xapi-project / xen-api / 11571363638
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CA-400199: open /dev/urandom on first use and use an input channel to reduce number of syscalls (#6085)

This is an alternative to
https://github.com/xapi-project/xen-api/pull/6077

2 recent optimizations have changed the Uuidx module to open
  /dev/urandom once on startup, instead of every time a value was
  requested.

However 'networkd_db' runs in the installer environment, inside a chroot
where /dev/urandom is not available.

Open /dev/urandom on first use instead.

Simplify the code and use a single implementation for both fast and
secure urandom generation:
* use a mutex to protect accesses to global urandom state
* use an input channel, rather than a Unix file descriptor, this allows
us to read many bytes in one go, and then generate multiple random
numbers without having to make syscalls that often

(syscalls are slow in this case because they require releasing the
runtime mutex, which gives another thread the opportunity to run for
50ms).

Fixes: a0176da73 ("CP-49135: open /dev/urandom just once")
Fixes: a2d9fbe39 ("IH-577 Implement v7 UUID generation")
Fixes: 6635a00d6 ("CP-49136: Introduce PRNG for generating non-secret
UUIDs")

This is slightly slower than before, but still fast enough:
```
│  uuidx creation/Uuidx.make            │             0.0004 mjw/run│            16.0001 mnw/run│            105.8801 ns/run│
│  uuidx creation/Uuidx.make_uuid_urnd  │             0.0004 mjw/run│            16.0001 mnw/run│            105.1474 ns/run│
```

Previously this used to take ~88ns, so in fact the difference is barely
noticable.

Also remove the feature flag: the previous change was feature flagged
too, but broke master anyway (I wouldn't have though anything *doesn't*
have /dev/urandom available, and didn't feature flag that part, because
in general it is not possible to feature flag startup code without
races).

`networkd_db` now doesn't try to open this anymore:
```
strace -e openat -e open networkd_db
open("/etc/ld.so.cache", O_RDONLY|O_CLO... (continued)

3462 of 4423 relevant lines covered (78.27%)

0.78 hits per line

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1 python3.11 - 11571363638.1 37
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