feat: async context (#24402)

We are switching to ContinuationPreservedEmbedderData. This allows
adding async context tracking to the various async operations that deno
provides.

Fixes: https://github.com/denoland/deno/issues/7010
Fixes: https://github.com/denoland/deno/issues/22886
Fixes: https://github.com/denoland/deno/issues/24368

1 files changed, 45 insertions, 0 deletions

diff --git a/runtime/js/01_async_context.js b/runtime/js/01_async_context.js
new file mode 100644
index 000000000..9c0236fbe
--- /dev/null
+++ b/runtime/js/01_async_context.js
@@ -0,0 +1,45 @@
+// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
+
+import { primordials } from "ext:core/mod.js";
+import { op_get_extras_binding_object } from "ext:core/ops";
+
+const {
+  SafeWeakMap,
+} = primordials;
+
+const {
+  getContinuationPreservedEmbedderData,
+  setContinuationPreservedEmbedderData,
+} = op_get_extras_binding_object();
+
+let counter = 0;
+
+export const getAsyncContext = getContinuationPreservedEmbedderData;
+export const setAsyncContext = setContinuationPreservedEmbedderData;
+
+export class AsyncVariable {
+  #id = counter++;
+  #data = new SafeWeakMap();
+
+  enter(value) {
+    const previousContextMapping = getAsyncContext();
+    const entry = { id: this.#id };
+    const asyncContextMapping = {
+      __proto__: null,
+      ...previousContextMapping,
+      [this.#id]: entry,
+    };
+    this.#data.set(entry, value);
+    setAsyncContext(asyncContextMapping);
+    return previousContextMapping;
+  }
+
+  get() {
+    const current = getAsyncContext();
+    const entry = current?.[this.#id];
+    if (entry) {
+      return this.#data.get(entry);
+    }
+    return undefined;
+  }
+}