[in_app_purchase_android] IllegalStateException "Reply already submitted": Play Billing can invoke a response listener twice for one call; only startConnection is guarded #191832

Steps to reproduce

Needs the Play Billing library to invoke the response listener of a single
async call twice — observed in production via its internal
watchdog/timeout path (zzaf.run → zzan.run), ~30 s after the app was
backgrounded with a queryProductDetailsAsync call still in flight:

1. App calls InAppPurchase.instance.queryProductDetails(...) (or any other
   async billing method).
2. App is backgrounded while the query is in flight.
3. Play Billing's internal watchdog fires a (timeout/error) response, AND the
   real response also arrives — the SAME ProductDetailsResponseListener is
   invoked twice.
4. The second invocation completes the pigeon callback a second time →
   reply.reply() throws IllegalStateException: Reply already submitted on
   the main thread → uncaught → process death (in background, so users see
   nothing; crash reporting and Play vitals do).

Expected results

A duplicate listener invocation is ignored; the first completion wins.

Actual results

java.lang.IllegalStateException: Reply already submitted
  DartMessenger$Reply.reply ← BasicMessageChannel reply (pigeon)
  Messages$InAppPurchaseApi.success
    ← MethodCallHandlerImpl.lambda$queryProductDetailsAsync (2nd completion)
  com.android.billingclient.api.zzan.run ← zzaf.run   # Play Billing watchdog

Analysis

A pigeon reply may be submitted exactly once. Upstream clearly knows this bug
class — startConnection carries its own alreadyFinished guard against a
double onBillingSetupFinished — but none of the other async methods have
one (verified on 0.5.2 and main, 2026-08-26): queryProductDetailsAsync,
queryPurchasesAsync, consumeAsync, acknowledgePurchase,
getBillingConfigAsync, showInAppMessages,
isAlternativeBillingOnlyAvailableAsync,
createAlternativeBillingOnlyReportingDetailsAsync,
showAlternativeBillingOnlyInformationDialog.

Prior reports of this crash class: #103432 (open since 2022), #134967 (closed
as its duplicate). For comparison, RevenueCat's Android SDK guards every
billing listener with an AtomicBoolean.getAndSet(true) single-shot for the
same reason.

Proposed fix

A small replyOnce(methodName, callback) wrapper
(AtomicBoolean.getAndSet(true); first completion wins, duplicates dropped
with a Log.d) applied to the pigeon callback in all 9 async methods, keeping
the method bodies otherwise unchanged. startConnection keeps its existing
guard. A PR implementing this (plus two JUnit regression tests that fire a
captured listener twice and assert exactly one pigeon completion holding the
first result) is attached.

Environment

• in_app_purchase_android: crash first captured in production 2026-05-14 on
  0.4.0+8; guard hole code-verified present on 0.5.2 (latest release) and
  main as of 2026-08-26.
• Flutter: 3.44.4 (stable), Dart 3.12.2

[✓] Flutter (Channel stable, 3.44.4, on Debian GNU/Linux 12 (bookworm) 6.1.0-37-amd64, locale en_US.UTF-8) [216ms]
    • Flutter version 3.44.4 on channel stable at /home/elinorapricot/devtools/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision ad70ec4617 (9 weeks ago), 2026-06-24 11:07:06 -0700
    • Engine revision a10d8ac38d
    • Dart version 3.12.2
    • DevTools version 2.57.0
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    • CHROME_EXECUTABLE = /usr/bin/chromium

[✓] Linux toolchain - develop for Linux desktop [1,879ms]
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[✓] Connected device (3 available) [1,932ms]
    • sdk gphone64 x86 64 (mobile) • emulator-5554 • android-x64    • Android 16 (API 36) (emulator)
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