These are available in sandboxes without access to files or
devices. Also [they are safer and more straightforward](https://en.wikipedia.org/wiki/Entropy-supplying_system_calls)
to use than `/dev/urandom` as reading from a file has quite a few edge
cases:
- Linux: `getrandom(buf, buflen, 0)`. [getrandom(2)](http://man7.org/linux/man-pages/man2/getrandom.2.html)
was introduced in version 3.17 of the Linux kernel.
- OpenBSD: `getentropy(buf, buflen)`. The [getentropy(2)](http://man.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man2/getentropy.2)
function appeared in OpenBSD 5.6.
- FreeBSD and NetBSD: `sysctl(KERN_ARND)`. Not sure when this was added
but it has existed for quite a while.
Alternatives:
- Linux has sysctl `CTL_KERN` / `KERN_RANDOM` / `RANDOM_UUID`
which gives 16 bytes of randomness. This may be available
on older kernels, however [sysctl is deprecated on Linux](https://lwn.net/Articles/605392/)
and even removed in some distros so we shouldn't use it.
Add tests for `GetOSRand()`:
- Test that no error happens (otherwise `RandFailure()` which aborts)
- Test that all 32 bytes are overwritten (initialize with zeros, try multiple times)
Discussion:
- When to use these? Currently they are always used when available.
Another option would be to use them only when `/dev/urandom` is not
available. But this would mean these code paths receive less testing,
and I'm not sure there is any reason to prefer `/dev/urandom`.
Closes: #9676
b471daf Adddress nits, use asyncio signal handling, create_task (Bob McElrath)
4bb7d1b Add python version checks and 3.4 example (Bob McElrath)
5406d51 Rewrite to not use Polling wrapper for asyncio, link to python2.7 example (Bob McElrath)
5ea5368 ZMQ example using python3 and asyncio (Bob McElrath)
3ddfe29 netbase: Do not print an error on connection timeouts through proxy (Wladimir J. van der Laan)
13f6085 netbase: Make InterruptibleRecv return an error code instead of bool (Wladimir J. van der Laan)
f6d18f5 Qt/Intro: Explain a bit more what will happen first time (Luke Dashjr)
50c5657 Qt/Intro: Storage shouldn't grow significantly with pruning enabled (Luke Dashjr)
9adb694 Qt/Intro: Move sizeWarningLabel text into C++ code (Luke Dashjr)
* Extends -dns parameter (via fNameLookup) to control these two new
parameters in addition to -addnode, -connect, and -seednode
* Moves fNameLookup assignment earlier as needed
* Changes -proxy and -onion to use Lookup() instead of LookupNumeric()
Remove "nLowestTimestamp <= chainActive.Tip()->GetBlockTimeMax()" check from
importmulti, which is always true because nLowestTimestamp is set to the
minimum of the most recent block time and all the imported key timestamps,
which is necessarily lower than the maximum block time.
e662af3 Use 2 hour grace period for key timestamps in importmulti rescans (Russell Yanofsky)
38d3e9e [qa] Extend import-rescan.py to test imports on pruned nodes. (Russell Yanofsky)
c28583d [qa] Extend import-rescan.py to test specific key timestamps (Russell Yanofsky)
8be0866 [qa] Simplify import-rescan.py (Russell Yanofsky)
