`wpdb->last_result` may not be countable

[desrosj]

In wpdb->get_col(), $wpdb->last_result may be null or an empty string, which are uncountable values.

This can be triggered by passing an empty query string as a query.

Props @spacedmonkey for pointing this out.

[spacedmonkey]

For a bit more detail, this notice error is generated when an invalid or empty query is passed to the get_col. It is kind of edge case but not to replicate.

The issue, is because get_row can return a last_result as null.
There are two fixes for this, either we change get_row, so it returns an empty array on failure. But this could be a breaking change as it changes functionality.

I have patched a patch for a much similar solution, that doesn't even require is_countable to be called.

[spacedmonkey]

While reviewing this bug, I found a second place, where last_result being null would be an issue.

See attached 45299-bug1.diff which is a simple fix. It might have to go in another ticket.

[dd32]

A visual review of the diffs looks good, marking as has-patch, needs-testing.

• 45299-bug1.diff​ matches the check used next block down for ARRAY_*.
• 45299.diff​ get_col() differs from the rest of the get_*() methods (which return null in this case) and always returns an array, so this also seems like a sane change.

[desrosj]

Related: #43583, #44123.

[desrosj]

45299.2.diff​ is a patch that combines the two changes and applies to the 5.0 branch.

Also, set up a ​test build on Travis. The changes are not causing any test failures, but I doubt there are tests that would trigger this specific edge case.

[desrosj]

Add some unit tests.

[pento]

In 43934:

WPDB: Check that $wpdb->last_result is countable before counting with it.

wpdb::get_col() iterates over $wpdb->last_result, which can be a non-countable value, should the preceeding query have failed.

Props spacedmonkey, desrosj.
See #45299.

[desrosj]

45299.3.diff​ applies cleanly to the 4.9 branch, if backporting is necessary (more details here).

[desrosj]

In 44272:

WPDB: Check that $wpdb->last_result is countable before counting with it.

wpdb::get_col() iterates over $wpdb->last_result, which can be a non-countable value, should the preceding query have failed.

Props spacedmonkey, desrosj, pento.

Merges [43934] into trunk.

See #45299.