Add method for downloading a file from a mailbox

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApi.java

@@ -77,6 +77,17 @@ interface MailboxApi {
 	List<MailboxFile> getFiles(MailboxProperties properties, String folderId)
 			throws IOException, ApiException;
 
+	/**
+	 * Used by owner and contacts to retrieve a file.
+	 * <p>
+	 * Returns 200 OK if successful with the files' raw bytes
+	 * in the response body.
+	 *
+	 * @param file the empty file the response bytes will be written into.
+	 */
+	void getFile(MailboxProperties properties, String folderId,
+			String fileId, File file) throws IOException, ApiException;
+
 	@Immutable
 	@JsonSerialize
 	class MailboxContact {

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiImpl.java

@@ -11,6 +11,7 @@ import org.briarproject.bramble.api.mailbox.MailboxProperties;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.File;
+import java.io.FileOutputStream;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -28,6 +29,7 @@ import okhttp3.ResponseBody;
 import static com.fasterxml.jackson.databind.MapperFeature.BLOCK_UNSAFE_POLYMORPHIC_BASE_TYPES;
 import static java.util.Objects.requireNonNull;
 import static okhttp3.internal.Util.EMPTY_REQUEST;
+import static org.briarproject.bramble.util.IoUtils.copyAndClose;
 import static org.briarproject.bramble.util.StringUtils.fromHexString;
 
 @NotNullByDefault
@@ -206,6 +208,19 @@ class MailboxApiImpl implements MailboxApi {
 		}
 	}
 
+	@Override
+	public void getFile(MailboxProperties properties, String folderId,
+			String fileId, File file) throws IOException, ApiException {
+		String path = "/files/" + folderId + "/" + fileId;
+		Response response = sendGetRequest(properties, path);
+		if (response.code() != 200) throw new ApiException();
+
+		ResponseBody body = response.body();
+		if (body == null) throw new ApiException();
+		FileOutputStream outputStream = new FileOutputStream(file);
+		copyAndClose(body.byteStream(), outputStream);
+	}
+
 	/* Helper Functions */
 
 	private Response sendGetRequest(MailboxProperties properties, String path)

bramble-core/src/test/java/org/briarproject/bramble/mailbox/MailboxApiTest.java

@@ -25,12 +25,14 @@ import okhttp3.OkHttpClient;
 import okhttp3.mockwebserver.MockResponse;
 import okhttp3.mockwebserver.MockWebServer;
 import okhttp3.mockwebserver.RecordedRequest;
+import okio.Buffer;
 
 import static java.util.Collections.singletonList;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static org.briarproject.bramble.test.TestUtils.getContactId;
 import static org.briarproject.bramble.test.TestUtils.getMailboxSecret;
 import static org.briarproject.bramble.test.TestUtils.getRandomBytes;
+import static org.briarproject.bramble.test.TestUtils.readBytes;
 import static org.briarproject.bramble.test.TestUtils.writeBytes;
 import static org.briarproject.bramble.util.StringUtils.getRandomString;
 import static org.junit.Assert.assertArrayEquals;
@@ -533,6 +535,53 @@ public class MailboxApiTest extends BrambleTestCase {
 		assertToken(request9, token);
 	}
 
+	@Test
+	public void testGetFile() throws Exception {
+		String name = getMailboxSecret();
+		File file1 = folder.newFile();
+		File file2 = folder.newFile();
+		File file3 = folder.newFile();
+		byte[] bytes = getRandomBytes(1337);
+
+		MockWebServer server = new MockWebServer();
+		server.enqueue(new MockResponse().setBody(new Buffer().write(bytes)));
+		server.enqueue(new MockResponse().setResponseCode(401));
+		server.enqueue(new MockResponse().setResponseCode(500));
+		server.start();
+		String baseUrl = getBaseUrl(server);
+		MailboxProperties properties =
+				new MailboxProperties(baseUrl, token, true);
+
+		// file gets downloaded as expected
+		api.getFile(properties, contactOutboxId, name, file1);
+		RecordedRequest request1 = server.takeRequest();
+		assertEquals("/files/" + contactOutboxId + "/" + name,
+				request1.getPath());
+		assertEquals("GET", request1.getMethod());
+		assertToken(request1, token);
+		assertArrayEquals(bytes, readBytes(file1));
+
+		// request is not successful
+		assertThrows(ApiException.class, () ->
+				api.getFile(properties, contactOutboxId, name, file2));
+		RecordedRequest request2 = server.takeRequest();
+		assertEquals("/files/" + contactOutboxId + "/" + name,
+				request2.getPath());
+		assertEquals("GET", request1.getMethod());
+		assertToken(request2, token);
+		assertEquals(0, readBytes(file2).length);
+
+		// server error
+		assertThrows(ApiException.class, () ->
+				api.getFile(properties, contactOutboxId, name, file3));
+		RecordedRequest request3 = server.takeRequest();
+		assertEquals("/files/" + contactOutboxId + "/" + name,
+				request3.getPath());
+		assertEquals("GET", request1.getMethod());
+		assertToken(request3, token);
+		assertEquals(0, readBytes(file3).length);
+	}
+
 	private String getBaseUrl(MockWebServer server) {
 		String baseUrl = server.url("").toString();
 		return baseUrl.substring(0, baseUrl.length() - 1);