Add require_started sender adaptor

[msimberg]

Fixes #790.

Comments on the name are especially welcome. I found require_started quite descriptive as it signals that the user is required to make sure that the sender is started. It also mirrors ensure_started which actually connects and starts a sender.

The require_started sender has an empty state to make it somewhat sensible to detect unstarted senders. E.g. just() is copyable, but it's also possible to connect it after moving it. This is not allowed by the require_started sender. The require_started sender stores the wrapped sender internally in an optional, and when moving a require_started sender the optional is emptied.

A require_started sender can be allowed to not be started, but it has to be explicitly asked for with discard.

When a require_started sender is not connected, or when it's connected but the operation state not started, the destructor will terminate (by default). Assigning to unstarted senders also leads to termination.

Mainly for testing purposes the behaviour on unstarted senders is customizable. The default behaviour is to terminate, since that reports the error immediately and cannot be ignored. The behaviour can be changed to throw instead with a flag passed to the sender constructor, or changed afterwards with set_mode. I don't expect this second mode to be used much in practice, but it's useful to have as an alternative at least for testing. The choice of mode is disabled with stdexec, since stdexec requires nothrow destructible senders. In this case terminate is always used.

[msimberg]

cscs-ci run

[pika-bot]

Performance test report

pika Performance

Comparison

BENCHMARK	NO-EXECUTOR
Future Overhead - Create Thread Hierarchical - Latch	-

Info

Property	Before	After
pika Datetime	2023-08-21T11:44:55+00:00	2023-11-24T16:29:13+00:00
pika Commit	02f9de8	562edc0
Hostname	nid01181	nid00459
Clustername	daint	daint
Envfile
Compiler	/apps/daint/SSL/pika/spack/lib/spack/env/clang/clang++ 11.0.1	/apps/daint/SSL/pika/spack/lib/spack/env/clang/clang++ 11.0.1
Datetime	2023-08-21T13:50:51.685166+02:00	2023-11-24T17:41:58.929351+01:00

Explanation of Symbols

Symbol	MEANING
=	No performance change (confidence interval within ±1%)
(=)	Probably no performance change (confidence interval within ±2%)
(+)/(-)	Very small performance improvement/degradation (≤1%)
+/-	Small performance improvement/degradation (>10%)
++/--	Large performance improvement/degradation (>10%)
+++/---	Very large performance improvement/degradation (>10%)
?	Probably no change, but quite large uncertainty (confidence interval with ±5%)
??	Unclear result, very large uncertainty (±10%)
???	Something unexpected…

[pika-bot]

Performance test report

pika Performance

Comparison

BENCHMARK	NO-EXECUTOR
Future Overhead - Create Thread Hierarchical - Latch	-

Info

Property	Before	After
pika Datetime	2023-08-21T11:44:55+00:00	2023-11-27T10:48:01+00:00
pika Commit	02f9de8	2d2f6d7
Compiler	/apps/daint/SSL/pika/spack/lib/spack/env/clang/clang++ 11.0.1	/apps/daint/SSL/pika/spack/lib/spack/env/clang/clang++ 11.0.1
Clustername	daint	daint
Envfile
Hostname	nid01181	nid01537
Datetime	2023-08-21T13:50:51.685166+02:00	2023-11-27T11:57:35.682158+01:00

Explanation of Symbols

Symbol	MEANING
=	No performance change (confidence interval within ±1%)
(=)	Probably no performance change (confidence interval within ±2%)
(+)/(-)	Very small performance improvement/degradation (≤1%)
+/-	Small performance improvement/degradation (>10%)
++/--	Large performance improvement/degradation (>10%)
+++/---	Very large performance improvement/degradation (>10%)
?	Probably no change, but quite large uncertainty (confidence interval with ±5%)
??	Unclear result, very large uncertainty (±10%)
???	Something unexpected…

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run

[pika-bot]

Performance test report

pika Performance

Comparison

BENCHMARK	NO-EXECUTOR
Future Overhead - Create Thread Hierarchical - Latch	-

Info

Property	Before	After
pika Datetime	2023-08-21T11:44:55+00:00	2023-12-04T12:47:56+00:00
pika Commit	02f9de8	a5d9162
Compiler	/apps/daint/SSL/pika/spack/lib/spack/env/clang/clang++ 11.0.1	/apps/daint/SSL/pika/spack/lib/spack/env/clang/clang++ 11.0.1
Datetime	2023-08-21T13:50:51.685166+02:00	2023-12-04T13:56:54.067011+01:00
Clustername	daint	daint
Hostname	nid01181	nid01181
Envfile

Explanation of Symbols

Symbol	MEANING
=	No performance change (confidence interval within ±1%)
(=)	Probably no performance change (confidence interval within ±2%)
(+)/(-)	Very small performance improvement/degradation (≤1%)
+/-	Small performance improvement/degradation (>10%)
++/--	Large performance improvement/degradation (>10%)
+++/---	Very large performance improvement/degradation (>10%)
?	Probably no change, but quite large uncertainty (confidence interval with ±5%)
??	Unclear result, very large uncertainty (±10%)
???	Something unexpected…

[msimberg]

cscs-ci run