async: add async_write(), a write primitive to write to a NONBLOCK:ing FD

async.c

@@ -0,0 +1,35 @@
+#include "async.h"
+
+#include <stdint.h>
+#include <errno.h>
+#include <unistd.h>
+
+#define LOG_MODULE "async"
+#define LOG_ENABLE_DBG 0
+#include "log.h"
+
+enum async_write_status
+async_write(int fd, const void *_data, size_t len, size_t *idx)
+{
+    const uint8_t *const data = _data;
+    size_t left = len - *idx;
+
+    while (left > 0) {
+        ssize_t ret = write(fd, &data[*idx], left);
+
+        if (ret < 0) {
+            if (errno == EAGAIN || errno == EWOULDBLOCK)
+                return ASYNC_WRITE_REMAIN;
+
+            return ASYNC_WRITE_ERR;
+        }
+
+        LOG_DBG("wrote %zd bytes of %zu (%zu left) to FD=%d",
+                ret, left, left - ret, fd);
+
+        *idx += ret;
+        left -= ret;
+    }
+
+    return ASYNC_WRITE_DONE;
+}