CA-400199: open /dev/urandom on first use and use an input channel to reduce number of syscalls

[edwintorok]

This is an alternative to #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: a0176da ("CP-49135: open /dev/urandom just once")
Fixes: a2d9fbe ("IH-577 Implement v7 UUID generation")
Fixes: 6635a00 ("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_CLOEXEC) = 3
open("/lib64/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/librt.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/var/lib/xcp/networkd.db", O_RDONLY) = 3
+++ exited with 0 +++

[robhoes]

This feels a bit pointless now, as there is only one "make" function. Can we just have

let make () = of_bytes (generate 16) |> Option.get

?

[robhoes]

I can't see any problems with this, apart from the small simplification mentioned above. In general, I think it is better to not open files when a module is loaded in a library, as it can lead to unintended consequences (as we have seen), so this is an improvement.

[lindig]

Another problem with side effects in module initialisation is the somewhat unknown order in which modules are initialised.