Provide host simulation start time as an option in shadow config file

[jerry855202]

Currently, the start time within each node is hard-coded by ‎EMUTIME_SIMULATION_START in emulated_time.rs to be 00:00:00 UTC, 1 January, 2000.
Right now if one have to change the start time within the node environment, they'd have to change this constant and recompile each time. It seems to me that it might be useful to add this as an environment option.
For example, this would probably be useful for people testing w/ things like SSL certificates.

[robgjansen]

Interesting idea, thanks for raising!

@stevenengler @sporksmith can you think of anything that would break inside of Shadow if we made EMUTIME_SIMULATION_START configurable and the user chose a weird time? Of course they might choose a time that breaks their own apps they are trying to run in Shadow, but I think that's OK as long as Shadow itself can gracefully handle different times.

[sporksmith]

I think we'll also need to plumb this through to the shim side, particularly for the shim logger. It'd probably make sense to add it to the Manager's shared memory struct.

[sporksmith]

For 3., could we validate the user input and consider the configuration invalid if the simulation end time overflows (is >=2038)?

I think this is somewhat orthogonal to the parent feature request; e.g. a user could already set a multi-year end time to end up in > 2038. FWIW I'd also lean towards continuing to allow it and to start shaking out those bugs since 2038 is only 14 years in the future (!!), and shadow could be useful for testing other software for 2038 bugs.

[stevenengler]

From looking at the code, it seems like a fair amount of work to plumb this option around everywhere (EMUTIME_SIMULATION_START is also aliased as EmulatedTime::SIMULATION_START). We also have some other constants that indirectly depend on this value:

// cbindgen refuses to do the arithmetic here, so we we pre-compute,
// and validate in the static assertion below.
pub const SIMTIME_MAX: CSimulationTime = 17500059273709551614u64;
const _: () =
    assert!(SIMTIME_MAX == emulated_time::EMUTIME_MAX - emulated_time::EMUTIME_SIMULATION_START);

If this option isn't needed often, I think it would be a much easier path to just recompile as needed, or to generate the TLS certs using something like faketime. Otherwise yeah I think making this a configuration option is possible, but it would be a bit painful and would take some work.

[jerry855202]

I'm not particularly familiar with the code base yet, so please bear with me if I asked something trivial.

If it would be easier to recompile on-deamnd, are we able to get the desired result just by changing the value for ‎SIMULATION_START_SEC and EMUTIME_SIMULATION_START‎ in emulated_time.rs, on line 27 and 36 respectrively?

I've searched the codebase very briefly for the two symbols, and also 946684800u64, and from what I can tell, everything else is referenced from these values.

[stevenengler]

If it would be easier to recompile on-deamnd, are we able to get the desired result just by changing the value for ‎SIMULATION_START_SEC and EMUTIME_SIMULATION_START‎ in emulated_time.rs, on line 27 and 36 respectrively?

I think you'll need to update those two values and the value of SIMTIME_MAX in src/lib/shadow-shim-helper-rs/src/simulation_time.rs, otherwise an assertion will fail. Otherwise I think that's all you need to change. Make sure to run ./setup test after to make sure nothing broke.

I'm not particularly familiar with the code base yet, so please bear with me if I asked something trivial.

It's not trivial so don't worry. The reason it's overly complicated is because we need to expose these constants in both Shadow's C code and Rust code (and to do so in a way that works with cbindgen). As Shadow moves more code to Rust, hopefully we can simplify this some day.

[jerry855202]

I think you'll need to update those two values and the value of SIMTIME_MAX in src/lib/shadow-shim-helper-rs/src/simulation_time.rs, otherwise an assertion will fail. Otherwise I think that's all you need to change. Make sure to run ./setup test after to make sure nothing broke.

Thank you. I'll try to patch these values out, and if anything interesting comes up, I'll update them in this thread.