Implement `maximum_results` option

[yamadapc]

When working on large mono-repos some queries might take a very long time due to listing all files on the project.

This waiting time is a combination of building the results array and serialising it back.

This commit adds a query option maximum_results, which allows clients to ask the server to truncate results above a certain threshold number.

On that case, a flag is sent back indicating to the client results have been truncated.

We could change this diff to read this value as a global setting rather than a per request setting. But then for any given instance we'd need to globally assume the setting is handled correctly, which may not be the case.

I still need to test direct BSER clients against this change.

I'd appreciate an opinion on the diff & where to write automated tests for this. I don't see unit-testing around this area, so I imagine those would be integration tests.

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[yamadapc]

Hello, could I get an opinion on this diff?

[yamadapc]

Hey :wave:

[genevievehelsel]

Hey 👋

followed up in the Sapling discord (nice idea asking there :) ) but copy-pasting here for wider visibility:

hi from the watchman team - I think this looks like a good idea 🙂 we've got a similar flag like @quark-zju mentioned for a watcher we use often (watcher/eden.cpp) for the same reasons you mentioned in your summary

on the making the eden_file_count_threshold_for_fresh_instannce more universal, the code pointer for that is here:

https://github.com/facebook/watchman/blob/main/watchman/watcher/eden.cpp#L760

It would be a lot cleaner IMO if we could just leverage the existing fresh_instance code for this, in a similar manner to how its done for the eden watcher, especially since some clients might already know how to handle fresh instances and they'd have to relearn how to handle a new flag if they wanted to use this feature

we have an integration test for this flag for the eden watcher here: https://github.com/facebook/watchman/blob/main/watchman/integration/eden/test_eden_since.py#L296

if you went the route of returning fresh instances if threshold is reached, you could just make a similar test to the above but for a non-eden watcher 🙂