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;
+}

async.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <stddef.h>
+
+enum async_write_status {ASYNC_WRITE_DONE, ASYNC_WRITE_REMAIN, ASYNC_WRITE_ERR};
+
+/*
+ * Primitive that writes data to a NONBLOCK:ing FD.
+ *
+ * _data: points to the beginning of the buffer
+ * len: total size of the data buffer
+ * idx: pointer to byte offset into data buffer - writing starts here.
+ *
+ * Thus, the total amount of data to write is (len - *idx). *idx is
+ * updated such that it points to the next unwritten byte in the data
+ * buffer.
+ *
+ * I.e. if the return value is:
+ *  - ASYNC_WRITE_DONE, then the *idx == len.
+ *  - ASYNC_WRITE_REMAIN, then *idx < len
+ *  - ASYNC_WRITE_ERR, there was an error, and no data was written
+ */
+enum async_write_status async_write(
+    int fd, const void *data, size_t len, size_t *idx);

meson.build

@@ -64,6 +64,7 @@ version = custom_target(
 
 executable(
   'foot',
+  'async.c', 'async.h',
   'base64.c', 'base64.h',
   'config.c', 'config.h',
   'commands.c', 'commands.h',