Thanks @squeek502 and @The-King-of-Toasters for the pointers...I've updated to use NtQueryVolumeInformationFile + FileFsDeviceInformation

Really appreciate the sleuthing help on this one, everyone. Based on this information I updated #24865 to also pre-set size_err when initializing in streaming mode. I think that's a nice optimization regardless, and it avoids these two issues enough to make them non release blockers. So at this point, what we're left with is fixing the case where a file is initialized in the default mode, it needs to detect properly when it should be downgraded to streaming mode.

It seems similar logic is in File.isCygwinPty. Maybe worth splitting it out into its own function?

I think the similarities are mostly superficial--it's basically just that they both call NtQueryVolumeInformationFile with FILE_FS_DEVICE_INFORMATION, so the potential consolidation would just be the addition of a thin wrapper around that particular combination which doesn't feel super necessary.

(I'm not opposed to it, though, that's just my initial reaction)

I'm vibing with your "initial reaction" squeek. Creating a thin wrapper function in this case feels it could be an "abstraction trap"...resist the urge.