Replace GetModificationTime with a file version scheme via WatchedFile events

[pepeiborra]

The idea is to replace GetModificationTime, which is expensive and called on every change, by a new rule GetFileVersion implemented entirely on top of watched file events. This would reduce I/O, eliminate timestamp serialization and comparison, and hopefully improve performance

One possible approach is to generalize the FileExistsMapVar in order to keep track of file versions.

[pepeiborra]

There's two reasons GetModificationTime is slow:

1. The non VFS code path is expensive as it needs to hit the file system, which can be addressed with WatchedFiles events
2. The rule is tagged as alwaysRerun, which could be addressed combining WatchedFiles and DidChangeTextDocument events.

It's likely that fixing 1 is good enough.

[pepeiborra]

Implemented both 1 and 2, but doesn't seem to pay off massively.

https://github.com/pepeiborra/ghcide/commits/get-file-modification

[ndmitchell]

How slow did you measure getmodificationtime as?

[pepeiborra]

I did not measure getModificationTime at all! It just seemed reasonable to reuse watched file notifications to avoid filesystem I/O here.

[jneira]

@pepeiborra does it worth keeping this opened? asking due to this comment:

Implemented both 1 and 2, but doesn't seem to pay off massively.

[pepeiborra]

Yes, I think it's still worth keeping this open. Back when I implemented 1 and 2 we didn't have a good benchmarking suite, and on paper this optimisation should be very much worth it.

[michaelpj]

We could also use the file change events that we get from watching files to update modification times.