wstream: write completion callback

Now modules using the wstream can find out what's happening to their writes.

src/nvim/os/wstream.c

@@ -21,6 +21,9 @@ struct wstream {
   // Number of pending requests
   size_t pending_reqs;
   bool freed;
+  // (optional) Write callback and data
+  wstream_cb cb;
+  void *data;
 };
 
 struct wbuffer {
@@ -57,6 +60,7 @@ WStream * wstream_new(size_t maxmem)
   rv->curmem = 0;
   rv->pending_reqs = 0;
   rv->freed = false;
+  rv->cb = NULL;
 
   return rv;
 }
@@ -83,6 +87,25 @@ void wstream_set_stream(WStream *wstream, uv_stream_t *stream)
   wstream->stream = stream;
 }
 
+/// Sets a callback that will be called on completion of a write request,
+/// indicating failure/success.
+///
+/// This affects all requests currently in-flight as well. Overwrites any
+/// possible earlier callback.
+///
+/// @note This callback will not fire if the write request couldn't even be
+///       queued properly (i.e.: when `wstream_write() returns an error`).
+///
+/// @param wstream The `WStream` instance
+/// @param cb The callback
+/// @param data User-provided data that will be passed to `cb`
+void wstream_set_write_cb(WStream *wstream, wstream_cb cb, void *data)
+  FUNC_ATTR_NONNULL_ARG(1)
+{
+  wstream->cb = cb;
+  wstream->data = data;
+}
+
 /// Queues data for writing to the backing file descriptor of a `WStream`
 /// instance. This will fail if the write would cause the WStream use more
 /// memory than specified by `maxmem`.
@@ -162,6 +185,14 @@ static void write_cb(uv_write_t *req, int status)
   release_wbuffer(data->buffer);
 
   data->wstream->pending_reqs--;
+
+  if (data->wstream->cb) {
+    data->wstream->cb(data->wstream,
+                      data->wstream->data,
+                      data->wstream->pending_reqs,
+                      status);
+  }
+
   if (data->wstream->freed && data->wstream->pending_reqs == 0) {
     // Last pending write, free the wstream;
     free(data->wstream);

src/nvim/os/wstream_defs.h

@@ -5,5 +5,17 @@ typedef struct wbuffer WBuffer;
 typedef struct wstream WStream;
 typedef void (*wbuffer_data_finalizer)(void *data);
 
+/// Type of function called when the WStream has information about a write
+/// request.
+///
+/// @param wstream The `WStream` instance
+/// @param data User-defined data
+/// @param pending The number of write requests that are still pending
+/// @param status 0 on success, anything else indicates failure
+typedef void (*wstream_cb)(WStream *wstream,
+                           void *data,
+                           size_t pending,
+                           int status);
+
 #endif  // NVIM_OS_WSTREAM_DEFS_H