Bug 627071 – g_output_stream_write() clarification

This patch guarantees that g_output_stream_write() can never fail with
G_IO_ERROR_WOULD_BLOCK. Without such a guarantee, we would need some
kind of GIOPollable interface or some way to get an event when the
stream is writable again. Which is mostly useless considering that
this method is asynchronous anyway.

Note: this patch just codifies existing behavior - GUnixOutputStream,
GSocketOutputStream and other implementations already work this way.

See also bug 626748 comment 5 for how the GDBus code relies on this
guarantee.

https://bugzilla.gnome.org/show_bug.cgi?id=627071

Signed-off-by: David Zeuthen <davidz@redhat.com>

gio/goutputstream.c

@@ -673,6 +673,10 @@ async_ready_close_flushed_callback_wrapper (GObject      *source_object,
  * requested size, as it can happen e.g. on a partial I/O error, 
  * but generally we try to write as many bytes as requested. 
  *
+ * You are guaranteed that this method will never fail with
+ * %G_IO_ERROR_WOULD_BLOCK - if @stream can't accept more data, the
+ * method will just wait until this changes.
+ *
  * Any outstanding I/O request with higher priority (lower numerical 
  * value) will be executed before an outstanding request with lower 
  * priority. Default priority is %G_PRIORITY_DEFAULT.