src/compact/fallback.rs

This file makes compaction unconditionally successful for you. The chain holds ordered compactor stages and runs them in sequence until one returns success: true.

With a non-failing terminal, an out-of-tokens condition stops surfacing as a failed pass. This file is always compiled.

Key items

FallbackCompactor::default_chain(client) gives you QA, then structured, then TerminalCapture(TruncatingCompactor), which is quality-descending, with blind truncation only as the last resort.

ChainBuilder gives you stage(name, compactor) for any Arc<dyn ContextCompactor> and terminal(&truncator) for your never-fail finale.

Call config before terminal(), because the floor is captured at terminal() time. The builder warns, and never errors, when the last stage’s name does not heuristically name a terminal.

TerminalCapture is the structural floor. It reconstructs your wrapped truncator with preserve_recent = max(its own, terminal_min_recent), which defaults to 4, and FallbackConfig clamps it to at least 1.

That is how your active turn and neighbors survive every stage including the last resort.

ChainReport, StageOutcome, and last_report() give you provenance: which stage ran, which won, and each stage’s error and token figure.

Your winning_stage is the seam you surface as "FallbackCompactor(TruncatingCompactor)".

Behavior notes

Every stage receives the same target_tokens and a cloned context whose tokens_before is recomputed with the context’s counter.

Every stage runs against a clone of your original input, so a failed stage’s returned messages are discarded rather than handed onward.

A misbehaving stage’s partial or emptied list therefore cannot corrupt its successors, and the all-failed outcome carries the original.

Past the loop, on a chain with no non-failing terminal, failure is honest: CompactionOutcome::failed("all compactor stages failed") with your originals intact and nothing evicted. An empty chain has the same shape.

Telemetry emits loopctl.compaction.passes, one event per successful pass where stage is the winner’s name, and loopctl.compaction.degradations, one per failed stage declined through, tagged from and to.

For the deep dive, see Compaction and the compactor family.