Bug: ggsave() does not work when called in mclapply() in RStudio IDE (same code works perfect at CLI)

RStudio IDE
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2

I can reproduce the "stall" in RStudio 1.1.453 on Linux with R 3.5.1.

A few comments:

  1. When the forked, child processes that parallel::mclapply() spawns off die / crash, the corresponding values will be NULL. This is a known behavior.

  2. The fact that you're observing NULL at random, suggests that the forked processes have crashed.

  3. Why did they crash? In ?parallel::mclapply, there a 'Warning' section that says "It is strongly discouraged to use these functions in GUI or embedded environments, because it leads to several processes sharing the same GUI which will likely cause chaos (and possibly crashes)." This could very well be the reason. I haven't investigate this, but I'd assume the RStudio GUI folks should be able to comment on whether the RStudio Console supports forked processing or not.

  4. That same 'Warning' section also says "Child processes should never use on-screen graphics devices." Note that with ggplot2 3.0.0, ggsave() actually opens an on-screen graphics device, e.g. https://twitter.com/ilarischeinin/status/1024643217315311616 and https://github.com/tidyverse/ggplot2/issues/2794. When I run your code in "terminal" R 3.5.1 on Linux with ggplot2 3.0.0, I get:

Error in (function (display = "", width, height, pointsize, gamma, bg,  : 
  a forked child should not open a graphics device

This can be avoided by wrapping ggsave(...) within R.devices::suppressGraphics() as the below example shows.

Now, instead of forked parallel processing, you can use background R session for parallelization using so called PSOCK clusters. Here's a simple way to do it:

library(ggplot2)

library(future.apply)
plan(multisession)  ## parallelize using background R sessions

y <- future_lapply(0:4, function(n) {
    df <- data.frame(x = runif(10), y = runif(10))
    p  <- ggplot(df, aes(x, y)) + geom_point()

    R.devices::suppressGraphics({
        ggsave(
            paste0('mmclapply-', n, '.png'),
            plot   = p,
            device = 'png',
            width  = 4,
            height = 4
        )
    })	    

    n
})

This works on any operating system.

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