lib: add ringbuf socket read function

Implement new ringbuf function to do the proper socket reads without
the need of intermediary buffers.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>

lib/ringbuf.c

@@ -131,3 +131,38 @@ void ringbuf_wipe(struct ringbuf *buf)
 	memset(buf->data, 0x00, buf->size);
 	ringbuf_reset(buf);
 }
+
+ssize_t ringbuf_read(struct ringbuf *buf, int sock)
+{
+	size_t to_read = ringbuf_space(buf);
+	size_t bytes_to_end = buf->size - buf->end;
+	ssize_t bytes_read;
+	struct iovec iov[2] = {};
+
+	/* Calculate amount of read blocks. */
+	if (to_read > bytes_to_end) {
+		iov[0].iov_base = buf->data + buf->end;
+		iov[0].iov_len = bytes_to_end;
+		iov[1].iov_base = buf->data;
+		iov[1].iov_len = to_read - bytes_to_end;
+	} else {
+		iov[0].iov_base = buf->data + buf->end;
+		iov[0].iov_len = to_read;
+	}
+
+	/* Do the system call. */
+	bytes_read = readv(sock, iov, 2);
+	if (bytes_read <= 0)
+		return bytes_read;
+
+	/* Calculate the new end. */
+	if ((size_t)bytes_read > bytes_to_end)
+		buf->end = bytes_read - bytes_to_end;
+	else
+		buf->end += bytes_read;
+
+	/* Set emptiness state. */
+	buf->empty = (buf->start == buf->end) && (buf->empty && !bytes_read);
+
+	return bytes_read;
+}

lib/ringbuf.h

@@ -126,6 +126,17 @@ void ringbuf_reset(struct ringbuf *buf);
  */
 void ringbuf_wipe(struct ringbuf *buf);
 
+/**
+ * Perform a socket/file `read()` in to the ring buffer.
+ *
+ * \param buf the ring buffer pointer.
+ * \param sock the file descriptor.
+ * \returns the number of bytes read, `0` on connection close or `-1` with
+ *          `errno` pointing the error (see `readv()` man page for more
+ *          information.)
+ */
+ssize_t ringbuf_read(struct ringbuf *buf, int sock);
+
 #ifdef __cplusplus
 }
 #endif