collector/filesystem: s/MNT_NOWAIT/MNT_WAIT

[rexagod]

getfsstat(2) spec mentions that using MNT_NOWAIT will return the information it has available without requesting an update from each file system. Hence, use MNT_WAIT in place of the earlier used mode, and make changes to the affected collectors to avoid being stuck for long intervals.

Fixes: #1498

[discordianfish]

I wonder if we could refactor the timeout logic implemented in https://github.com/prometheus/node_exporter/pull/997/files and re-use it here?

[rexagod]

@discordianfish Sure! Do you think it'd be better to do that in a separate PR?

[discordianfish]

@rexagod I think you can just do it here and use to to address the MNT_WAIT change

[rexagod]

@discordianfish I revisited the machinery introduced in #997, and although it seems fitting for that use case, I've had to drop it entirely in this patch to accommodate for the varying design patterns for different filesystems. As such, I believe a per-filesystem case makes sense here since encapsulating that logic such that it fits all filesystems can get non-scalable in the future as more corner cases are added to satisfy the various implementations. Please note that while the previous machinery is being dropped here, the expected behavior is still the same.

LMK if you think otherwise, though. :)

[discordianfish]

Would be nice if we could avoid duplicating the channel/goroutine/timeout logic

[rexagod]

So I ended up taking a couple of stabs at this, but I believe that the refactor (that essentially takes the goroutine, channel, and the timeout logic into a common place may add up to the maintenance complexity, since there are different APIs being used between linux|bsd|{open,free}bsd (since they may not be available on other operating systems) that makes it harder to abstract things more. Owing to this, I am able to establish a common pattern between {free,open}bsd, but not linux and bsd as they differ significantly.

PLMK there's anything I'm missing that could help address your earlier comment.

[discordianfish]

@SuperQ If you're ok with this, I'd say we merge if @rexagod can rebase it