[Xen-API] timing loops

[Dave Scott <Dave.Scott@xxxxxxxxxxxxx>]

Hi,

With all the recent xapi disaggregation work, are we now more vulnerable to 
failures induced by moving the system clock around, affecting timeout logic in 
our async-style interfaces where we wait for 'n' seconds for an event 
notification?

I've recently added 'oclock' as a dependency which gives us access to a 
monotonic clock source, which is perfect (I believe) for reliably 'timing out'. 
I started a patch to convert the whole codebase over but it was getting much 
too big and hard to test because sometimes we really do want a calendar date, 
and other times we really want a point in time.

Maybe I should make a subset of my patch which fixes all the new timing loops 
that have been introduced. What do you think? Would you like to confess to 
having written:

let start = Unix.gettimeofday () in
while (not p && (Unix.gettimeofday () -. start < timeout) do Thread.delay 1. 
done

I've got a nice higher-order function to replace this which does:

let until p timeout interval =
  let start = Oclock.gettime Oclock.monotonic in
  while (not p && (Int64.(to_float (sub (Oclock.gettime Oclock.monotonic) 
start) / 1e9) < timeout) do Thread.delay 1. Done

I believe this is one of many things that lwt (and JS core) does a nice job of.

Cheers,
Dave

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