLS support.

TLS 1.3 is supported for Mbed TLS
WolfSSL is now supported without an extra module. (You need to honor the licensing terms of WolfSSL itself however.)
TLS support for PSK (pre-shared keys)
A new nng_id_visit API is available for iterating over all nodes in an ID hash map.
An experimental (undocumented) but exposed "public" API for UDP is available

There are significant fixes for race conditions around close, and use-after-free.  Everyone is recommended to update.
More details:
What's Changed

Move supplemental platform APIs to core. by @gdamore in Move supplemental platform APIs to core. nanomsg/nng#1821
fixes #1808 nng_msg_insert: munmap_chunk(): invalid pointer by @gdamore in fixes #1808 nng_msg_insert: munmap_chunk(): invalid pointer nanomsg/nng#1823
macOS - fix deadlock on reqrep socket close by @alzix in macOS - fix deadlock on reqrep socket close nanomsg/nng#1824
Gdamore/win clock fix by @gdamore in Gdamore/win clock fix nanomsg/nng#1834
fixes #1827 Windows a deadlock on nng_close() by @gdamore in fixes #1827 Windows a deadlock on nng_close() nanomsg/nng#1828
Fixes typo that will cause error when comipling zerotier transport. by @PWisPeterWang in Fixes typo that will cause error when comipling zerotier transport. nanomsg/nng#1829
UDP: Introduce an experimental (undocumented for now) public API for … by @gdamore in UDP: Introduce an experimental (undocumented for now) public API for … nanomsg/nng#1838
fix testcase crash on NUTTX When log_buf is null for the first time. by @Meissi-jian in fix testcase crash on NUTTX When log_buf is null for the first time. nanomsg/nng#1844
fixes #1846 Add support for TLS PSK by @gdamore in fixes #1846 Add support for TLS PSK nanomsg/nng#1847
Merge WolfSSL support into mainstream, and relicense it under MIT. by @gdamore in Merge WolfSSL support into mainstream, and relicense it under MIT. nanomsg/nng#1850
idhash: add nng_id_visit API by @gdamore in idhash: add nng_id_visit API nanomsg/nng#1863
trial to fix winipc crash by @gdamore in trial to fix winipc crash nanomsg/nng#1848
TLS: enable TLS 1.3 for Mbed TLS. by @gdamore in TLS: enable TLS 1.3 for Mbed TLS. nanomsg/nng#1854
fixes #1866 clock_gettime is not used on Android, even though available by @gdamore in fixes #1866 clock_gettime is not used on Android, even though available nanomsg/nng#1870

New Contributors

@alzix made their first contribution in macOS - fix deadlock on reqrep socket close nanomsg/nng#1824
@PWisPeterWang made their first contribution in Fixes typo that will cause error when comipling zerotier transport. nanomsg/nng#1829
@Meissi-jian made their first contribution in fix testcase crash on NUTTX When log_buf is null for the first time. nanomsg/nng#1844

Full Changelog: nanomsg/nng@v1.8.0...v1.9.0