Add contract resolution code, which checks release requirements

Change-type: minor
Signed-off-by: Cameron Diver <cameron@balena.io>

src/lib/contracts.ts

@@ -0,0 +1,66 @@
+import * as _ from 'lodash';
+
+import { Blueprint, Contract, ContractObject } from '@balena/contrato';
+
+import constants = require('./constants');
+import { InternalInconsistencyError } from './errors';
+import * as osRelease from './os-release';
+import supervisorVersion = require('./supervisor-version');
+
+export async function containerContractsFulfilled(
+	containers: ContractObject[],
+): Promise<boolean> {
+	const osContract = new Contract({
+		slug: 'balenaOS',
+		type: 'sw.os',
+		name: 'balenaOS',
+		version: await osRelease.getOSSemver(constants.hostOSVersionPath),
+	});
+
+	const supervisorContract = new Contract({
+		slug: 'balena-supervisor',
+		type: 'sw.supervisor',
+		name: 'balena-supervisor',
+		version: supervisorVersion,
+	});
+
+	const blueprint = new Blueprint(
+		{
+			'sw.os': 1,
+			'sw.supervisor': 1,
+			'sw.container': '1+',
+		},
+		{
+			type: 'sw.runnable.configuration',
+			slug: '{{children.sw.container.slug}}',
+		},
+	);
+
+	const universe = new Contract({
+		type: 'meta.universe',
+	});
+
+	universe.addChildren(
+		[osContract, supervisorContract].concat(
+			containers.map(c => new Contract(c)),
+		),
+	);
+
+	const solution = blueprint.reproduce(universe);
+
+	if (solution.length > 1) {
+		throw new InternalInconsistencyError(
+			'More than one solution available for container contracts when only one is expected!',
+		);
+	}
+	if (solution.length === 0) {
+		return false;
+	}
+
+	// Detect how many containers are present in the resulting
+	// solution
+	const children = solution[0].getChildren({
+		types: new Set(['sw.container']),
+	});
+	return children.length === containers.length;
+}