release-manifests: allow overriding fetch implementation

This allows us to use yarn's built in HTTP request
utilities in the Backstage yarn plugin, thereby
benefiting from the built-in caching and proxy
configuration utilities.

Signed-off-by: MT Lewis <mtlewis@users.noreply.github.com>

.changeset/violet-snakes-laugh.md

@@ -0,0 +1,5 @@
+---
+'@backstage/release-manifests': patch
+---
+
+Allow overriding the fetch function used inside getManifestByVersion

packages/release-manifests/report.api.md

@@ -21,6 +21,12 @@ export function getManifestByVersion(
 // @public
 export type GetManifestByVersionOptions = {
   version: string;
+  fetch?: (
+    url: string,
+    options?: {
+      signal?: AbortSignal;
+    },
+  ) => Promise<Pick<Response, 'status' | 'json' | 'url'>>;
 };
 
 // @public

packages/release-manifests/src/manifest.test.ts

@@ -55,6 +55,37 @@ describe('Release Manifests', () => {
         getManifestByVersion({ version: '999.0.1' }),
       ).rejects.toThrow('No release found for 999.0.1 version');
     });
+
+    it('should allow overriding the fetch implementation', async () => {
+      const mockFetch = jest.fn().mockImplementation(async url => ({
+        status: 200,
+        url,
+        json: () => ({
+          packages: [{ name: '@backstage/core', version: '2.3.4' }],
+        }),
+      }));
+
+      const pkgs = await getManifestByVersion({
+        version: '0.0.0',
+        fetch: mockFetch,
+      });
+
+      expect(pkgs.packages).toEqual([
+        {
+          name: '@backstage/core',
+          version: '2.3.4',
+        },
+      ]);
+
+      mockFetch.mockImplementation(async url => ({
+        status: 404,
+        url,
+      }));
+
+      await expect(
+        getManifestByVersion({ version: '0.0.0', fetch: mockFetch }),
+      ).rejects.toThrow('No release found for 0.0.0 version');
+    });
   });
 
   describe('getManifestByReleaseLine', () => {

packages/release-manifests/src/manifest.ts

@@ -33,6 +33,10 @@ export type ReleaseManifest = {
  */
 export type GetManifestByVersionOptions = {
   version: string;
+  fetch?: (
+    url: string,
+    options?: { signal?: AbortSignal },
+  ) => Promise<Pick<Response, 'status' | 'json' | 'url'>>;
 };
 
 // Wait for waitMs, or until signal is aborted.
@@ -82,15 +86,21 @@ export async function getManifestByVersion(
   options: GetManifestByVersionOptions,
 ): Promise<ReleaseManifest> {
   const versionEnc = encodeURIComponent(options.version);
+
+  const fetchFn = options.fetch ?? fetch;
+
   const res = await withFallback(
     signal =>
-      fetch(`${VERSIONS_BASE_URL}/v1/releases/${versionEnc}/manifest.json`, {
+      fetchFn(`${VERSIONS_BASE_URL}/v1/releases/${versionEnc}/manifest.json`, {
         signal,
       }),
     signal =>
-      fetch(`${GITHUB_RAW_BASE_URL}/v1/releases/${versionEnc}/manifest.json`, {
-        signal,
-      }),
+      fetchFn(
+        `${GITHUB_RAW_BASE_URL}/v1/releases/${versionEnc}/manifest.json`,
+        {
+          signal,
+        },
+      ),
     500,
   );
   if (res.status === 404) {