Use fatrw binary to read/write from boot partition

Change-type: patch

src/lib/host-utils.ts

@@ -1,6 +1,7 @@
+import { spawn } from 'child_process';
 import * as path from 'path';
 import * as constants from './constants';
-import * as fsUtils from './fs-utils';
+import { exec } from './fs-utils';
 
 // Returns an absolute path starting from the hostOS root partition
 // This path is accessible from within the Supervisor container
@@ -14,10 +15,52 @@ export function pathOnBoot(relPath: string) {
 	return pathOnRoot(path.join(constants.bootMountPoint, relPath));
 }
 
-// Receives an absolute path for a file under the boot partition (e.g. `/mnt/root/mnt/boot/config.txt`)
+class CodedError extends Error {
-// and writes the given data. This function uses the best effort to write a file trying to minimize corruption
+	constructor(msg: string, readonly code: number) {
-// due to a power cut. Given that the boot partition is a vfat filesystem, this means
+		super(msg);
-// using write + sync
+	}
-export async function writeToBoot(file: string, data: string | Buffer) {
+}
-	return await fsUtils.writeAndSyncFile(file, data);
+
+// Receives an absolute path for a file (assumed to be under the boot partition, e.g. `/mnt/root/mnt/boot/config.txt`)
+// and reads from the given location. This function uses fatrw to safely read from a FAT filesystem
+// https://github.com/balena-os/fatrw
+export async function readFromBoot(
+	fileName: string,
+	encoding: 'utf-8',
+): Promise<string>;
+export async function readFromBoot(
+	fileName: string,
+	encoding?: string,
+): Promise<Buffer>;
+export async function readFromBoot(
+	fileName: string,
+	encoding = 'buffer',
+): Promise<string | Buffer> {
+	const cmd = ['fatrw', 'read', fileName].join(' ');
+	const { stdout } = await exec(cmd, { encoding });
+	return stdout;
+}
+
+// Receives an absolute path for a file (assumed to be under the boot partition, e.g. `/mnt/root/mnt/boot/config.txt`)
+// and writes the given data. This function uses fatrw to safely write from a FAT filesystem
+// https://github.com/balena-os/fatrw
+export async function writeToBoot(fileName: string, data: string | Buffer) {
+	const fatrw = spawn('fatrw', ['write', fileName], { stdio: 'pipe' });
+
+	// Write to the process stdinput
+	fatrw.stdin.write(data);
+	fatrw.stdin.end();
+
+	// We only care about stderr
+	let error = '';
+	for await (const chunk of fatrw.stderr) {
+		error += chunk;
+	}
+	const exitCode: number = await new Promise((resolve) => {
+		fatrw.on('close', resolve);
+	});
+
+	if (exitCode) {
+		throw new CodedError(`Write failed with error: ${error}`, exitCode);
+	}
 }