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controller: simplify reconciler generation increment logic (#8560)

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## Problem

This code was confusing, untested and covered:
- an impossible case, where intent state is AttacheStale (we never do
this)
- a rare edge case (going from AttachedMulti to Attached), which we were
not testing, and in any case the pageserver internally does the same
Tenant reset in this transition as it would do if we incremented
generation.

Closes: https://github.com/neondatabase/neon/issues/8367

## Summary of changes

- Simplify the logic to only skip incrementing the generation if the
location already has the expected generation and the exact same mode.
This commit is contained in:
John Spray
2024-07-31 18:37:47 +01:00
committed by GitHub
parent ff51b565d3
commit 2f9ada13c4
1 changed files with 7 additions and 17 deletions
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24
storage_controller/src/reconciler.rs
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@@ -656,11 +656,8 @@ impl Reconciler {
// reconcile this location. This includes locations with different configurations, as well
// as locations with unknown (None) observed state.
// The general case is to increment the generation. However, there are cases
// where this is not necessary:
// - if we are only updating the TenantConf part of the location
// - if we are only changing the attachment mode (e.g. going to attachedmulti or attachedstale)
// and the location was already in the correct generation
// Incrementing generation is the safe general case, but is inefficient for changes that only
// modify some details (e.g. the tenant's config).
let increment_generation = match observed {
None => true,
Some(ObservedStateLocation { conf: None }) => true,
@@ -669,18 +666,11 @@ impl Reconciler {
}) => {
let generations_match = observed.generation == wanted_conf.generation;
use LocationConfigMode::*;
let mode_transition_requires_gen_inc =
match (observed.mode, wanted_conf.mode) {
// Usually the short-lived attachment modes (multi and stale) are only used
// in the case of [`Self::live_migrate`], but it is simple to handle them correctly
// here too. Locations are allowed to go Single->Stale and Multi->Single within the same generation.
(AttachedSingle, AttachedStale) => false,
(AttachedMulti, AttachedSingle) => false,
(lhs, rhs) => lhs != rhs,
};
!generations_match || mode_transition_requires_gen_inc
// We may skip incrementing the generation if the location is already in the expected mode and
// generation. In principle it would also be safe to skip from certain other modes (e.g. AttachedStale),
// but such states are handled inside `live_migrate`, and if we see that state here we're cleaning up
// after a restart/crash, so fall back to the universally safe path of incrementing generation.
!generations_match || (observed.mode != wanted_conf.mode)
}
};