use SystemTime instead of Instant

[laptou]

This branch is based on the master branch in my fork, so it also contains the changes from #2405.
In this PR, I changed all instances of Instant to SystemTime in the Kademlia behaviour, because I am trying to make a RecordStore that serializes the records to an SQLite database, but Instants are extremely resistant to serialization. For now, I have only modified the structures used by Kademlia and their transitive dependencies, so there are still many uses of Instant in the ping, mDNS, and gossipsub behaviours.

[thomaseizinger]

I think we only use Instant across the codebase because SystemTime is a very recent addition.

I can't think of any downside on using SystemTime over Instant so feel free to go ahead with these changes :)

[thomaseizinger]

IF we end up merging this, we need to use instant::SystemTime here.

[thomaseizinger]

This seems to be an erroneous change :D

[thomaseizinger]

This too.

[mxinden]

RecordStore that serializes the records to an SQLite database

Great to see this happening @laptou!

I think we only use Instant across the codebase because SystemTime is a very recent addition.

SystemTime is not guaranteed to be monotonic:

Distinct from the Instant type, this time measurement is not monotonic. This means that you can save a file to the file system, then save another file to the file system, and the second file has a SystemTime measurement earlier than the first. In other words, an operation that happens after another operation in real time may have an earlier SystemTime!

https://doc.rust-lang.org/nightly/std/time/struct.SystemTime.html

I am not sure whether we rely on this property anywhere within the codebase.

I am trying to make a RecordStore that serializes the records to an SQLite database

Would it not be enough to use SystemTime in the serialization logic only, i.e. transform an Instant to a SystemTime via Duration when serializing and deserializing a record?

[tomaka]

Replacing Instant with SystemTime is most certainly wrong. At any point, the value returned by SystemTime::now() can change. Maybe one moment SystemTime::now() returns December 29th 2021, and the next moment it returns June 2nd 1995, you can't know.

In order to serialize an Instant, one proper way is to do something like:

let now = Instant::now();
let now_system_time = SystemTime::now();
if instant > now {
    now_system_time + (instant - now)
} else {
    now_system_time - (now - instant)
}

[thomaseizinger]

Learned something today, thanks for the comments!