fix(execution): correct sends_stopped in bulk and schedule_from completion signatures

[arpittkhandelwal]

Problem Summary

bulk_sender and schedule_from_sender incorrectly hard-coded:

static constexpr bool sends_stopped = false;

However, both internally propagate cancellation (set_stopped)
from upstream senders to downstream receivers.

This creates a mismatch between the static contract (completion signatures)
and runtime behavior, violating the P2300 std::execution specification
and leading to undefined behavior.

Why This Matters

Completion signatures define what signals a sender may emit.

If sends_stopped is declared false:

• Downstream receivers may NOT implement set_stopped()
• But at runtime, set_stopped() is still invoked
• This leads to calls into receivers that are not prepared

=> Undefined behavior

Fix: bulk.hpp

bulk should transparently propagate stopping behavior
from its upstream sender.

BEFORE:
static constexpr bool sends_stopped = false;

AFTER:
static constexpr bool sends_stopped =
sends_stopped_of_v<Sender, Env>;

Fix: schedule_from.hpp

schedule_from can receive cancellation from:

1. Predecessor sender
2. Scheduler sender

So it must reflect both.

BEFORE:
static constexpr bool sends_stopped = false;

AFTER:
static constexpr bool sends_stopped =
sends_stopped_of_v<Sender, Env> ||
sends_stopped_of_v<scheduler_sender_type, Env>;

Not Changed

as_sender, keep_future:

These wrap HPX futures, which:

• Do NOT support cancellation
• Never call set_stopped()

So this remains correct:

static constexpr bool sends_stopped = false;

Tests Added

File:
libs/core/execution/tests/unit/algorithm_bulk.cpp

--- Static Tests ---

1. Non-stopping upstream -> remains false
   static_assert(
   !check_sends_stopped<
   bulk(just(42), {})

   ()
   );

2. Stopping upstream -> propagates true
   static_assert(
   check_sends_stopped<
   bulk(stopped_sender_with_value_type{}, {})

   ()
   );

3. Chained bulk layers propagate correctly
   static_assert(
   check_sends_stopped<
   bulk(
   bulk(
   stopped_sender_with_value_type{},
   {}
   ),
   {}
   )

   ()
   );

--- Runtime Test ---

Ensure set_stopped is actually delivered

{
auto sender =
bulk(
stopped_sender_with_value_type{},
{}
);

auto op = connect(sender, expect_stopped_receiver{});
start(op);

}

Key Insight

Senders must NEVER lie in their completion signatures.

If set_stopped can happen at runtime,
sends_stopped MUST be true at compile time.

bulk and schedule_from are transparent propagators,
so their sends_stopped must reflect upstream behavior.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[Copilot]

Pull request overview

This PR fixes incorrect completion signature contracts in HPX std::execution-style algorithms by ensuring bulk and schedule_from correctly advertise sends_stopped when they can propagate set_stopped at runtime, aligning the static sender contract with actual behavior and avoiding undefined behavior.

Changes:

• Update bulk_sender completion signatures to propagate sends_stopped from the upstream sender.
• Update schedule_from_sender completion signatures to propagate sends_stopped from both the predecessor sender and the scheduler’s schedule() sender.
• Add unit coverage in algorithm_bulk.cpp to validate bulk’s corrected sends_stopped behavior (static checks + runtime stopped delivery).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
libs/core/execution/include/hpx/execution/algorithms/bulk.hpp	Fix bulk_sender completion signatures to reflect upstream sends_stopped.
libs/core/execution/include/hpx/execution/algorithms/schedule_from.hpp	Fix schedule_from_sender completion signatures to reflect stopped propagation from predecessor and scheduler sender.
libs/core/execution/tests/unit/algorithm_bulk.cpp	Add tests verifying bulk now correctly advertises and forwards set_stopped (non-stdexec path).