Use built-in callback cancellation from later

[wch]

Closes #114.

Previously, instead of cancelling the callback immediately, the canceller function would simply assign func <<- NULL. That would make the actual callback (which in turn calls func()) effectively a no-op, but it would still exist and consume memory.

This change uses the built-in later callback cancellation added in 1.0.0.

With this change, the test code from #114 (comment) does not leak.

library(pryr)
library(pool)
library(dplyr)

pool <- dbPool(RSQLite::SQLite(), dbname = ":memory:")

# Use dplyr syntax to copy mtcars to object
db_desc(pool)
copy_to(pool, mtcars, "mtcars", temporary = FALSE)

old.mem <- pryr::mem_used()
highest.mem <- old.mem
when <- Sys.time()
i <- 0
while (TRUE) {
  Sys.sleep(0.5)
  x <- dbGetQuery(pool, "SELECT * FROM mtcars LIMIT 5;")
  new.mem <- pryr::mem_used()
  if (new.mem > highest.mem) {
    highest.mem <- new.mem
    when <- as.character(Sys.time())
  }
  diff <- new.mem - old.mem
  old.mem <- new.mem
  
  cat(as.character(Sys.time()), ' Highest: ', highest.mem, ' at ', when, '. Mem now: ', new.mem, ' (', diff, ')\n', sep='')
}

Output:

2020-12-10 10:14:35 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095472 (11408)

2020-12-10 10:14:35 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095312 (-160)
2020-12-10 10:14:36 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (120)
2020-12-10 10:14:36 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095368 (-64)
2020-12-10 10:14:37 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (64)
2020-12-10 10:14:37 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)
2020-12-10 10:14:38 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)
2020-12-10 10:14:39 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)
2020-12-10 10:14:39 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)
2020-12-10 10:14:40 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)
2020-12-10 10:14:40 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)
2020-12-10 10:14:41 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)
2020-12-10 10:14:41 Highest: 79095472 at 2020-12-10 10:14:35. Mem now: 79095432 (0)

---

Note: Using the CRAN version of pool (0.1.5), I tried using a smaller idleTimeout, as in the following:

pool <- dbPool(RSQLite::SQLite(), dbname = ":memory:", idleTimeout = 2)

I expected it to release the memory after 2 seconds, but that doesn't seem to be the case, and I'm not sure why.

[jcheng5]

Nice work @wch!

[wch]

Oh, I know why the callback was never invoked even with the shorter timeout: later::run_now() is never called and the console isn't idle, so there's not an opportunity for the callbacks to be invoked. That shouldn't be a problem in a Shiny app, though.

In a Shiny app, when pool is used with the default settings, it should release the memory after the 60 seconds.