Note that some stream types may not be able to implement this 100%
reliably, and it is possible that a call to OutputStream.write
after this returns TRUE would still block. To guarantee
non-blocking behavior, you should always use
g_pollable_output_stream_write_nonblocking(), which will return a
G_IO_ERROR_WOULD_BLOCK error rather than blocking.
TRUE if stream is writable, FALSE if not. If an error
has occurred on stream, this will result in
g_pollable_output_stream_is_writable() returning TRUE, and the
next attempt to write will return the error.
Checks if stream can be written.
Note that some stream types may not be able to implement this 100% reliably, and it is possible that a call to OutputStream.write after this returns TRUE would still block. To guarantee non-blocking behavior, you should always use g_pollable_output_stream_write_nonblocking(), which will return a G_IO_ERROR_WOULD_BLOCK error rather than blocking.