From 2940b80563dc86c935542bb04acd32a4c0608e3a Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonyg@leastfixedpoint.com>
Date: Sun, 12 Dec 2021 23:03:22 +0100
Subject: [PATCH] Supervision

---
 packages/core/src/index.ts             |  1 +
 packages/core/src/runtime/supervise.ts | 77 ++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)
 create mode 100644 packages/core/src/runtime/supervise.ts

diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index a2aa724..5810b0f 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -16,6 +16,7 @@ export * as QuasiValue from './runtime/quasivalue.js';
 export * from './runtime/randomid.js';
 export * as Rewrite from './runtime/rewrite.js';
 export * as Skeleton from './runtime/skeleton.js';
+export * from './runtime/supervise.js';
 export * as Task from './runtime/task.js';
 
 export * as Cryptography from './transport/cryptography.js';
diff --git a/packages/core/src/runtime/supervise.ts b/packages/core/src/runtime/supervise.ts
new file mode 100644
index 0000000..95edcf0
--- /dev/null
+++ b/packages/core/src/runtime/supervise.ts
@@ -0,0 +1,77 @@
+/// SPDX-License-Identifier: GPL-3.0-or-later
+/// SPDX-FileCopyrightText: Copyright © 2021 Tony Garnock-Jones <tonyg@leastfixedpoint.com>
+
+import { State } from '../gen/service';
+import { Actor, AnyValue, LocalAction, Turn } from './actor';
+import { Field } from './dataflow';
+
+type StateName = State['_variant'];
+
+export enum SupervisorRestartPolicy {
+    ALWAYS,
+    ON_ERROR_ONLY,
+}
+
+export type SupervisorConfiguration = {
+    intensity: number,
+    period: number, /* seconds */
+    pauseTime: number, /* seconds */
+    sleepTime: number, /* seconds */
+    restartPolicy: SupervisorRestartPolicy,
+};
+
+export const DEFAULT_CONFIG: SupervisorConfiguration = {
+    intensity: 1,
+    period: 5,
+    pauseTime: 0.2,
+    sleepTime: 10,
+    restartPolicy: SupervisorRestartPolicy.ON_ERROR_ONLY,
+};
+
+export class Supervisor {
+    readonly config: SupervisorConfiguration;
+    readonly nameFunction: () => AnyValue;
+    readonly bootFunction: LocalAction;
+    readonly restarts: number[] = []; /* timestamps */
+    readonly state: Field<StateName> = Turn.active.field('started');
+    supervisee: Actor | null = null;
+
+    constructor(config: Partial<SupervisorConfiguration>,
+                nameFunction: () => AnyValue,
+                bootFunction: LocalAction)
+    {
+        this.config = Object.assign({}, DEFAULT_CONFIG, config);
+        this.nameFunction = nameFunction;
+        this.bootFunction = bootFunction;
+        this.startSupervisee();
+    }
+
+    startSupervisee() {
+        Turn.active.facet(() => {
+            this.state.value = 'started';
+            this.supervisee = Turn.active.spawnLink(this.bootFunction);
+            if (this.supervisee) this.supervisee.name = this.nameFunction();
+            Turn.activeFacet.onStop(() => {
+                const exitReason = this.supervisee?.exitReason;
+                if (!exitReason) {
+                    throw new Error("Expected supervisee to have terminated");
+                }
+                if (exitReason.ok && (this.config.restartPolicy === SupervisorRestartPolicy.ON_ERROR_ONLY)) {
+                    this.state.value = 'complete';
+                } else {
+                    this.state.value = exitReason.ok ? 'complete' : 'failed';
+                    const now = +(new Date());
+                    const oldestToKeep = now - this.config.period * 1000.0;
+                    this.restarts.push(now);
+                    while (this.restarts.length && (this.restarts[0] < oldestToKeep)) {
+                        this.restarts.shift();
+                    }
+                    const waitTime = (this.restarts.length > this.config.intensity)
+                        ? this.config.sleepTime
+                        : this.config.pauseTime;
+                    Turn.active.after(waitTime * 1000.0, () => this.startSupervisee());
+                }
+            });
+        });
+    }
+}