Progress bar not completing on Windows

[mihaiconstantin]

The following code will fail on Windows.

# Create a specification object.
specification <- Specification$new()

# Set the cluster requirements.
specification$set_cores(2)
specification$set_type("psock")

# Create an asynchronous backend.
backend <- AsyncBackend$new()

# Create a progress-tracking context.
context <- ProgressDecorator$new()

# Create a bar.
bar <- ModernBar$new()

# Set the backend.
context$set_backend(backend)

# Start the backend.
context$start(specification)

# Set the bar.
context$set_bar(bar)

# Configure the bar.
context$configure_bar(show_after = 0)

# Run a task.
context$sapply(1:1000, function(x) { Sys.sleep(0.01); x })

# Get the output.
output <- context$get_output()

# Close the backend.
context$stop()

It most likely has to do with the private method .decorate() in the
ProgressDecorator class. It seems that on Unix systems, the injection

parabar/R/ProgressDecorator.R

Lines 153 to 161 in e0da7d9

	fun_body[[length_fun_body + 1]] <- substitute(
	# The injected expression.
	on.exit({
	cat("\n", file = log, sep = "", append = TRUE)
	}),
	# The environment to use for substitution.
	parent.frame(n = 1)
	)

works as expected. However, on Windows, it leads to race conditions. Therefore,
the temporary log file does not contain as many new lines as the tasks that were
executed. As a result, the task finishes executing, but the progress bar is not
updated to reflect this. Then, the function hangs, waiting for the progress bar
to be terminated.

Two possible ways I can think to tackle this:

1. Ensure the log file is locked while a process writes to it (e.g., see
   filelock package). Will have to understand how much overhead this adds.
   Worst-case scenario, it may only be used for Windows systems.

2. In the .show_progress() of the ProgressDecorator class, check the task
   state and break out of the loop to update the progress bar if the task
   is finished. This is not desirable, however, as it would mean that the
   progress bar would not reflect the actual progress of the task.

parabar/R/ProgressDecorator.R

Lines 192 to 207 in e0da7d9

	while (tasks_processed < total) {
	# Get the current number of tasks processed.
	current_tasks_processed <- length(readLines(log, warn = FALSE))
	# Redraw the bar only if the number of tasks processed has increased.
	if (current_tasks_processed > tasks_processed) {
	# Update the number of tasks processed.
	tasks_processed <- current_tasks_processed
	# Update the progress bar to completed state.
	private$.bar$update(tasks_processed)
	}
	# Wait a bit.
	Sys.sleep(timeout)
	}

Thanks to @eschat for uncovering this.