device: allow to use dummy as a way to test streamer

This adds loopback tests via `tests/`

device/dummy/buffer.c

@@ -1,11 +1,18 @@
 #include "dummy.h"
 #include "device/buffer.h"
+#include "device/buffer_list.h"
+#include "util/opts/log.h"
 
 #include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
 
 int dummy_buffer_open(buffer_t *buf)
 {
   buf->dummy = calloc(1, sizeof(buffer_dummy_t));
+  buf->start = buf->buf_list->dummy->data;
+  buf->used = buf->buf_list->dummy->length;
+  buf->length = buf->buf_list->dummy->length;
   return 0;
 }
 
@@ -16,15 +23,39 @@ void dummy_buffer_close(buffer_t *buf)
 
 int dummy_buffer_enqueue(buffer_t *buf, const char *who)
 {
-  return -1;
+  unsigned index = buf->index;
+  if (write(buf->buf_list->dummy->fds[1], &index, sizeof(index)) != sizeof(index)) {
+    return -1;
+  }
+  return 0;
 }
 
 int dummy_buffer_list_dequeue(buffer_list_t *buf_list, buffer_t **bufp)
 {
-  return -1;
+  unsigned index = 0;
+  int n = read(buf_list->dummy->fds[0], &index, sizeof(index));
+  if (n != sizeof(index)) {
+    LOG_INFO(buf_list, "Received invalid result from `read`: %d", n);
+    return -1;
+  }
+
+  if (index >= (unsigned)buf_list->nbufs) {
+    LOG_INFO(buf_list, "Received invalid index from `read`: %d >= %d", index, buf_list->nbufs);
+    return -1;
+  }
+
+  *bufp = buf_list->bufs[index];
+  return 0;
 }
 
 int dummy_buffer_list_pollfd(buffer_list_t *buf_list, struct pollfd *pollfd, bool can_dequeue)
 {
-  return -1;
+  int count_enqueued = buffer_list_count_enqueued(buf_list);
+  pollfd->fd = buf_list->dummy->fds[0]; // write end
+  pollfd->events = POLLHUP;
+  if (can_dequeue && count_enqueued > 0) {
+    pollfd->events |= POLLIN;
+  }
+  pollfd->revents = 0;
+  return 0;
 }