[ARROW-15841] [R] Implement SafeCallIntoR to safely call the R API from another thread

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 8.0.0
• Component/s: R
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/20138
• Assignee: Dewey Dunnington
• Reporter: Dewey Dunnington
• Created: 03/Mar/22 18:14
• Updated: 11/Jan/23 11:40
• Resolved: 07/Apr/22 16:50

Description

In ARROW-9235 we have a problem where we need to evaluate code (may be C++ that calls the R API in a way that may allocate R memory or literal R code) on the R main thread. As discussed in the pull request ( https://github.com/apache/arrow/pull/12323#issuecomment-1054726309 ), there are other times that this has come up and will come up in the future (e.g., evaluating user-defined R functions from the query engine, implementing extension types from R).

After discussing with westonpace, it seems like it is worth investing in a SafeCallFromR() C++ function that lives in the R bindings. The initial approach (that may need to be altered as we go) will be to

• have a global object that is initialized at the beginning of a C++ call from R to specific functions (and released on exit) that maintains some list of tasks that need to run on the R main thread and an event loop to periodically evaluate them while background tasks are running.
• define a SafeCallFromR() C++ function that adds a task to the global object and waits for the result to come back.

(with apologies to Weston if I mischaracterized our conversation!)

Issue Links

• links to: GitHub Pull Request #12558