Ability to cache subsonic artist images locally on disk (#61)

tests/subsonic.test.ts

@@ -1,5 +1,8 @@
 import { Md5 } from "ts-md5/dist/md5";
 import { v4 as uuid } from "uuid";
+import tmp from "tmp";
+import fse from "fs-extra";
+import path from "path";
 
 import {
   isDodgyImage,
@@ -11,6 +14,7 @@ import {
   appendMimeTypeToClientFor,
   asURLSearchParams,
   splitCoverArtId,
+  cachingImageFetcher,
 } from "../src/subsonic";
 import encryption from "../src/encryption";
 
@@ -158,6 +162,74 @@ describe("asURLSearchParams", () => {
   });
 });
 
+describe("cachingImageFetcher", () => {
+  const delegate = jest.fn();
+  const url = "http://test.example.com/someimage.jpg";
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    jest.resetAllMocks();
+  });
+
+  describe("when there is no image in the cache", () => {
+    it.only("should fetch the image from the source and then cache and return it", async () => {
+      const dir = tmp.dirSync();
+      const cacheFile = path.join(dir.name, `${Md5.hashStr(url)}.png`);
+      const jpgImage = Buffer.from("jpg-image", 'utf-8');
+      const pngImage = Buffer.from("png-image", 'utf-8');
+
+      delegate.mockResolvedValue({ contentType: "image/jpeg", data: jpgImage });
+      const png = jest.fn();
+      (sharp as unknown as jest.Mock).mockReturnValue({ png });
+      png.mockReturnValue({
+        toBuffer: () => Promise.resolve(pngImage),
+      });
+
+      const result = await cachingImageFetcher(dir.name, delegate)(url);
+
+      expect(result!.contentType).toEqual("image/png");
+      expect(result!.data).toEqual(pngImage);
+
+      expect(delegate).toHaveBeenCalledWith(url);
+      expect(fse.existsSync(cacheFile)).toEqual(true);
+      expect(fse.readFileSync(cacheFile)).toEqual(pngImage);
+    });
+  });
+
+  describe("when the image is already in the cache", () => {
+    it.only("should fetch the image from the cache and return it", async () => {
+      const dir = tmp.dirSync();
+      const cacheFile = path.join(dir.name, `${Md5.hashStr(url)}.png`);
+      const data = Buffer.from("foobar2", "utf-8");
+
+      fse.writeFileSync(cacheFile, data);
+
+      const result = await cachingImageFetcher(dir.name, delegate)(url);
+
+      expect(result!.contentType).toEqual("image/png");
+      expect(result!.data).toEqual(data);
+
+      expect(delegate).not.toHaveBeenCalled();
+    });
+  });
+
+  describe("when the delegate returns undefined", () => {
+    it.only("should return undefined", async () => {
+      const dir = tmp.dirSync();
+      const cacheFile = path.join(dir.name, `${Md5.hashStr(url)}.png`);
+
+      delegate.mockResolvedValue(undefined);
+
+      const result = await cachingImageFetcher(dir.name, delegate)(url);
+
+      expect(result).toBeUndefined();
+
+      expect(delegate).toHaveBeenCalledWith(url);
+      expect(fse.existsSync(cacheFile)).toEqual(false);
+    });
+  });
+});
+
 const ok = (data: string) => ({
   status: 200,
   data,