RNG state not preserved in callbacks

[wch]

The RNG state is not always preserved when executing callbacks. This is similar to rstudio/shiny#1768.

This example sets the value of .Random.seed in a callback, but it appears to not have an effect:

{
  set.seed(100)
  oldseed <- .GlobalEnv$.Random.seed
  set.seed(1234)

  later(function() {
    .GlobalEnv$.Random.seed <- oldseed
  })
  run_now()
}
identical(oldseed, .GlobalEnv$.Random.seed)
# [1] FALSE

If you call httpuv::getRNGState() in the callback, it fixes the problem. (That function was added to deal with problems just like this).

{
  set.seed(100)
  oldseed <- .GlobalEnv$.Random.seed
  set.seed(1234)

  later(function() {
    .GlobalEnv$.Random.seed <- oldseed
    httpuv::getRNGState()
  })
  run_now()
}
identical(oldseed, .GlobalEnv$.Random.seed)
# [1] TRUE

When this is fixed, we also need to make sure that it fixes it when the input handler executes the callback. To test it, run this at the console. Just run the first part, wait for a moment, and then run the last line.

set.seed(100)
oldseed <- .GlobalEnv$.Random.seed
set.seed(1234)

later(function() {
  .GlobalEnv$.Random.seed <- oldseed
})

# Wait for a moment before running this
identical(oldseed, .GlobalEnv$.Random.seed)