saw a project log in production where this kernelspec was called 466 times at once

- surely due to the accumulation from bad clients, etc.
- but since "jupyter kernelspec list" on a cocalc project could take 2-3
  seconds, this could break a lot
- so reuseInFlight!

Author: williamstein

src/packages/jupyter/kernel/kernel-data.ts

@@ -13,9 +13,9 @@ Specs: https://jupyter-client.readthedocs.io/en/stable/kernels.html#kernel-specs
 
 import { findAll } from "kernelspecs";
 import LRU from "lru-cache";
-
 import type { KernelSpec } from "@cocalc/jupyter/types/types";
 import { field_cmp } from "@cocalc/util/misc";
+import { reuseInFlight } from "@cocalc/util/reuse-in-flight";
 
 const cache = new LRU<"kernel_data", KernelSpec[]>({
   ttl: 15 * 1000,
@@ -44,35 +44,39 @@ function spec2language(spec): string {
   }
 }
 
-export async function get_kernel_data(): Promise<KernelSpec[]> {
-  let x = cache.get("kernel_data");
-  if (x != null) {
-    return x;
-  }
-  const kernel_data = await findAll();
-  const v: KernelSpec[] = [];
-  for (const kernel in kernel_data) {
-    const value = kernel_data[kernel];
-    v.push({
-      name: kernel.toLowerCase(),
-      display_name: value.spec.display_name,
-      language: spec2language(value.spec),
-      // @ts-ignore
-      interrupt_mode: value.spec.interrupt_mode,
-      env: value.spec.env ?? {},
-      // @ts-ignore
-      metadata: value.spec.metadata,
-      // kernelspecs incorrectly calls it resources_dir instead of resource_dir.
-      // See https://github.com/nteract/kernelspecs/issues/25
-      // @ts-ignore
-      resource_dir: value.resource_dir ?? value.resources_dir,
-      argv: value.spec.argv,
-    });
-  }
-  v.sort(field_cmp("display_name"));
-  cache.set("kernel_data", v);
-  return v;
-}
+// this is very expensive and can get called many times at once before
+// the cache is set, which is very bad... so reuseInFlight.
+export const get_kernel_data = reuseInFlight(
+  async (): Promise<KernelSpec[]> => {
+    let x = cache.get("kernel_data");
+    if (x != null) {
+      return x;
+    }
+    const kernel_data = await findAll();
+    const v: KernelSpec[] = [];
+    for (const kernel in kernel_data) {
+      const value = kernel_data[kernel];
+      v.push({
+        name: kernel.toLowerCase(),
+        display_name: value.spec.display_name,
+        language: spec2language(value.spec),
+        // @ts-ignore
+        interrupt_mode: value.spec.interrupt_mode,
+        env: value.spec.env ?? {},
+        // @ts-ignore
+        metadata: value.spec.metadata,
+        // kernelspecs incorrectly calls it resources_dir instead of resource_dir.
+        // See https://github.com/nteract/kernelspecs/issues/25
+        // @ts-ignore
+        resource_dir: value.resource_dir ?? value.resources_dir,
+        argv: value.spec.argv,
+      });
+    }
+    v.sort(field_cmp("display_name"));
+    cache.set("kernel_data", v);
+    return v;
+  },
+);
 
 export async function getLanguage(kernelName: string): Promise<string> {
   const kernelSpec = await get_kernel_data_by_name(kernelName);