Add cached_markdown_version_for_bulk_clear helper

zlirequested to merge
zl/607937-target-version into master

What does this MR do and why?

Adds Gitlab::MarkdownCache.cached_markdown_version_for_bulk_clear, which resolves the lowest cached_markdown_version that should still be treated as current:

def self.cached_markdown_version_for_bulk_clear(local_version: nil)
  previous_cached_markdown_version(local_version: local_version) ||
    cached_markdown_version_for_write(local_version: local_version)
end

Anything strictly below the returned version is stale: its cached HTML will be re-rendered on the next read anyway, so it is safe to clear in bulk. This composes the two existing rollout states:

  • Mid-rollout: previous_cached_markdown_version is non-nil, so the bound is the previous version. A bulk clear then only touches true stragglers below it, leaving the previous and current populations alone since they are still rolling forward through the read path.
  • Steady state: there is no rollout, previous_cached_markdown_version returns nil, and the bound falls back to cached_markdown_version_for_write, the current version, so everything below it is stale.

Why a separate method, and this name

latest_cached_markdown_version is already documented as the staleness check, but it is the per-read one: during a rollout it rolls stochastically, which is the load-shedding mechanism for read traffic. A bulk clear needs the opposite property, a bound that never rolls, so the two cannot share a method. The name follows the existing cached_markdown_version_for_<purpose> shape used by cached_markdown_version_for_write, naming the operation rather than claiming to be the general answer to "is this row stale".

This cannot live on the CacheMarkdownField model concern, because background operations iterate a dynamic batchable model (Gitlab::Database::DynamicModelHelpers) that does not include it. Putting the fallback in Gitlab::MarkdownCache keeps the version-resolution rules in one place instead of duplicating them per caller.

This MR adds only the helper and its spec. There is no caller on master yet; Gitlab::BackgroundOperation::MergeRequestsClearStaleCachedHtml will be the first, once it is rebased on this.

Nothing user-facing changes, so no changelog entry is warranted.

References

Edited by zli

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