Degrade an optional stage in tiers, and report what survived

Engines that take their processing graph as a string validate the whole string at once: one unknown stage name and nothing installs — not "that stage is skipped", the entire graph is rejected. So a chain mixing a stage that ships everywhere with one that is an optional build-time dependency has a failure mode where the mandatory half disappears because of the optional half. Attempt, fall back by tiers, and hand the caller a record rather than a boolean:

/** What the install actually got past the engine. Callers must not drive a stage that is false. */
data class Chain(val sweep: Boolean, val pitchShift: Boolean) {
    companion object { val NONE = Chain(sweep = false, pitchShift = false) }
}

fun installChain(sweep: Sweep?, startHz: Float, pitchShift: Boolean): Chain {
    if (sweep == null && !pitchShift) { clearChain(); return Chain.NONE }
    if (apply(sweep, startHz, pitchShift)) return Chain(sweep != null, pitchShift)
    // The pitch stage is an OPTIONAL build-time dependency; the filter stages ship with every build
    // of the underlying codec library. Drop the optional one rather than lose the sweep too.
    if (pitchShift && sweep != null && apply(sweep, startHz, pitchShift = false)) {
        log.w(TAG, "pitch stage unavailable; keeping the sweep without the pitch match")
        return Chain(sweep = true, pitchShift = false)
    }
    log.w(TAG, "could not install the chain; falling back to a plain volume fade")
    clearChain(); return Chain.NONE
}

Traps

A boolean return cannot express the interesting outcome. "Did the install land" has three answers here, not two, and the middle one — partial — is the one the caller has to behave differently for. A boolean forces the caller to re-derive which stages exist, usually by re-checking the same condition that was already wrong.

Per-step commands against a stage that is not there fail on every frame. The reason the record matters is that the follow-up updates — retuning a live filter as an animation advances — are addressed to a stage by name. Against a missing stage they fail at animation frame rate: a log flood, an animation that visibly does nothing, and nothing that looks like an error to the user. Gate every per-step call on the tier flag, at the caller.

Install the stages at a neutral setting, so the install itself is inaudible. A sweep armed at its own start frequency and a pitch stage armed at ratio 1.0 change nothing until the first per-step command arrives — which is what lets you build the graph once, up front, and pay the expensive rebuild outside the animation. It also means "present" and "doing something" are different questions: the record answers the first, and a caller that treats it as the second will report a feature as active before it has ever been given a target.

Do not probe by asking the engine what it was built with. A capability or feature list answers a different question from "will this exact graph build" — the stage can be present but reject your parameters, or be listed under a name your string does not use. Attempt the real string and read the real result; the attempt is the probe. This is the same discipline as upstream-lib-bug-workaround-template: pin the behaviour you observed, not the version you think has it.

Order the tiers by what is lost, and make the last tier a real fallback. Optional stage first, mandatory stage second, no chain third — and the third must actually clear, not leave whatever the failed attempt deposited. A half-built graph that nobody clears is worse than no graph, because the next feature to write the property inherits it.

Log the degradation once, at install, and never per step. One warning naming the tier that was lost is diagnosable; the same warning at frame rate is what makes the log unusable in exactly the session you need it. Callers gated on the tier flag will not produce the per-step lines at all.

A degraded tier is not an error to surface to the user. The feature still works, with less. A dialog here trains users to dismiss dialogs; the log line plus the disabled sub-control is the right pair. Do surface it if the mandatory tier fails, because that is a behaviour change.

Clearing a stage also clears whatever that stage was holding. The optional stage usually carries state of its own — a pitch ratio, a wet/dry mix, a filter cutoff — and dropping it from the chain resets that silently. Say so where clearChain() is defined, and enumerate what callers must re-assert afterwards; otherwise the reset shows up two features away, as "the speed reverts when the transition ends".

Remember that this composes with the other owner of the chain property. Retrying a tier means writing the chain string more than once, and if a second feature also has entries in there, every one of those writes has to carry both. See filter-chain-two-owners-one-writer — a tiered install that writes the property directly is the fastest way to discover that skill the hard way.

The optional stage's absence is platform-shaped, not user-shaped. It depends on how the native bundle for that platform was built, so it is stable per build and invisible on the developer's own machine — the classic "works here, silently does nothing in the shipped artifact". Record which bundle you verified the stage in, next to the fallback.

Verifying it

The fallback path never runs on a machine where the stage is present, so force it:

# what the shipped bundle actually contains — run against the artifact, not the dev machine
# usage: bash probe.sh <optional-stage-name>
LIB=$(find natives -type f \( -name 'lib*.so*' -o -name '*.dll' -o -name '*.dylib' \) 2>/dev/null | head -1)
[ -n "$LIB" ] || { echo "no native bundle staged under natives/"; exit 1; }
STAGE="${1:?name the optional stage, e.g. the pitch-shift filter}"
strings "$LIB" | grep -i "$STAGE" | head -3

Many builds embed their own configure line, so this often answers outright — a hit reading -Dsomestage=disabled is the whole diagnosis, and it is the case a developer's machine never shows.

Then exercise all three tiers deliberately:

• install with a deliberately misspelt optional stage name and assert the mandatory stage is still installed and the returned record says optional = false;
• misspell both and assert the record is NONE and that the chain property is empty afterwards — the clear on the last tier is the half that gets forgotten;
• with the optional stage reported absent, run the animation and assert zero per-step commands were issued. A test that only checks the install return value passes while the caller still hammers a stage that is not there.