refactor: extract Watchable<T> base class for SDK effect wrappers

Eliminates boilerplate across 7 wrapper classes (GetContainerIp,
GetHostInfo, GetOutboundGateway, GetServiceManifest, GetSslCertificate,
GetStatus, GetSystemSmtp) by moving shared const/once/watch/onChange/
waitFor logic into an abstract Watchable<T> base class.

sdk/base/lib/util/Watchable.ts

@@ -0,0 +1,107 @@
+import { Effects } from '../Effects'
+import { AbortedError } from './AbortedError'
+import { DropGenerator, DropPromise } from './Drop'
+
+export abstract class Watchable<T> {
+  constructor(readonly effects: Effects) {}
+
+  protected abstract call(callback?: () => void): Promise<T>
+  protected abstract readonly label: string
+
+  /**
+   * Returns the value. Reruns the context from which it has been called if the underlying value changes
+   */
+  const(): Promise<T> {
+    return this.call(
+      this.effects.constRetry &&
+        (() => this.effects.constRetry && this.effects.constRetry()),
+    )
+  }
+
+  /**
+   * Returns the value. Does nothing if the value changes
+   */
+  once(): Promise<T> {
+    return this.call()
+  }
+
+  private async *watchGen(abort?: AbortSignal) {
+    const resolveCell = { resolve: () => {} }
+    this.effects.onLeaveContext(() => {
+      resolveCell.resolve()
+    })
+    abort?.addEventListener('abort', () => resolveCell.resolve())
+    while (this.effects.isInContext && !abort?.aborted) {
+      let callback: () => void = () => {}
+      const waitForNext = new Promise<void>((resolve) => {
+        callback = resolve
+        resolveCell.resolve = resolve
+      })
+      yield await this.call(() => callback())
+      await waitForNext
+    }
+    return new Promise<never>((_, rej) => rej(new AbortedError()))
+  }
+
+  /**
+   * Watches the value. Returns an async iterator that yields whenever the value changes
+   */
+  watch(abort?: AbortSignal): AsyncGenerator<T, never, unknown> {
+    const ctrl = new AbortController()
+    abort?.addEventListener('abort', () => ctrl.abort())
+    return DropGenerator.of(this.watchGen(ctrl.signal), () => ctrl.abort())
+  }
+
+  /**
+   * Watches the value. Takes a custom callback function to run whenever the value changes
+   */
+  onChange(
+    callback: (
+      value: T | undefined,
+      error?: Error,
+    ) => { cancel: boolean } | Promise<{ cancel: boolean }>,
+  ) {
+    ;(async () => {
+      const ctrl = new AbortController()
+      for await (const value of this.watch(ctrl.signal)) {
+        try {
+          const res = await callback(value)
+          if (res.cancel) {
+            ctrl.abort()
+            break
+          }
+        } catch (e) {
+          console.error(
+            `callback function threw an error @ ${this.label}.onChange`,
+            e,
+          )
+        }
+      }
+    })()
+      .catch((e) => callback(undefined, e))
+      .catch((e) =>
+        console.error(
+          `callback function threw an error @ ${this.label}.onChange`,
+          e,
+        ),
+      )
+  }
+
+  /**
+   * Watches the value. Returns when the predicate is true
+   */
+  waitFor(pred: (value: T) => boolean): Promise<T> {
+    const ctrl = new AbortController()
+    return DropPromise.of(
+      Promise.resolve().then(async () => {
+        for await (const next of this.watchGen(ctrl.signal)) {
+          if (pred(next)) {
+            return next
+          }
+        }
+        throw new AbortedError()
+      }),
+      () => ctrl.abort(),
+    )
+  }
+}