Add readTag function to AbstractMailboxSession

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

@@ -247,4 +247,35 @@ public abstract class AbstractMailboxSession implements Runnable {
 		return os.toByteArray();
 	}
 
+
+	/**
+	 * Reads a Tag from the given InputStream and returns the matching StreamContext
+	 * if available
+	 *
+	 * @param in InputStream to read the tag from
+	 * @return StreamContext matching the tag read from the InputStream
+	 * @throws IOException
+	 * @throws DbException
+	 */
+	@Nullable
+	protected StreamContext readTag(InputStream in)
+			throws IOException, DbException {
+
+		byte[] tag = new byte[TAG_LENGTH];
+		// read tag from input stream
+		int offset = 0;
+		while (offset < tag.length) {
+			int read = in.read(tag, offset, tag.length - offset);
+			if (read == -1) throw new EOFException();
+			offset += read;
+		}
+
+		StreamContext ctx =
+				keyManager.getStreamContext(MailboxConstants.ID, tag);
+
+		if (ctx == null)
+			throw new IOException(
+					"Received stream with unrecognisable tag");
+		return ctx;
+	}
 }