atheriel / sigterm

An R package for capturing and detecting SIGTERM to enable graceful shutdown.

sigterm

On Unix-like operating systems, a process can be sent the SIGTERM signal to initiate a graceful shutdown. This is widely used by container platforms like Docker and Kubernetes. But for R, this abruptly ends the process – no on.exit() handlers will run for the current function, no finalizers will be triggered, nor will the .Last() handler be called.

This makes it hard to write R programs that clean up after themselves when asked, for example closing network or database connections, writing intermediate outputs to file, or finishing in-progress work that is difficult to restart.

sigterm is an extremely simple package that provides a way to implement just these kinds of operations in response to SIGTERM: when the package is loaded, it will install a signal handler that prevents R from immediately exiting. Users can then periodically check whether a SIGTERM signal has been received and respond appropriately.

Installation

sigterm is not yet available on CRAN. You can install it from GitHub with:

# install.packages("devtools")
devtools::install_github("atheriel/sigterm")

Usage

sigterm has only one exported function:

library(sigterm)
#> Installed the SIGTERM handler.
has_sigterm_flag()
#> [1] FALSE

This function will return TRUE when R has received the signal, for example when sent through the shell:

system2("kill", c("-TERM", Sys.getpid()))
has_sigterm_flag()
#> [1] TRUE

Keep in mind that this package prevents R from exiting at all. You may want to call quit() to do so instead, or simply return from the current function or loop; it will depend on the context.

For example:

for (i in 1:100) {
  # Do some important work that should not be interrupted mid-task.
  
  if (has_sigterm_flag()) {
    message("Shutting down after task ", i, "...")
    break
  }
}
#> Shutting down after task 1...

Simply refusing to shut down will probably result in the process being sent SIGKILL after some timeout (30 seconds by default in Kubernetes), which cannot be caught and will terminate R immediately.

License

Copyright 2021 Aaron Jacobs

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.