Create mechanism to read files from host /

The supervisor needs to read some files from the host root since there
are no alternative interfaces that the host provides. These files are

- /etc/os-release to read the OS version
- /run/openvpn/vpn_status/active to check the VPN status

While we are working to introduce alternative interfaces for reading the
information provided by these files, the supervisor still needs a
backwards compatible way to get that info.

This PR uses the fact that the supervisor has access to the docker
socket to mount a file into a container and read the file from there.

Change-type: minor

src/lib/host-utils.ts

@@ -2,6 +2,8 @@ import { spawn } from 'child_process';
 import * as path from 'path';
 import * as constants from './constants';
 import { exec, exists } from './fs-utils';
+import { PassThrough } from 'stream';
+import { docker } from './docker-utils';
 
 // Returns an absolute path starting from the hostOS root partition
 // This path is accessible from within the Supervisor container
@@ -21,6 +23,72 @@ class CodedError extends Error {
 	}
 }
 
+/**
+ * This function is used to read a file from the hostOS
+ *
+ * This function launches a docker container with a mount into
+ * the hostOS root partition, and then reads the file from there.
+ * This is an expensive operation and should be used ideally never.
+ *
+ * TODO: remove this when the OS has a dbus API for reading the OS version
+ * and the VPN status
+ */
+export async function readFromRoot(
+	fileName: string,
+	encoding: 'utf8' | 'utf-8',
+): Promise<string>;
+export async function readFromRoot(fileName: string): Promise<Buffer>;
+export async function readFromRoot(
+	fileName: string,
+	encoding?: 'utf8' | 'utf-8',
+) {
+	const stdout = new PassThrough();
+
+	return new Promise(async (resolve) => {
+		const chunks: Buffer[] = [];
+		stdout.on('data', (chunk) => {
+			chunks.push(Buffer.from(chunk));
+		});
+
+		stdout.on('end', () => {
+			const buf = Buffer.concat(chunks);
+			if (encoding) {
+				resolve(buf.toString(encoding));
+			} else {
+				resolve(buf);
+			}
+		});
+
+		const [output] = await docker.run(
+			// TODO: use the supervisor image for this. We need first to get
+			// the image from the current container
+			'alpine:latest',
+			['cat', fileName],
+			stdout,
+			{
+				HostConfig: {
+					Mounts: [
+						{
+							type: 'bind',
+							source: fileName,
+							target: fileName,
+							readonly: true,
+						},
+					],
+					AutoRemove: true,
+				},
+			},
+		);
+
+		if (output.StatusCode !== 0) {
+			throw new CodedError(
+				`Failed to read ${fileName} from the hostOS`,
+				'ENOENT',
+			);
+		}
+	});
+}
+
 // 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