Long stacker towers spam erroneous 'extension is blocked' errors

Issue description

Long stacker towers take a while to scan their zone for spaces to fill... because they rely on a live scan, back and forth. While that does cause the tower to slow down over time, and there are plenty of ways to make it more responsive without appreciably increasing memory or CPU load, that's not the issue here- the issue is that the 'stacker extension is blocked' error appears to be on a timer, rather than waiting for an actual, full scan of the zone to say 'nothing to fill, cannot place extension'. As a result, these long stacker towers spam errors into the log- and that sound that should be reserved for actionable issues, making it a Boy Who Cried Wolf. This is unforgivable in a set-and-forget machine.

Reproduction steps

1. Build a stacker tower with full zone width. Will need a very long area filled at least as far as the tower would. First appearance is at just a few extensions, starts getting serious around 25-30, and downright unsalvageable at 60, especially when the stacker's internal conveyor is large enough to buffer an entire extension's worth of resources.
2. Feed resources into the tower.
3. The tower will frequently take several seconds to perform its scan before creating an extension or moving to a fresh dump spot (scanning all down one side and back up), and if that scan takes longer than about a second... "The rail extension of Stacker tower is blocked" when it manifestly isn't. The error will clear itself when the scan finds something to do- and if you hide it, well, it forgets that it was hidden and comes back with a vengeance.

Game version and build number

Update 4.1 | v0.8.3a (b578)

Operating system

Windows

Attachments

Also note the holes visible on the rock pad next to the selected tower. For some reason that strikes me as related, long stacker towers will periodically order the next extension immediately after the prior is completed, while still moving to the first work site of the new extension. That tower has since been removed- but it's worth note that it "stepped over" a mountain, such that either end of the extension was at a valid height above terrain, but the middle of the rail intersected. That rail correctly collapsed, but the stacker simply reordered it- and somehow managed to build two more after it before I caught it and ordered the demolition.

Just confirmed, the "stacker tower over-extended" issue happens whenever an extension finishes building, and the placement location for the next is already valid- even if the stacker has work to do in the newly-unlocked space. It creates an untidy pit in whatever is being dumped, especially when the towers are being used to create a flat platform, as I'm using them for- though I have trucks later, adding a single-deep layer of dirt so I can use it for farming, or other purposes. I don't think it really needs its own issue post, since it strikes me as a consequence of the search behavior, which the main bug for this post is exactly that. Judging by the regularity of the pits, and how I've had it happen on much shorter towers as well, it happens every time, and is probably a data race effect.

This issue has been fixed, can confirm. Reading back through the patchnotes, it appears to have been v0.8.4a.