Implement fs.readFile / fs.writeFile

Summary: Per title. Made an explicit distinction between binary and non binary files, since they need to be encoded differently. This keeps both the implementation and API simpler (in terms of overloading / type checking)

Reviewed By: aigoncharov

Differential Revision: D33016031

fbshipit-source-id: 3c99956eb016849a908a171d88a7a64a88b76268

desktop/flipper-server-core/src/FlipperServerImpl.tsx

@@ -48,8 +48,10 @@ import {commandDownloadFileStartFactory} from './commands/DownloadFile';
 import {promises} from 'fs';
 // Electron 11 runs on Node 12 which does not support fs.promises.rm
 import rm from 'rimraf';
+import assert from 'assert';
 
-const {access, copyFile, mkdir, unlink, stat, readlink} = promises;
+const {access, copyFile, mkdir, unlink, stat, readlink, readFile, writeFile} =
+  promises;
 
 export const SERVICE_FLIPPER = 'flipper.oAuthToken';
 
@@ -258,6 +260,22 @@ export class FlipperServerImpl implements FlipperServer {
       };
     },
     'node-api-fs-readlink': readlink,
+    'node-api-fs-readfile': async (path, options) => {
+      const contents = await readFile(path, options ?? 'utf8');
+      assert(
+        typeof contents === 'string',
+        `File ${path} was not read with a string encoding`,
+      );
+      return contents;
+    },
+    'node-api-fs-readfile-binary': async (path) => {
+      const contents = await readFile(path);
+      return Base64.fromUint8Array(contents);
+    },
+    'node-api-fs-writefile': (path, contents, options) =>
+      writeFile(path, contents, options ?? 'utf8'),
+    'node-api-fs-writefile-binary': (path, base64contents) =>
+      writeFile(path, Base64.toUint8Array(base64contents), 'binary'),
     // TODO: Do we need API to cancel an active download?
     'download-file-start': commandDownloadFileStartFactory(
       this.emit.bind(this),