Having such a function in WP would be useful, but not that frequently, so it’s understandable if there’s not one yet.
Speaking generally, I’d think it’s not too complex to make such a function. First it would support one or two most common setups, and support for additional ones would be added when needed. However, I don’t see a single party that would benefit of this enough to be bothered to write it, since those able to write these kind of patches have other means of monitoring disk space handy.
Which, as an aside, I suspect is also the reason why some of the most basic things are clumsy to do in Drupal; the core developers have learned to work around those issues efficiently and don’t feel much of a need to make eg. updating a site’s Drupal core to be doable via GUI. It’s a tedious command line task, which can be automated via a shell script, so I suspect the core devs have set up shell scripts for themselves. They have no direct incentive to make a GUI update system, as that would deprecate their current ones. However, having a GUI update system would benefit the Drupal ecosystem as a whole, which would indirectly benefit the core devs, who could land more fame and consulting jobs.
As an end to that aside, my contributions also typically limit to reports and suggestions in WP forums and trackers. Having a free_space_available() function is, for me, worth a shot by writing a forum post about it, but not the full working day or two that would be spent planning, writing, testing and promoting the patch, that might then get completely ignored as those with the commit access to core have higher priorities themselves.
A pluginland solution could also make sense, but so far I haven’t seen any library plugins get much traction for WP, and this one I don’t see pressing enough to make much sense as a standalone. With an ideal pluginland solution WPide could just use a library when available like this:
if ( $Library ) {
if ( ! $Library->free_space_available() ) {
// handle the error and exit
}
}
// Attempt the saving of the file
