Add support for BALENA_HOST_SPLASH_IMAGE config

Setting this this variable to a base64 encoded string will replace the splash
image on the device by rewriting `/mnt/boot/splash/balena-logo.png`.
This will also make a copy of the default balena logo so the splash can
be restored if the variable is removed.

Change-type: minor
Signed-off-by: Felipe Lalanne <felipe@balena.io>

src/config/backends/backend.ts

@@ -12,7 +12,7 @@ export const bootMountPoint = `${constants.rootMountPoint}${constants.bootMountP
 
 export async function remountAndWriteAtomic(
 	file: string,
-	data: string,
+	data: string | Buffer,
 ): Promise<void> {
 	// Here's the dangerous part:
 	await child_process.exec(

src/config/backends/index.ts

@@ -5,6 +5,7 @@ import { ExtraUEnv } from './extra-uEnv';
 import { ConfigTxt } from './config-txt';
 import { ConfigFs } from './config-fs';
 import { Odmdata } from './odmdata';
+import { SplashImage } from './splash-image';
 
 export const allBackends = [
 	new Extlinux(),
@@ -12,6 +13,7 @@ export const allBackends = [
 	new ConfigTxt(),
 	new ConfigFs(),
 	new Odmdata(),
+	new SplashImage(),
 ];
 
 export function matchesAnyBootConfig(envVar: string): boolean {

src/config/backends/splash-image.ts

@@ -0,0 +1,206 @@
+import * as Bluebird from 'bluebird';
+import * as _ from 'lodash';
+import { fs } from 'mz';
+import * as path from 'path';
+
+import * as constants from '../../lib/constants';
+import log from '../../lib/supervisor-console';
+import {
+	bootMountPoint,
+	ConfigBackend,
+	ConfigOptions,
+	remountAndWriteAtomic,
+} from './backend';
+
+export class SplashImage extends ConfigBackend {
+	private static readonly BASEPATH = path.join(bootMountPoint, 'splash');
+	private static readonly DEFAULT = path.join(
+		SplashImage.BASEPATH,
+		'balena-logo-default.png',
+	);
+	private static readonly FILENAMES = ['balena-logo.png', 'resin-logo.png'];
+	private static readonly PREFIX = `${constants.hostConfigVarPrefix}SPLASH_`;
+	private static readonly CONFIGS = ['image'];
+	private static readonly DATA_URI_REGEX = /^data:(.+);base64,(.*)$/;
+
+	// Check the first 8 bytes of a buffer for a PNG header
+	// Source: https://github.com/sindresorhus/is-png/blob/master/index.js
+	private isPng(buffer: Buffer) {
+		if (!buffer || buffer.length < 8) {
+			return false;
+		}
+
+		return (
+			buffer[0] === 0x89 &&
+			buffer[1] === 0x50 &&
+			buffer[2] === 0x4e &&
+			buffer[3] === 0x47 &&
+			buffer[4] === 0x0d &&
+			buffer[5] === 0x0a &&
+			buffer[6] === 0x1a &&
+			buffer[7] === 0x0a
+		);
+	}
+
+	// Get the file path for the splash image for the underlying
+	// system
+	private async getSplashPath(): Promise<string> {
+		// TODO: this is not perfect, if the file was supposed to be
+		// resin-logo.png and for some reason is not present on the folder
+		// the supervisor will try to write new images from the API into
+		// balena-logo.png to no effect. Ideally this should be configurable
+		// as a constant that is provided on supervisor container launch.
+		const [
+			// If no logo is found, assume the file is `balena-logo.png`
+			splashFile = path.join(SplashImage.BASEPATH, 'balena-logo.png'),
+		] = (
+			await Bluebird.resolve(fs.readdir(SplashImage.BASEPATH))
+				// Read the splash dir (will throw if the path does not exist)
+				// And filter valid filenames
+				.filter((filename) => SplashImage.FILENAMES.includes(filename))
+		)
+			// Sort by name, so in case both files are defined, balena-logo will
+			// be chosen
+			.sort()
+			// Convert to full path
+			.map((filename) => path.join(SplashImage.BASEPATH, filename));
+
+		return splashFile;
+	}
+
+	// Returns the base64 contents of the splash image
+	private async readSplashImage(where?: string): Promise<string> {
+		// Read from defaultPath unless where is defined
+		where = where ?? (await this.getSplashPath());
+
+		// read the image file...
+		return (await fs.readFile(where)).toString('base64');
+	}
+
+	// Write a splash image provided as a base64 string
+	private async writeSplashImage(image: string, where?: string): Promise<void> {
+		// Write to defaultPath unless where is defined
+		where = where ?? (await this.getSplashPath());
+
+		const buffer = Buffer.from(image, 'base64');
+		if (this.isPng(buffer)) {
+			// Write the buffer to the given location
+			await remountAndWriteAtomic(where, buffer);
+		} else {
+			throw new Error('Splash image should be a base64 encoded PNG image');
+		}
+	}
+
+	private stripPrefix(name: string): string {
+		if (!name.startsWith(SplashImage.PREFIX)) {
+			return name;
+		}
+		return name.substr(SplashImage.PREFIX.length);
+	}
+
+	public createConfigVarName(name: string): string {
+		return `${SplashImage.PREFIX}${name}`;
+	}
+
+	public async getBootConfig(): Promise<ConfigOptions> {
+		try {
+			const defaultImg = await this.readSplashImage(SplashImage.DEFAULT);
+			const img = await this.readSplashImage();
+
+			// If the image is the same as the default image
+			// return nothing
+			if (img !== defaultImg) {
+				return {
+					image: `data:image/png;base64,${img}`,
+				};
+			}
+		} catch (e) {
+			log.warn('Failed to read splash image:', e);
+		}
+		return {};
+	}
+
+	public async initialise(): Promise<SplashImage> {
+		try {
+			await super.initialise();
+
+			// The default boot image file has already
+			// been created
+			if (await fs.exists(SplashImage.DEFAULT)) {
+				return this;
+			}
+
+			// read the existing image file...
+			const image = await this.readSplashImage();
+
+			// write the image to the DEFAULT path
+			await this.writeSplashImage(image, SplashImage.DEFAULT);
+
+			log.success('Initialised splash image backend');
+		} catch (error) {
+			log.warn('Could not initialise splash image backend', error);
+		}
+		return this;
+	}
+
+	public ensureRequiredConfig(_deviceType: string, conf: ConfigOptions) {
+		// If the value from the cloud is empty, it is the same as no definition
+		if (
+			!_.isUndefined(conf.image) &&
+			_.isEmpty((conf.image as string).trim())
+		) {
+			delete conf.image;
+		}
+		return conf;
+	}
+
+	public isSupportedConfig(name: string): boolean {
+		return SplashImage.CONFIGS.includes(this.stripPrefix(name).toLowerCase());
+	}
+
+	public isBootConfigVar(name: string): boolean {
+		return SplashImage.CONFIGS.includes(this.stripPrefix(name).toLowerCase());
+	}
+
+	public async matches(_deviceType: string): Promise<boolean> {
+		// all device types
+		return true;
+	}
+
+	public processConfigVarName(name: string): string {
+		return this.stripPrefix(name).toLowerCase();
+	}
+
+	public processConfigVarValue(
+		_name: string,
+		value: string,
+	): string | string[] {
+		// check data url regex
+		const matches = value.match(SplashImage.DATA_URI_REGEX);
+		if (!_.isNull(matches)) {
+			const [, media, data] = matches;
+			const [type] = media.split(';'); // discard mediatype parameters
+
+			return `data:${type};base64,${data}`;
+		}
+
+		if (!_.isEmpty(value.trim())) {
+			// Assume data is base64 encoded. If is not, setBootConfig will fail
+			return `data:image/png;base64,${value}`;
+		}
+		return value.trim();
+	}
+
+	public async setBootConfig(opts: ConfigOptions): Promise<void> {
+		// If no splash image is defined, revert to the default splash image
+		const value = !_.isEmpty(opts.image)
+			? (opts.image as string)
+			: await this.readSplashImage(SplashImage.DEFAULT);
+
+		// If it is a data URI get only the data part
+		const [, image] = value.startsWith('data:') ? value.split(',') : [, value];
+
+		// Rewrite the splash image
+		await this.writeSplashImage(image);
+	}
+}

src/device-config.ts

@@ -321,9 +321,11 @@ export function formatConfigKeys(conf: {
 		confFromLegacyNamespace,
 		noNamespaceConf,
 	);
-	return _.pickBy(confWithoutNamespace, (_v, k) => {
+
-		return _.includes(validKeys, k) || matchesAnyBootConfig(k);
+	return _.pickBy(
-	});
+		confWithoutNamespace,
+		(_v, k) => _.includes(validKeys, k) || matchesAnyBootConfig(k),
+	);
 }
 
 export function getDefaults() {

src/lib/fs-utils.ts

@@ -1,18 +1,28 @@
 import * as Bluebird from 'bluebird';
+import * as _ from 'lodash';
 import { fs } from 'mz';
 import * as Path from 'path';
 import * as constants from './constants';
 import { ENOENT } from './errors';
 
-export function writeAndSyncFile(path: string, data: string): Bluebird<void> {
+export function writeAndSyncFile(
+	path: string,
+	data: string | Buffer,
+): Bluebird<void> {
 	return Bluebird.resolve(fs.open(path, 'w')).then((fd) => {
-		fs.write(fd, data, 0, 'utf8')
+		_.isString(data)
-			.then(() => fs.fsync(fd))
+			? fs.write(fd, data, 0, 'utf8')
-			.then(() => fs.close(fd));
+			: fs
+					.write(fd, data, 0, data.length)
+					.then(() => fs.fsync(fd))
+					.then(() => fs.close(fd));
 	});
 }
 
-export function writeFileAtomic(path: string, data: string): Bluebird<void> {
+export function writeFileAtomic(
+	path: string,
+	data: string | Buffer,
+): Bluebird<void> {
 	return Bluebird.resolve(writeAndSyncFile(`${path}.new`, data)).then(() =>
 		fs.rename(`${path}.new`, path),
 	);

test/12-device-config.spec.ts

@@ -10,6 +10,7 @@ import { Extlinux } from '../src/config/backends/extlinux';
 import { ConfigTxt } from '../src/config/backends/config-txt';
 import { Odmdata } from '../src/config/backends/odmdata';
 import { ConfigFs } from '../src/config/backends/config-fs';
+import { SplashImage } from '../src/config/backends/splash-image';
 import * as constants from '../src/lib/constants';
 import * as config from '../src/config';
 
@@ -19,6 +20,7 @@ const extlinuxBackend = new Extlinux();
 const configTxtBackend = new ConfigTxt();
 const odmdataBackend = new Odmdata();
 const configFsBackend = new ConfigFs();
+const splashImageBackend = new SplashImage();
 
 describe('Device Backend Config', () => {
 	let logSpy: SinonSpy;
@@ -524,4 +526,151 @@ describe('Device Backend Config', () => {
 			).to.equal(false);
 		});
 	});
+
+	describe('Boot splash image', () => {
+		const defaultLogo =
+			'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX/wQDLA+84AAAACklEQVR4nGNiAAAABgADNjd8qAAAAABJRU5ErkJggg==';
+		const png =
+			'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX/TQBcNTh/AAAAAXRSTlPM0jRW/QAAAApJREFUeJxjYgAAAAYAAzY3fKgAAAAASUVORK5CYII=';
+		const uri = `data:image/png;base64,${png}`;
+
+		beforeEach(() => {
+			// Setup stubs
+			stub(fsUtils, 'writeFileAtomic').resolves();
+			stub(child_process, 'exec').resolves();
+		});
+
+		afterEach(() => {
+			// Restore stubs
+			(fsUtils.writeFileAtomic as SinonStub).restore();
+			(child_process.exec as SinonStub).restore();
+		});
+
+		it('should correctly write to resin-logo.png', async () => {
+			// Devices with balenaOS < 2.51 use resin-logo.png
+			stub(fs, 'readdir').resolves(['resin-logo.png']);
+
+			const current = {};
+			const target = {
+				HOST_SPLASH_image: png,
+			};
+
+			// This should work with every device type, but testing on a couple
+			// of options
+			expect(
+				deviceConfig.bootConfigChangeRequired(
+					splashImageBackend,
+					current,
+					target,
+					'fincm3',
+				),
+			).to.equal(true);
+
+			await deviceConfig.setBootConfig(splashImageBackend, target);
+
+			expect(child_process.exec).to.be.calledOnce;
+			expect(logSpy).to.be.calledTwice;
+			expect(logSpy.getCall(1).args[2]).to.equal('Apply boot config success');
+			expect(fsUtils.writeFileAtomic).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/resin-logo.png',
+			);
+
+			// restore the stub
+			(fs.readdir as SinonStub).restore();
+		});
+
+		it('should correctly write to balena-logo.png', async () => {
+			// Devices with balenaOS >= 2.51 use balena-logo.png
+			stub(fs, 'readdir').resolves(['balena-logo.png']);
+
+			const current = {};
+			const target = {
+				HOST_SPLASH_image: png,
+			};
+
+			// This should work with every device type, but testing on a couple
+			// of options
+			expect(
+				deviceConfig.bootConfigChangeRequired(
+					splashImageBackend,
+					current,
+					target,
+					'raspberrypi4-64',
+				),
+			).to.equal(true);
+
+			await deviceConfig.setBootConfig(splashImageBackend, target);
+
+			expect(child_process.exec).to.be.calledOnce;
+			expect(logSpy).to.be.calledTwice;
+			expect(logSpy.getCall(1).args[2]).to.equal('Apply boot config success');
+			expect(fsUtils.writeFileAtomic).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+			);
+
+			// restore the stub
+			(fs.readdir as SinonStub).restore();
+		});
+
+		it('should correctly write to balena-logo.png if no default logo is found', async () => {
+			// Devices with balenaOS >= 2.51 use balena-logo.png
+			stub(fs, 'readdir').resolves([]);
+
+			const current = {};
+			const target = {
+				HOST_SPLASH_image: png,
+			};
+
+			// This should work with every device type, but testing on a couple
+			// of options
+			expect(
+				deviceConfig.bootConfigChangeRequired(
+					splashImageBackend,
+					current,
+					target,
+					'raspberrypi3',
+				),
+			).to.equal(true);
+
+			await deviceConfig.setBootConfig(splashImageBackend, target);
+
+			expect(child_process.exec).to.be.calledOnce;
+			expect(logSpy).to.be.calledTwice;
+			expect(logSpy.getCall(1).args[2]).to.equal('Apply boot config success');
+			expect(fsUtils.writeFileAtomic).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+			);
+
+			// restore the stub
+			(fs.readdir as SinonStub).restore();
+		});
+
+		it('should correctly read the splash logo if different from the default', async () => {
+			stub(fs, 'readdir').resolves(['balena-logo.png']);
+
+			const readFileStub: SinonStub = stub(fs, 'readFile').resolves(
+				Buffer.from(png, 'base64') as any,
+			);
+			readFileStub
+				.withArgs('test/data/mnt/boot/splash/balena-logo-default.png')
+				.resolves(Buffer.from(defaultLogo, 'base64') as any);
+
+			expect(
+				await deviceConfig.getBootConfig(splashImageBackend),
+			).to.deep.equal({
+				HOST_SPLASH_image: uri,
+			});
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+			);
+
+			// Restore stubs
+			(fs.readdir as SinonStub).restore();
+			(fs.readFile as SinonStub).restore();
+			readFileStub.restore();
+		});
+	});
 });

test/16-config-utils.spec.ts

@@ -8,6 +8,7 @@ import { ExtraUEnv } from '../src/config/backends/extra-uEnv';
 import { Extlinux } from '../src/config/backends/extlinux';
 import { ConfigTxt } from '../src/config/backends/config-txt';
 import { ConfigFs } from '../src/config/backends/config-fs';
+import { SplashImage } from '../src/config/backends/splash-image';
 import { ConfigBackend } from '../src/config/backends/backend';
 
 describe('Config Utilities', () => {
@@ -16,8 +17,9 @@ describe('Config Utilities', () => {
 		const configStub = stub(config, 'get').resolves('raspberry');
 		// Get list of backends
 		const devices = await configUtils.getSupportedBackends();
-		expect(devices.length).to.equal(1);
+		expect(devices.length).to.equal(2);
 		expect(devices[0].constructor.name).to.equal('ConfigTxt');
+		expect(devices[1].constructor.name).to.equal('SplashImage');
 		// Restore stub
 		configStub.restore();
 		// TO-DO: When we have a device that will match for multiple backends
@@ -46,6 +48,7 @@ const BACKENDS: Record<string, ConfigBackend> = {
 	extlinux: new Extlinux(),
 	configtxt: new ConfigTxt(),
 	configfs: new ConfigFs(),
+	splashImage: new SplashImage(),
 };
 
 const CONFIGS = {
@@ -88,6 +91,14 @@ const CONFIGS = {
 			foobar: 'baz',
 		},
 	},
+	splashImage: {
+		envVars: {
+			HOST_SPLASH_image: 'data:image/png;base64,aaa',
+		},
+		bootConfig: {
+			image: 'data:image/png;base64,aaa',
+		},
+	},
 	// TO-DO: Config-FS is commented out because it behaves differently and doesn't
 	// add value to the Config Utilities if we make it work but would like to add it
 	// configfs: {

test/43-splash-image.spec.ts

@@ -0,0 +1,306 @@
+import { fs, child_process } from 'mz';
+import { SinonStub, stub } from 'sinon';
+
+import { expect } from './lib/chai-config';
+import * as fsUtils from '../src/lib/fs-utils';
+import { SplashImage } from '../src/config/backends/splash-image';
+import log from '../src/lib/supervisor-console';
+
+describe('Splash image configuration', () => {
+	const backend = new SplashImage();
+	const defaultLogo =
+		'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX/wQDLA+84AAAACklEQVR4nGNiAAAABgADNjd8qAAAAABJRU5ErkJggg==';
+	const logo =
+		'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX/TQBcNTh/AAAAAXRSTlPM0jRW/QAAAApJREFUeJxjYgAAAAYAAzY3fKgAAAAASUVORK5CYII=';
+	const uri = `data:image/png;base64,${logo}`;
+	let readDirStub: SinonStub;
+	let readFileStub: SinonStub;
+	let writeFileAtomicStub: SinonStub;
+
+	beforeEach(() => {
+		// Setup stubs
+		writeFileAtomicStub = stub(fsUtils, 'writeFileAtomic').resolves();
+		stub(child_process, 'exec').resolves();
+		readFileStub = stub(fs, 'readFile').resolves(
+			Buffer.from(logo, 'base64') as any,
+		);
+		readFileStub
+			.withArgs('test/data/mnt/boot/splash/balena-logo-default.png')
+			.resolves(Buffer.from(defaultLogo, 'base64') as any);
+		readDirStub = stub(fs, 'readdir').resolves(['balena-logo.png']);
+	});
+
+	afterEach(() => {
+		// Restore stubs
+		writeFileAtomicStub.restore();
+		(child_process.exec as SinonStub).restore();
+		readFileStub.restore();
+		readDirStub.restore();
+	});
+
+	describe('initialise', () => {
+		it('should make a copy of the existing boot image on initialise if not yet created', async () => {
+			stub(fs, 'exists').resolves(false);
+
+			// Do the initialization
+			await backend.initialise();
+
+			expect(fs.readFile).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+			);
+
+			// Should make a copy
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+				Buffer.from(logo, 'base64'),
+			);
+
+			(fs.exists as SinonStub).restore();
+		});
+
+		it('should skip initialization if the default image already exists', async () => {
+			stub(fs, 'exists').resolves(true);
+
+			// Do the initialization
+			await backend.initialise();
+
+			expect(fs.exists).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(fs.readFile).to.not.have.been.called;
+
+			(fs.exists as SinonStub).restore();
+		});
+
+		it('should fail initialization if there is no default image on the device', async () => {
+			stub(fs, 'exists').resolves(false);
+			readDirStub.resolves([]);
+			readFileStub.rejects();
+			stub(log, 'warn');
+
+			// Do the initialization
+			await backend.initialise();
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(fs.readFile).to.have.been.calledOnce;
+			expect(log.warn).to.be.calledOnce;
+
+			(log.warn as SinonStub).restore();
+		});
+	});
+
+	describe('getBootConfig', () => {
+		it('should return an empty object if the current logo matches the default logo', async () => {
+			readDirStub.resolves(['resin-logo.png']);
+
+			// The default logo resolves to the same value as the current logo
+			readFileStub
+				.withArgs('test/data/mnt/boot/splash/balena-logo-default.png')
+				.resolves(logo);
+
+			expect(await backend.getBootConfig()).to.deep.equal({});
+			expect(readDirStub).to.be.calledOnce;
+			expect(readFileStub).to.be.calledTwice;
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/resin-logo.png',
+			);
+		});
+
+		it('should read the splash image from resin-logo.png if available', async () => {
+			readDirStub.resolves(['resin-logo.png']);
+
+			expect(await backend.getBootConfig()).to.deep.equal({
+				image: uri,
+			});
+			expect(readDirStub).to.be.calledOnce;
+			expect(readFileStub).to.be.calledTwice;
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/resin-logo.png',
+			);
+		});
+
+		it('should read the splash image from balena-logo.png if available', async () => {
+			readDirStub.resolves(['balena-logo.png']);
+
+			expect(await backend.getBootConfig()).to.deep.equal({
+				image: uri,
+			});
+			expect(readDirStub).to.be.calledOnce;
+			expect(readFileStub).to.be.calledTwice;
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+			);
+		});
+
+		it('should read the splash image from balena-logo.png even if resin-logo.png exists', async () => {
+			readDirStub.resolves(['balena-logo.png', 'resin-logo.png']);
+
+			expect(await backend.getBootConfig()).to.deep.equal({
+				image: uri,
+			});
+			expect(readFileStub).to.be.calledTwice;
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(readFileStub).to.be.calledWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+			);
+		});
+
+		it('should catch readDir errors', async () => {
+			stub(log, 'warn');
+			readDirStub.rejects();
+
+			expect(await backend.getBootConfig()).to.deep.equal({});
+			expect(readDirStub).to.be.calledOnce;
+			expect(log.warn).to.be.calledOnce;
+
+			(log.warn as SinonStub).restore();
+		});
+
+		it('should catch readFile errors', async () => {
+			stub(log, 'warn');
+			readDirStub.resolves([]);
+			readFileStub.rejects();
+
+			expect(await backend.getBootConfig()).to.deep.equal({});
+			expect(log.warn).to.be.calledOnce;
+
+			(log.warn as SinonStub).restore();
+		});
+	});
+
+	describe('setBootConfig', () => {
+		it('should write the given image to resin-logo.png if set', async () => {
+			readDirStub.resolves(['resin-logo.png']);
+
+			await backend.setBootConfig({ image: uri });
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/resin-logo.png',
+				Buffer.from(logo, 'base64'),
+			);
+		});
+
+		it('should write the given image to balena-logo.png if set', async () => {
+			readDirStub.resolves(['balena-logo.png']);
+
+			await backend.setBootConfig({ image: uri });
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+				Buffer.from(logo, 'base64'),
+			);
+		});
+
+		it('should write the given image to balena-logo.png by default', async () => {
+			readDirStub.resolves([]);
+
+			await backend.setBootConfig({ image: uri });
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+				Buffer.from(logo, 'base64'),
+			);
+		});
+
+		it('should accept just a base64 as an image', async () => {
+			readDirStub.resolves(['balena-logo.png']);
+
+			await backend.setBootConfig({ image: logo });
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+				Buffer.from(logo, 'base64'),
+			);
+		});
+
+		it('should restore balena-logo.png if image arg is unset', async () => {
+			readDirStub.resolves(['balena-logo.png']);
+			await backend.setBootConfig({});
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(readFileStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+				Buffer.from(defaultLogo, 'base64'),
+			);
+		});
+
+		it('should restore resin-logo.png if image arg is unset', async () => {
+			readDirStub.resolves(['resin-logo.png']);
+			await backend.setBootConfig({});
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(readFileStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/resin-logo.png',
+				Buffer.from(defaultLogo, 'base64'),
+			);
+		});
+
+		it('should restore balena-logo.png if image arg is empty', async () => {
+			readDirStub.resolves(['balena-logo.png']);
+			await backend.setBootConfig({ image: '' });
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(readFileStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo.png',
+				Buffer.from(defaultLogo, 'base64'),
+			);
+		});
+
+		it('should restore resin-logo.png if image arg is empty', async () => {
+			readDirStub.resolves(['resin-logo.png']);
+			await backend.setBootConfig({ image: '' });
+
+			expect(readDirStub).to.be.calledOnce;
+			expect(readFileStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/balena-logo-default.png',
+			);
+			expect(writeFileAtomicStub).to.be.calledOnceWith(
+				'test/data/mnt/boot/splash/resin-logo.png',
+				Buffer.from(defaultLogo, 'base64'),
+			);
+		});
+
+		it('should throw if arg is not a valid base64 string', async () => {
+			expect(backend.setBootConfig({ image: 'somestring' })).to.be.rejected;
+			expect(writeFileAtomicStub).to.not.be.called;
+		});
+
+		it('should throw if image is not a valid PNG file', async () => {
+			expect(backend.setBootConfig({ image: 'aGVsbG8=' })).to.be.rejected;
+			expect(writeFileAtomicStub).to.not.be.called;
+		});
+	});
+
+	describe('isBootConfigVar', () => {
+		it('Accepts any case variable names', () => {
+			expect(backend.isBootConfigVar('HOST_SPLASH_IMAGE')).to.be.true;
+			expect(backend.isBootConfigVar('HOST_SPLASH_image')).to.be.true;
+			expect(backend.isBootConfigVar('HOST_SPLASH_Image')).to.be.true;
+			expect(backend.isBootConfigVar('HOST_SPLASH_ImAgE')).to.be.true;
+		});
+	});
+});