io: add MultiWriter (#10021)

vlib/io/multi_writer.v

@@ -0,0 +1,33 @@
+module io
+
+// new_multi_writer returns a Writer that writes to all writers. The write
+// function of the returned Writer writes to all writers of the MultiWriter,
+// returns the length of bytes written, and if any writer fails to write the
+// full length an error is returned and writing to other writers stops, and if
+// any writer returns an error the error is returned immediately and writing to
+// other writers stops.
+pub fn new_multi_writer(writers []Writer) Writer {
+	return &MultiWriter{
+		writers: writers
+	}
+}
+
+// MultiWriter writes to all its writers.
+pub struct MultiWriter {
+pub:
+	writers []Writer
+}
+
+// write writes to all writers of the MultiWriter. Returns the length of bytes
+// written. If any writer fails to write the full length an error is returned
+// and writing to other writers stops. If any writer returns an error the error
+// is returned immediately and writing to other writers stops.
+pub fn (m MultiWriter) write(buf []byte) ?int {
+	for w in m.writers {
+		n := w.write(buf) ?
+		if n != buf.len {
+			return error('io: incomplete write to writer of MultiWriter')
+		}
+	}
+	return buf.len
+}