Defer importing ky-universal until actually needed

This accomplishes two things:

* It prevents a dangling promise with an import in it. If the module is
  loaded (queuing up the import promise) and then no clients are ever
  used, and thus never awaited, the import can happen too late. In
  particular, Jest will break if the import happens after the test is
  complete.

* It avoids a dynamic import altogether when the client isn't needed. In
  particular, Jest uses Node's VM API which only experimentally supports
  dynamic imports (and ESM modules altogether). This change means that
  merely loading the http-client module doesn't require enabling that
  experimental support.

lib/deferred.js

@@ -0,0 +1,16 @@
+export function deferred(f) {
+  let promise;
+
+  return {
+    then(
+      onfulfilled,
+      onrejected
+    ) {
+      promise ||= new Promise(resolve => resolve(f()));
+      return promise.then(
+        onfulfilled,
+        onrejected
+      );
+    },
+  };
+}

lib/httpClient.js

@@ -2,9 +2,10 @@
  * Copyright (c) 2020-2022 Digital Bazaar, Inc. All rights reserved.
  */
 import {convertAgent} from './agentCompatibility.js';
+import {deferred} from './deferred.js';
 
-export const kyOriginalPromise = import('ky-universal')
-  .then(({default: ky}) => ky);
+export const kyOriginalPromise = deferred(() => import('ky-universal')
+  .then(({default: ky}) => ky));
 
 export const DEFAULT_HEADERS = {
   Accept: 'application/ld+json, application/json'
@@ -33,7 +34,7 @@ export function createInstance({
   params = convertAgent(params);
 
   // create new ky instance that will asynchronously resolve
-  const kyPromise = parent.then(kyBase => {
+  const kyPromise = deferred(() => parent.then(kyBase => {
     let ky;
     if(parent === kyOriginalPromise) {
       // ensure default headers, allow overrides
@@ -46,7 +47,7 @@ export function createInstance({
       ky = kyBase.extend({headers, ...params});
     }
     return ky;
-  });
+  }));
 
   return _createHttpClient(kyPromise);
 }

tests/deferred.spec.js

@@ -0,0 +1,50 @@
+import {deferred} from '../lib/deferred.js';
+
+describe('deferred()', () => {
+  it('resolves to the return value of its function', async () => {
+    const d = deferred(() => {
+      return 'return value';
+    });
+
+    const ret = await d;
+    ret.should.equal('return value');
+  });
+
+  it('defers execution until awaited', async () => {
+    let executionCount = 0;
+    executionCount.should.equal(0);
+
+    const d = deferred(() => {
+      executionCount++;
+      return 'return value';
+    });
+
+    executionCount.should.equal(0);
+    await d;
+    executionCount.should.equal(1);
+  });
+
+  it('only executes once', async () => {
+    let executionCount = 0;
+    executionCount.should.equal(0);
+
+    const d = deferred(() => {
+      executionCount++;
+      return 'return value';
+    });
+
+    await d;
+    await d;
+    executionCount.should.equal(1);
+  });
+
+  it('unwraps returned promises', async () => {
+    const d = deferred(() => {
+      return Promise.resolve('return value');
+    });
+
+    const ret = await d;
+    ret.should.equal('return value');
+  });
+});
+