Cancellation propagation

Mircea Baja - 18 November 2025

# Cancellation propagation

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---

# Motivation

We're going to look at cases where cancellation is initiated
- who initiates it and why
- how it propagates:
  - who is told about the cancellation
  - in what order
  - and in what form

- Follows on:
  - the structured concurrency discussion
  - the cancellation at leaves
  - the description of stop_source, stop_token and stop_callback

---

# Kinds of result

When an asynchronous activity completes it can complete:
- **with a value**: (if you include `void` as a value) i.e. successfully
- **with an error**: the activity encountered an error
- **with a stop**: the activity was cancelled and the cancellation is complete

- when completing with a value or an error: the activity completed **with a
  result**

---

# Forms of propagation

- A `stop_source` has `.request_stop()` called. The users of the associated
  `stop_token` (children) can find out by:
  - polling the `stop_token`
  - or by a function being called via a `stop_callback` registration
- A parent is told by child that the child completed with a value, an error or
  a stop

- There can be (and usually is) a gap between a parent calling
  `stop_source.request_stop()` which calls the registered `stop_callback`
  functions inside AND the child telling the parent that the child completed

---

# From root

- Initiation and propagation from root

---

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  <text class="svg20251118-7-t1" x="30" y="45">stop_source: false</text>
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- root means e.g. a `run` blocking function that initiates the chain
- chain means a sequence of asynchronous work with a strict parent-child
  relationship:
  - the child is started by a parent
  - the child communicates back to the parent when it completes (with a result
    or with a stop)
  - the child does not outlive the parent
- `run` has a `stop_source` and some boolean to track that its first child
  completed
- if/when `run` decides to stop the chain it started

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- `request_stop()` is called on the `stop_source`. The leaf of the chain is the
  one that will react to this:
  - if it has a `stop_callback` registered it will be called (the only way for
    a blocked one in a single threaded case) OR when it tries to register a
    `stop_callback`
  - or when it checks `stop_token.stop_requested()`

---

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- when the leaf's `stop_callback` is called, the leaf initiates the
  cancellation, but it might return before the cancellation completes
- but when it does: it informs its parent that it completed with a stop

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  <text class="svg20251118-10-t1" x="20" y="25">run</text>
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- the parent destroys the child, it becomes itself the leaf of the chain
- in turn it propagates the completion with a stop to its parent and so on
- until we reach `run`

---

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  <text class="svg20251118-11-t1" x="20" y="25">run</text>
  <text class="svg20251118-11-t2" x="30" y="45">stop_source: true</text>
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- when we reach `run` the completion provided by `run` is called by the child
  of `run` (the last one remaining)
- run then sets the `done` flag
- if `run` is blocked waiting for the flag OR runs a loop processing work from
  a queue: it can exit that condition_variable::wait OR the loop
- and then it returns to the caller (e.g. a std::nullopt to indicate that it
  was stopped before it had a result)

---

# Through coroutines

- Propagation downhill through coroutines

---

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      fill: #ffffff;
    }
    .svg20251118-12-box_orange {
      stroke: #000000;
      stroke-width: 2;
      fill: #f5bb00;
    }
  </style>
  <defs>
    <marker id="svg20251118-12-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-12-arrow1"/>
    </marker>
    <marker id="svg20251118-12-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-12-arrow2"/>
    </marker>
  </defs>
  <rect x="10" y="5" width="180" height="78" class="svg20251118-12-box_white"/>
  <rect x="210" y="52" width="60" height="30" class="svg20251118-12-box_white"/>
  <path class="svg20251118-12-l1" d="M 170 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-12-arrow1)"/>
  <rect x="280" y="52" width="60" height="30" class="svg20251118-12-box_orange"/>
  <path class="svg20251118-12-l1" d="M 240 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-12-arrow1)"/>
  <rect x="350" y="52" width="60" height="30" class="svg20251118-12-box_orange"/>
  <path class="svg20251118-12-l2" d="M 310 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-12-arrow2)"/>
  <rect x="420" y="52" width="60" height="30" class="svg20251118-12-box_white"/>
  <path class="svg20251118-12-l2" d="M 380 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-12-arrow2)"/>
  <rect x="490" y="52" width="60" height="30" class="svg20251118-12-box_white"/>
  <path class="svg20251118-12-l1" d="M 450 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-12-arrow1)"/>
  <text class="svg20251118-12-t1" x="20" y="25">run</text>
  <text class="svg20251118-12-t1" x="30" y="45">stop_source: false</text>
  <text class="svg20251118-12-t1" x="30" y="65">done: false</text>
</svg>
</div>

- So we have a chain as before, but this time some of the activities are
  coroutines (marked in yellow)
- Children of a coroutine start at a `co_await` (marked with a purple arrow)
- Normally when a child of a coroutine indicates completion this results in the
  parent coroutine being resumed, but this is different when we want to stop

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-13-box" width="100%" viewBox="0 0 800 110">
  <style>
    #svg20251118-13-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-13-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-13-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-13-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-13-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-13-l2 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-13-arrow2 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-13-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-13-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
    .svg20251118-13-box_orange {
      stroke: #000000;
      stroke-width: 2;
      fill: #f5bb00;
    }
  </style>
  <defs>
    <marker id="svg20251118-13-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-13-arrow1"/>
    </marker>
    <marker id="svg20251118-13-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-13-arrow2"/>
    </marker>
  </defs>
  <rect x="10" y="5" width="180" height="78" class="svg20251118-13-box_white"/>
  <rect x="210" y="52" width="60" height="30" class="svg20251118-13-box_white"/>
  <path class="svg20251118-13-l1" d="M 170 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-13-arrow1)"/>
  <rect x="280" y="52" width="60" height="30" class="svg20251118-13-box_orange"/>
  <path class="svg20251118-13-l1" d="M 240 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-13-arrow1)"/>
  <rect x="350" y="52" width="60" height="30" class="svg20251118-13-box_orange"/>
  <path class="svg20251118-13-l2" d="M 310 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-13-arrow2)"/>
  <rect x="420" y="52" width="60" height="30" class="svg20251118-13-box_white"/>
  <path class="svg20251118-13-l2" d="M 380 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-13-arrow2)"/>
  <rect x="490" y="52" width="60" height="30" class="svg20251118-13-box_white"/>
  <path class="svg20251118-13-l1" d="M 520 83 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-13-arrow1)"/>
  <line class="svg20251118-13-l4" x1="435" y1="65" x2="445" y2="55"/>
  <line class="svg20251118-13-l4" x1="435" y1="55" x2="445" y2="65"/>
  <text class="svg20251118-13-t1" x="20" y="25">run</text>
  <text class="svg20251118-13-t2" x="30" y="45">stop_source: true</text>
  <text class="svg20251118-13-t1" x="30" y="65">done: false</text>
</svg>
</div>

When the `stop_source.request_stop()` is called, the leaf responds to that
and eventually completes to its parent with a stop

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-14-box" width="100%" viewBox="0 0 800 110">
  <style>
    #svg20251118-14-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-14-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-14-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-14-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-14-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-14-l2 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-14-arrow2 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-14-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-14-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
    .svg20251118-14-box_orange {
      stroke: #000000;
      stroke-width: 2;
      fill: #f5bb00;
    }
  </style>
  <defs>
    <marker id="svg20251118-14-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-14-arrow1"/>
    </marker>
    <marker id="svg20251118-14-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-14-arrow2"/>
    </marker>
  </defs>
  <rect x="10" y="5" width="180" height="78" class="svg20251118-14-box_white"/>
  <rect x="210" y="52" width="60" height="30" class="svg20251118-14-box_white"/>
  <path class="svg20251118-14-l1" d="M 170 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-14-arrow1)"/>
  <rect x="280" y="52" width="60" height="30" class="svg20251118-14-box_orange"/>
  <path class="svg20251118-14-l1" d="M 240 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-14-arrow1)"/>
  <rect x="350" y="52" width="60" height="30" class="svg20251118-14-box_orange"/>
  <path class="svg20251118-14-l2" d="M 310 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-14-arrow2)"/>
  <rect x="420" y="52" width="60" height="30" class="svg20251118-14-box_white"/>
  <path class="svg20251118-14-l1" d="M 450 83 c -5 30 -200 30 -206 -16" marker-end="url(#svg20251118-14-arrow1)"/>
  <line class="svg20251118-14-l4" x1="225" y1="65" x2="235" y2="55"/>
  <line class="svg20251118-14-l4" x1="225" y1="55" x2="235" y2="65"/>
  <text class="svg20251118-14-t1" x="20" y="25">run</text>
  <text class="svg20251118-14-t2" x="30" y="45">stop_source: true</text>
  <text class="svg20251118-14-t1" x="30" y="65">done: false</text>
</svg>
</div>

When we reach a coroutine, we propagate the completion with a stop to the
parent of the coroutine segment: which by destroying that coroutine will
destroy recursively the whole coroutine segment

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-15-box" width="100%" viewBox="0 0 800 110">
  <style>
    #svg20251118-15-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-15-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-15-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-15-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-15-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-15-l2 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-15-arrow2 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-15-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-15-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
    .svg20251118-15-box_orange {
      stroke: #000000;
      stroke-width: 2;
      fill: #f5bb00;
    }
  </style>
  <defs>
    <marker id="svg20251118-15-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-15-arrow1"/>
    </marker>
    <marker id="svg20251118-15-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-15-arrow2"/>
    </marker>
  </defs>
  <rect x="10" y="5" width="180" height="78" class="svg20251118-15-box_white"/>
  <rect x="210" y="52" width="60" height="30" class="svg20251118-15-box_white"/>
  <path class="svg20251118-15-l1" d="M 240 83 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-15-arrow1)"/>
  <line class="svg20251118-15-l4" x1="155" y1="65" x2="165" y2="55"/>
  <line class="svg20251118-15-l4" x1="155" y1="55" x2="165" y2="65"/>
  <text class="svg20251118-15-t1" x="20" y="25">run</text>
  <text class="svg20251118-15-t2" x="30" y="45">stop_source: true</text>
  <text class="svg20251118-15-t2" x="30" y="65">done: true</text>
</svg>
</div>

---

# From branched peers

- Initiation and propagation from branch peers
- Uses `wait_any` as an example

---

# From branched peers

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-1-box" width="100%" viewBox="0 0 800 180">
  <style>
    #svg20251118-1-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-1-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: middle;
      dominant-baseline: middle;
    }
    .svg20251118-1-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-1-arrow {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-1-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-1-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-1-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-1-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-1-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-1-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-1-arrow" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-1-arrow"/>
    </marker>
    <marker id="svg20251118-1-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-1-arrow2"/>
    </marker>
    <marker id="svg20251118-1-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-1-arrow3"/>
    </marker>
  </defs>
  <path class="svg20251118-1-l1" d="M 140 30 c 0 20 -10 20 -50 20 s -50 0 -50 20" marker-end="url(#svg20251118-1-arrow)"/>
  <path class="svg20251118-1-l2" d="M 140 30 v 40" marker-end="url(#svg20251118-1-arrow2)"/>
  <path class="svg20251118-1-l3" d="M 140 30 c 0 20 10 20 50 20 s 50 0 50 20" marker-end="url(#svg20251118-1-arrow3)"/>
  <path class="svg20251118-1-l1" d="M 40 113 c 0 20 10 20 50 20 s 50 0 50 20"/>
  <path class="svg20251118-1-l3" d="M 240 113 c 0 20 -10 20 -50 20 s -50 0 -50 20"/>
  <path class="svg20251118-1-l2" d="M 140 113 v 40" marker-end="url(#svg20251118-1-arrow2)"/>
  <path class="svg20251118-1-box_white" d="M 10 82 h 60 v 30 l -60 10 z"/>
  <rect x="110" y="82" width="60" height="30" class="svg20251118-1-box_white"/>
  <path class="svg20251118-1-box_white" d="M 210 82 h 60 v 30 l -60 10 z"/>
  <line class="svg20251118-1-l4" x1="15" y1="115" x2="25" y2="105"/>
  <line class="svg20251118-1-l4" x1="15" y1="105" x2="25" y2="115"/>
  <line class="svg20251118-1-l4" x1="215" y1="115" x2="225" y2="105"/>
  <line class="svg20251118-1-l4" x1="215" y1="105" x2="225" y2="115"/>
  <text class="svg20251118-1-t1" x="140" y="15">wait_any</text>
</svg>
</div>

- `wait_any` starts/branches into a number of children chains
- when one of the children completes, then the others need to be told to stop
- when all the children complete, then `wait_any` continues its own
  parent with the same kind of completion as the first completing child (e.g.
  value into value, error into error, stop into stop)

- another way of looking at this is to rotate the picture 90 degrees and look
  at the children chains of `wait_any`

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-3-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20251118-3-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-3-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-3-l0 {
      stroke: #f5bb00;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-3-arrow0 {
      stroke: #f5bb00;
      stroke-width: 1;
      fill: #f5bb00;
    }
    .svg20251118-3-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-3-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-3-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-3-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-3-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-3-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-3-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-3-arrow0" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-3-arrow0"/>
    </marker>
    <marker id="svg20251118-3-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-3-arrow1"/>
    </marker>
    <marker id="svg20251118-3-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-3-arrow2"/>
    </marker>
    <marker id="svg20251118-3-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-3-arrow3"/>
    </marker>
  </defs>
  <rect x="-20" y="152" width="60" height="30" class="svg20251118-3-box_white"/>
  <rect x="70" y="5" width="180" height="310" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l0" d="M 10 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow0)"/>
  <rect x="270" y="52" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l1" d="M 230 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow1)"/>
  <rect x="340" y="52" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l1" d="M 300 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow1)"/>
  <rect x="410" y="52" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l1" d="M 370 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow1)"/>
  <rect x="270" y="152" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l2" d="M 230 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow2)"/>
  <rect x="340" y="152" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l2" d="M 300 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow2)"/>
  <rect x="410" y="152" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l2" d="M 370 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow2)"/>
  <rect x="270" y="252" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l3" d="M 230 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow3)"/>
  <rect x="340" y="252" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l3" d="M 300 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow3)"/>
  <rect x="410" y="252" width="60" height="30" class="svg20251118-3-box_white"/>
  <path class="svg20251118-3-l3" d="M 370 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-3-arrow3)"/>
  <text class="svg20251118-3-t1" x="10" y="85">parent</text>
  <text class="svg20251118-3-t1" x="10" y="105">chain</text>
  <text class="svg20251118-3-t1" x="80" y="25">wait_any</text>
  <text class="svg20251118-3-t1" x="90" y="45">stop_source: false</text>
  <text class="svg20251118-3-t1" x="90" y="65">pending: 3</text>
  <text class="svg20251118-3-t1" x="490" y="65">child chain 1</text>
  <text class="svg20251118-3-t1" x="490" y="165">child chain 2</text>
  <text class="svg20251118-3-t1" x="490" y="265">child chain 3</text>
</svg>
</div>

- `wait_any` is part of a parent chain
- it starts a chain for each child
- `wait_any` has a `stop_source`
- and a counter for pending children (3 in this case, once the children were
  started)

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-4-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20251118-4-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-4-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-4-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-4-l0 {
      stroke: #f5bb00;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-4-arrow0 {
      stroke: #f5bb00;
      stroke-width: 1;
      fill: #f5bb00;
    }
    .svg20251118-4-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-4-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-4-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-4-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-4-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-4-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-4-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-4-arrow0" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-4-arrow0"/>
    </marker>
    <marker id="svg20251118-4-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-4-arrow1"/>
    </marker>
    <marker id="svg20251118-4-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-4-arrow2"/>
    </marker>
    <marker id="svg20251118-4-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-4-arrow3"/>
    </marker>
  </defs>
  <rect x="-20" y="152" width="60" height="30" class="svg20251118-4-box_white"/>
  <rect x="70" y="5" width="180" height="310" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l0" d="M 10 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-4-arrow0)"/>
  <rect x="270" y="52" width="60" height="30" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l1" d="M 230 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-4-arrow1)"/>
  <rect x="340" y="52" width="60" height="30" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l1" d="M 300 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-4-arrow1)"/>
  <rect x="410" y="52" width="60" height="30" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l1" d="M 370 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-4-arrow1)"/>
  <rect x="270" y="152" width="60" height="30" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l2" d="M 300 183 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-4-arrow2)"/>
  <rect x="270" y="252" width="60" height="30" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l3" d="M 230 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-4-arrow3)"/>
  <rect x="340" y="252" width="60" height="30" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l3" d="M 300 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-4-arrow3)"/>
  <rect x="410" y="252" width="60" height="30" class="svg20251118-4-box_white"/>
  <path class="svg20251118-4-l3" d="M 370 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-4-arrow3)"/>
  <circle cx="220" cy="146" r="7" class="svg20251118-4-l2"/>
  <text class="svg20251118-4-t1" x="10" y="85">parent</text>
  <text class="svg20251118-4-t1" x="10" y="105">chain</text>
  <text class="svg20251118-4-t1" x="80" y="25">wait_any</text>
  <text class="svg20251118-4-t2" x="90" y="45">stop_source: true</text>
  <text class="svg20251118-4-t1" x="90" y="65">pending: 2</text>
  <text class="svg20251118-4-t1" x="490" y="65">child chain 1</text>
  <text class="svg20251118-4-t1" x="490" y="165">child chain 2</text>
  <text class="svg20251118-4-t1" x="490" y="265">child chain 3</text>
</svg>
</div>

- child chain 2 happens to be the first child that completes, e.g. with a value
- the `stop_source.request_stop()` is called: this tells the other children to
  hurry up completing: the leaves on those chains will usually have the
  `stop_callback` function called and they will arrange to complete with a stop
  ASAP
- the pending counter is decremented, but it's not zero

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-5-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20251118-5-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-5-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-5-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-5-l0 {
      stroke: #f5bb00;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-5-arrow0 {
      stroke: #f5bb00;
      stroke-width: 1;
      fill: #f5bb00;
    }
    .svg20251118-5-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-5-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-5-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-5-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-5-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-5-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-5-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-5-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-5-arrow0" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-5-arrow0"/>
    </marker>
    <marker id="svg20251118-5-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-5-arrow1"/>
    </marker>
    <marker id="svg20251118-5-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-5-arrow2"/>
    </marker>
    <marker id="svg20251118-5-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-5-arrow3"/>
    </marker>
  </defs>
  <rect x="-20" y="152" width="60" height="30" class="svg20251118-5-box_white"/>
  <rect x="70" y="5" width="180" height="310" class="svg20251118-5-box_white"/>
  <path class="svg20251118-5-l0" d="M 10 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-5-arrow0)"/>
  <line class="svg20251118-5-l4" x1="210" y1="65" x2="220" y2="55"/>
  <line class="svg20251118-5-l4" x1="210" y1="55" x2="220" y2="65"/>
  <rect x="270" y="52" width="60" height="30" class="svg20251118-5-box_white"/>
  <path class="svg20251118-5-l1" d="M 300 83 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-5-arrow1)"/>
  <rect x="270" y="252" width="60" height="30" class="svg20251118-5-box_white"/>
  <path class="svg20251118-5-l3" d="M 230 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-5-arrow3)"/>
  <rect x="340" y="252" width="60" height="30" class="svg20251118-5-box_white"/>
  <path class="svg20251118-5-l3" d="M 300 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-5-arrow3)"/>
  <rect x="410" y="252" width="60" height="30" class="svg20251118-5-box_white"/>
  <path class="svg20251118-5-l3" d="M 370 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-5-arrow3)"/>
  <circle cx="220" cy="146" r="7" class="svg20251118-5-l2"/>
  <text class="svg20251118-5-t1" x="10" y="85">parent</text>
  <text class="svg20251118-5-t1" x="10" y="105">chain</text>
  <text class="svg20251118-5-t1" x="80" y="25">wait_any</text>
  <text class="svg20251118-5-t2" x="90" y="45">stop_source: true</text>
  <text class="svg20251118-5-t1" x="90" y="65">pending: 1</text>
  <text class="svg20251118-5-t1" x="490" y="65">child chain 1</text>
  <text class="svg20251118-5-t1" x="490" y="265">child chain 3</text>
</svg>
</div>

- another child chain completes (e.g. with a stop): child chain 1 in this case
- the pending counter is decremented, but it's not zero

- there could be case where the (not the first) child of `wait_any` completes
  with a result rather than a stop. This does not impact the completion of
  `wait_any` which will be the one of the first

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-6-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20251118-6-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-6-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-6-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-6-l0 {
      stroke: #f5bb00;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-6-arrow0 {
      stroke: #f5bb00;
      stroke-width: 1;
      fill: #f5bb00;
    }
    .svg20251118-6-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-6-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-6-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-6-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-6-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-6-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-6-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-6-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-6-arrow0" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-6-arrow0"/>
    </marker>
    <marker id="svg20251118-6-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-6-arrow1"/>
    </marker>
    <marker id="svg20251118-6-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-6-arrow2"/>
    </marker>
    <marker id="svg20251118-6-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-6-arrow3"/>
    </marker>
  </defs>
  <rect x="-20" y="152" width="60" height="30" class="svg20251118-6-box_white"/>
  <rect x="70" y="5" width="180" height="310" class="svg20251118-6-box_white"/>
  <path class="svg20251118-6-l0" d="M 100 183 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-6-arrow0)"/>
  <rect x="270" y="252" width="60" height="30" class="svg20251118-6-box_white"/>
  <path class="svg20251118-6-l3" d="M 300 283 q -100 40 -190 -90" marker-end="url(#svg20251118-6-arrow3)"/>
  <line class="svg20251118-6-l4" x1="100" y1="175" x2="110" y2="165"/>
  <line class="svg20251118-6-l4" x1="100" y1="165" x2="110" y2="175"/>
  <circle cx="20" cy="165" r="6" class="svg20251118-6-l2"/>
  <text class="svg20251118-6-t1" x="10" y="85">parent</text>
  <text class="svg20251118-6-t1" x="10" y="105">chain</text>
  <text class="svg20251118-6-t1" x="80" y="25">wait_any</text>
  <text class="svg20251118-6-t2" x="90" y="45">stop_source: true</text>
  <text class="svg20251118-6-t2" x="90" y="65">pending: 0</text>
  <text class="svg20251118-6-t1" x="490" y="265">child chain 3</text>
</svg>
</div>

- when the last child chain completes (e.g. with a stop): child chain 3 in this
  case
- the pending counter is decremented
- it reaches zero
- the parent chain is resumed (with the value of the first chain that completed)

- similarly if the first child completed

---

# Through branches

- Propagation uphill through branches

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-16-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20251118-16-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-16-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-16-l0 {
      stroke: #f5bb00;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-16-arrow0 {
      stroke: #f5bb00;
      stroke-width: 1;
      fill: #f5bb00;
    }
    .svg20251118-16-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-16-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-16-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-16-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-16-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-16-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-16-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-16-arrow0" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-16-arrow0"/>
    </marker>
    <marker id="svg20251118-16-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-16-arrow1"/>
    </marker>
    <marker id="svg20251118-16-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-16-arrow2"/>
    </marker>
    <marker id="svg20251118-16-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-16-arrow3"/>
    </marker>
  </defs>
  <rect x="10" y="105" width="180" height="78" class="svg20251118-16-box_white"/>
  <rect x="210" y="152" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l0" d="M 160 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow0)"/>
  <text class="svg20251118-16-t1" x="20" y="125">run</text>
  <text class="svg20251118-16-t1" x="30" y="145">stop_source: false</text>
  <text class="svg20251118-16-t1" x="30" y="165">done: false</text>
  <rect x="280" y="5" width="180" height="310" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l0" d="M 230 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow0)"/>
  <rect x="480" y="52" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l1" d="M 440 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow1)"/>
  <rect x="550" y="52" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l1" d="M 510 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow1)"/>
  <rect x="620" y="52" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l1" d="M 580 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow1)"/>
  <rect x="480" y="152" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l2" d="M 440 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow2)"/>
  <rect x="550" y="152" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l2" d="M 510 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow2)"/>
  <rect x="620" y="152" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l2" d="M 580 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow2)"/>
  <rect x="480" y="252" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l3" d="M 440 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow3)"/>
  <rect x="550" y="252" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l3" d="M 510 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow3)"/>
  <rect x="620" y="252" width="60" height="30" class="svg20251118-16-box_white"/>
  <path class="svg20251118-16-l3" d="M 580 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-16-arrow3)"/>
  <text class="svg20251118-16-t1" x="200" y="85">parent</text>
  <text class="svg20251118-16-t1" x="200" y="105">chain</text>
  <text class="svg20251118-16-t1" x="290" y="25">wait_any</text>
  <text class="svg20251118-16-t1" x="300" y="45">stop_source: false</text>
  <text class="svg20251118-16-t1" x="300" y="65">pending: 3</text>
  <text class="svg20251118-16-t1" x="690" y="65">child chain 1</text>
  <text class="svg20251118-16-t1" x="690" y="165">child chain 2</text>
  <text class="svg20251118-16-t1" x="690" y="265">child chain 3</text>
</svg>
</div>

- here, from `run`, we have a chain reaching a `wait_any` that then branches into
  three child chains
- we look at what should happen if the parent chain of `wait_any` initiates the
  cancellation: how does it propagate through the child chains of `wait_any`?

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-17-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20251118-17-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-17-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-17-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-17-l0 {
      stroke: #f5bb00;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-17-arrow0 {
      stroke: #f5bb00;
      stroke-width: 1;
      fill: #f5bb00;
    }
    .svg20251118-17-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-17-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-17-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-17-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-17-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-17-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-17-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-17-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-17-arrow0" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-17-arrow0"/>
    </marker>
    <marker id="svg20251118-17-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-17-arrow1"/>
    </marker>
    <marker id="svg20251118-17-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-17-arrow2"/>
    </marker>
    <marker id="svg20251118-17-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-17-arrow3"/>
    </marker>
  </defs>
  <rect x="10" y="105" width="180" height="78" class="svg20251118-17-box_white"/>
  <rect x="210" y="152" width="60" height="30" class="svg20251118-17-box_white"/>
  <path class="svg20251118-17-l0" d="M 160 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-17-arrow0)"/>
  <text class="svg20251118-17-t1" x="20" y="125">run</text>
  <text class="svg20251118-17-t2" x="30" y="145">stop_source: true</text>
  <text class="svg20251118-17-t1" x="30" y="165">done: false</text>
  <rect x="280" y="5" width="180" height="310" class="svg20251118-17-box_white"/>
  <path class="svg20251118-17-l0" d="M 230 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-17-arrow0)"/>
  <rect x="480" y="152" width="60" height="30" class="svg20251118-17-box_white"/>
  <path class="svg20251118-17-l2" d="M 510 183 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-17-arrow2)"/>
  <line class="svg20251118-17-l4" x1="420" y1="165" x2="430" y2="155"/>
  <line class="svg20251118-17-l4" x1="420" y1="155" x2="430" y2="165"/>
  <rect x="480" y="252" width="60" height="30" class="svg20251118-17-box_white"/>
  <path class="svg20251118-17-l3" d="M 440 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-17-arrow3)"/>
  <rect x="550" y="252" width="60" height="30" class="svg20251118-17-box_white"/>
  <path class="svg20251118-17-l3" d="M 510 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-17-arrow3)"/>
  <rect x="620" y="252" width="60" height="30" class="svg20251118-17-box_white"/>
  <path class="svg20251118-17-l3" d="M 580 258 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-17-arrow3)"/>
  <text class="svg20251118-17-t1" x="200" y="85">parent</text>
  <text class="svg20251118-17-t1" x="200" y="105">chain</text>
  <text class="svg20251118-17-t1" x="290" y="25">wait_any</text>
  <text class="svg20251118-17-t2" x="300" y="45">stop_source: true</text>
  <text class="svg20251118-17-t1" x="300" y="65">pending: 2</text>
  <text class="svg20251118-17-t1" x="690" y="165">child chain 2</text>
  <text class="svg20251118-17-t1" x="690" y="265">child chain 3</text>
</svg>
</div>

- the `wait_any` got a `stop_token` from its parent chain (from `run` in this
  case)
- when the parent `stop_source.request_stop()` is called
- `wait_any` is notified via a `stop_callback`
- and in turn it calls `stop_source.request_stop()` on the stop source
  controlling its children chains
- the children chains then one by one complete (e.g. with a stop)

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-18-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20251118-18-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-18-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-18-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-18-l0 {
      stroke: #f5bb00;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-18-arrow0 {
      stroke: #f5bb00;
      stroke-width: 1;
      fill: #f5bb00;
    }
    .svg20251118-18-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-18-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-18-l2 {
      stroke: #8ea604;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-18-arrow2 {
      stroke: #8ea604;
      stroke-width: 1;
      fill: #8ea604;
    }
    .svg20251118-18-l3 {
      stroke: #9665ac;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-18-arrow3 {
      stroke: #9665ac;
      stroke-width: 1;
      fill: #9665ac;
    }
    .svg20251118-18-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-18-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-18-arrow0" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-18-arrow0"/>
    </marker>
    <marker id="svg20251118-18-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-18-arrow1"/>
    </marker>
    <marker id="svg20251118-18-arrow2" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-18-arrow2"/>
    </marker>
    <marker id="svg20251118-18-arrow3" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-18-arrow3"/>
    </marker>
  </defs>
  <rect x="10" y="105" width="180" height="78" class="svg20251118-18-box_white"/>
  <rect x="210" y="152" width="60" height="30" class="svg20251118-18-box_white"/>
  <path class="svg20251118-18-l0" d="M 160 158 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-18-arrow0)"/>
  <text class="svg20251118-18-t1" x="20" y="125">run</text>
  <text class="svg20251118-18-t2" x="30" y="145">stop_source: true</text>
  <text class="svg20251118-18-t1" x="30" y="165">done: false</text>
  <rect x="280" y="5" width="180" height="310" class="svg20251118-18-box_white"/>
  <path class="svg20251118-18-l0" d="M 310 183 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-18-arrow0)"/>
  <rect x="480" y="252" width="60" height="30" class="svg20251118-18-box_white"/>
  <path class="svg20251118-18-l3" d="M 510 283 q -100 40 -190 -90" marker-end="url(#svg20251118-18-arrow3)"/>
  <line class="svg20251118-18-l4" x1="310" y1="175" x2="320" y2="165"/>
  <line class="svg20251118-18-l4" x1="310" y1="165" x2="320" y2="175"/>
  <line class="svg20251118-18-l4" x1="225" y1="165" x2="235" y2="155"/>
  <line class="svg20251118-18-l4" x1="225" y1="155" x2="235" y2="165"/>
  <text class="svg20251118-18-t1" x="200" y="85">parent</text>
  <text class="svg20251118-18-t1" x="200" y="105">chain</text>
  <text class="svg20251118-18-t1" x="290" y="25">wait_any</text>
  <text class="svg20251118-18-t2" x="300" y="45">stop_source: true</text>
  <text class="svg20251118-18-t2" x="300" y="65">pending: 0</text>
  <text class="svg20251118-18-t1" x="690" y="265">child chain 3</text>
</svg>
</div>

- when the last child of `wait_any` completes
- then the pending count reaches zero
- then `wait_any` itself completes with a stop (ignoring any result from a
  child that maybe completed earlier with a result)

---

# From leaf

- Initiation from leaf and containing it

---

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20251118-19-box" width="100%" viewBox="0 0 800 110">
  <style>
    #svg20251118-19-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20251118-19-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
    }
    .svg20251118-19-t2 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: left;
      dominant-baseline: middle;
      fill: #ee6055;
    }
    .svg20251118-19-l1 {
      stroke: #2d93ad;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-19-arrow1 {
      stroke: #2d93ad;
      stroke-width: 1;
      fill: #2d93ad;
    }
    .svg20251118-19-l4 {
      stroke: #ee6055;
      stroke-width: 3;
      fill: none;
    }
    .svg20251118-19-box_white {
      stroke: #000000;
      stroke-width: 2;
      fill: #ffffff;
    }
  </style>
  <defs>
    <marker id="svg20251118-19-arrow1" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z" class="svg20251118-19-arrow1"/>
    </marker>
  </defs>
  <rect x="10" y="5" width="180" height="78" class="svg20251118-19-box_white"/>
  <rect x="210" y="52" width="60" height="30" class="svg20251118-19-box_white"/>
  <path class="svg20251118-19-l1" d="M 170 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-19-arrow1)"/>
  <rect x="280" y="52" width="60" height="30" class="svg20251118-19-box_white"/>
  <path class="svg20251118-19-l1" d="M 240 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-19-arrow1)"/>
  <rect x="350" y="52" width="60" height="30" class="svg20251118-19-box_white"/>
  <path class="svg20251118-19-l1" d="M 310 58 c 20 -70 70 -40 70 -18" marker-end="url(#svg20251118-19-arrow1)"/>
  <rect x="420" y="52" width="60" height="30" class="svg20251118-19-box_white"/>
  <path class="svg20251118-19-l1" d="M 450 83 c -5 30 -40 30 -66 -16" marker-end="url(#svg20251118-19-arrow1)"/>
  <line class="svg20251118-19-l4" x1="365" y1="65" x2="375" y2="55"/>
  <line class="svg20251118-19-l4" x1="365" y1="55" x2="375" y2="65"/>
  <text class="svg20251118-19-t1" x="20" y="25">run</text>
  <text class="svg20251118-19-t1" x="30" y="45">stop_source: false</text>
  <text class="svg20251118-19-t1" x="30" y="65">done: false</text>
</svg>
</div>

- a leaf of a chain can complete with a stop (instead of a result) without
  being triggered via the `stop_token` of the chain it belongs to
  - `just_stopped` is such a primitive that "just completes with a stop"
- most links in a chain propagate this further down the chain by completing
  with a stop themselves
  - this is the behaviour of `wait_any` if the first child to complete
    completes with a stop
- this would lead to the stop being propagated all the way to the `run` that
  initiated the chains
  - however a primitive like `stopped_as_optional` converts a child completion
    with a stop into a completion with a `std::nullopt` value, allowing control
    of how deep a child completion with stop propagates down the chain

---

# Questions?