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4150. The preconditions on run_loop::run() are too strict

Section: 33.11.1.4 [exec.run.loop.members] Status: New Submitter: Eric Niebler Opened: 2024-09-05 Last modified: 2024-09-06

Priority: Not Prioritized

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Discussion:

From sender-receiver/issues #280:

The precondition on run_loop::run() is that the state is starting. Given that run_loop::finish() puts the loop in the finishing state, the implication is that one cannot call finish() before calling run().

However, in most cases sync_wait is calling finish() on its run_loop before calling run(), violating this precondition. sync_wait does the following:

run_loop loop;
auto op = connect(sndr, sync_wait_receiver{&loop});
start(op);

loop.run();

If sndr completes synchronously, the sync_wait_receiver will call finish() on the loop before loop.run() is called, violating run's precondition.

Proposed resolution:

This wording is relative to N4988.

1. Modify 33.11.1.4 [exec.run.loop.members] as indicated:

   void run();
   

   -5- Preconditions: state is starting or finishing.

   -6- Effects: Sets the state to running. Then, equivalent to:

   while (auto* op = pop-front()) {
     op->execute();
   }
   

   -7- Remarks: When state changes, it does so without introducing data races.