Requested by Moritz Hoffmann · Slack thread

Motivation

First half of a two-PR stack. #38208 (cluster- and replica-scoped parameters in the config-sync file) is stacked on this PR. This part changes how the existing flat ConfigMap is read and nothing about what it means, so it can be reviewed and landed on its own.

Before: the file-backed frontend re-read and re-parsed the file once per synchronized parameter on every tick, and logged a warning per parameter per tick while the file was missing or malformed. A missing or unparseable file was handed to the scoped reconcile as a complete, empty desired state, which pruned every scoped override.

After: the file is read once per tick, and the parse is cached until the file's bytes change, so the warnings fire once per edit. A tick whose read failed or did not parse skips the scoped reconcile, so an unreadable file leaves the scoped overrides alone.

Description

• SystemParameterFrontend::pull reads the file once, through refresh_config_file, which compares the bytes against the cached read and re-parses only on a change.
• has_scoped_desired_state reports whether the latest read parsed (always true for LaunchDarkly). sync_scoped_params returns early while it is false.
• Values for a flat file come out as they do today: the same JSON-to-string conversion, null means no opinion, and a parameter the file does not mention, or every parameter while the file is unreadable, keeps its current value.

On main the file client produces no scoped overrides, so skipping the reconcile only matters for rows written some other way. It matters once #38208 gives the file scoped rules, and it belongs with the read path.

Verification

New unit tests in src/adapter/src/config/frontend.rs:

• test_parse_flat_file_is_environment_wide, test_parse_rejects_non_object_document: flat-file parsing and the JSON-to-string conversion.
• test_config_file_cached_until_it_changes: unchanged bytes reuse the parse.
• test_unreadable_file_has_no_scoped_desired_state: a missing file, invalid JSON, or a non-object document reports no desired state. An empty object does report one.
• test_pull_from_file: pull against a real file, covering values, null, and a file that turns unparseable and then goes missing.

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https://claude.ai/code/session_01KdNahqJ7ASK2z2EZVEvr36

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