From fdfd71bbd099645bd1cd99bb9b754ea6cc48800a Mon Sep 17 00:00:00 2001
From: Jonatan Rhodin <jonatan.rhodin@mullvad.net>
Date: Mon, 27 Nov 2023 15:13:54 +0100
Subject: [PATCH] Add exponential back off for api verification of purchases

---
 .../mullvadvpn/constant/PaymentConstant.kt    |  5 +++
 .../mullvadvpn/usecase/PaymentUseCase.kt      | 25 +++++++++----
 .../net/mullvad/mullvadvpn/util/FlowUtils.kt  | 35 +++++++++++++++++++
 .../mullvad/mullvadvpn/util/LongExtensions.kt |  5 +++
 4 files changed, 64 insertions(+), 6 deletions(-)
 create mode 100644 android/app/src/main/kotlin/net/mullvad/mullvadvpn/constant/PaymentConstant.kt
 create mode 100644 android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/LongExtensions.kt

diff --git a/android/app/src/main/kotlin/net/mullvad/mullvadvpn/constant/PaymentConstant.kt b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/constant/PaymentConstant.kt
new file mode 100644
index 000000000000..1cf1c32b2cf5
--- /dev/null
+++ b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/constant/PaymentConstant.kt
@@ -0,0 +1,5 @@
+package net.mullvad.mullvadvpn.constant
+
+const val VERIFICATION_MAX_ATTEMPTS = 4
+const val VERIFICATION_INITIAL_BACK_OFF_MILLISECONDS = 3000L
+const val VERIFICATION_BACK_OFF_FACTOR = 3L
diff --git a/android/app/src/main/kotlin/net/mullvad/mullvadvpn/usecase/PaymentUseCase.kt b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/usecase/PaymentUseCase.kt
index bda53bcaf29e..5a50e8040151 100644
--- a/android/app/src/main/kotlin/net/mullvad/mullvadvpn/usecase/PaymentUseCase.kt
+++ b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/usecase/PaymentUseCase.kt
@@ -4,11 +4,15 @@ import android.app.Activity
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.asStateFlow
+import net.mullvad.mullvadvpn.constant.VERIFICATION_BACK_OFF_FACTOR
+import net.mullvad.mullvadvpn.constant.VERIFICATION_INITIAL_BACK_OFF_MILLISECONDS
+import net.mullvad.mullvadvpn.constant.VERIFICATION_MAX_ATTEMPTS
 import net.mullvad.mullvadvpn.lib.payment.PaymentRepository
 import net.mullvad.mullvadvpn.lib.payment.model.PaymentAvailability
 import net.mullvad.mullvadvpn.lib.payment.model.ProductId
 import net.mullvad.mullvadvpn.lib.payment.model.PurchaseResult
 import net.mullvad.mullvadvpn.lib.payment.model.VerificationResult
+import net.mullvad.mullvadvpn.util.retryWithExponentialBackOff
 
 interface PaymentUseCase {
     val paymentAvailability: Flow<PaymentAvailability?>
@@ -43,13 +47,22 @@ class PlayPaymentUseCase(private val paymentRepository: PaymentRepository) : Pay
     }
 
     override suspend fun verifyPurchases(onSuccessfulVerification: () -> Unit) {
-        paymentRepository.verifyPurchases().collect {
-            if (it == VerificationResult.Success) {
-                // Update the payment availability after a successful verification.
-                queryPaymentAvailability()
-                onSuccessfulVerification()
+        paymentRepository
+            .verifyPurchases()
+            .retryWithExponentialBackOff(
+                maxAttempts = VERIFICATION_MAX_ATTEMPTS,
+                initialBackOffDelay = VERIFICATION_INITIAL_BACK_OFF_MILLISECONDS,
+                backOffDelayFactor = VERIFICATION_BACK_OFF_FACTOR
+            ) {
+                it is VerificationResult.Error
+            }
+            .collect {
+                if (it == VerificationResult.Success) {
+                    // Update the payment availability after a successful verification.
+                    queryPaymentAvailability()
+                    onSuccessfulVerification()
+                }
             }
-        }
     }
 }
 
diff --git a/android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt
index a754b5a6c29e..9be4b13b593b 100644
--- a/android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt
+++ b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt
@@ -5,9 +5,13 @@ import kotlin.coroutines.EmptyCoroutineContext
 import kotlinx.coroutines.Deferred
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.channels.awaitClose
+import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.callbackFlow
+import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.flatMapLatest
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.retryWhen
 import kotlinx.coroutines.flow.take
 import kotlinx.coroutines.withTimeoutOrNull
 import net.mullvad.mullvadvpn.lib.common.util.safeOffer
@@ -129,3 +133,34 @@ inline fun <T1, T2, T3, T4, T5, T6, T7, T8, R> combine(
 
 suspend inline fun <T> Deferred<T>.awaitWithTimeoutOrNull(timeout: Long) =
     withTimeoutOrNull(timeout) { await() }
+
+@Suppress("UNCHECKED_CAST")
+suspend inline fun <T> Flow<T>.retryWithExponentialBackOff(
+    maxAttempts: Int,
+    initialBackOffDelay: Long,
+    backOffDelayFactor: Long,
+    crossinline predicate: (T) -> Boolean,
+): Flow<T> =
+    map {
+            if (predicate(it)) {
+                throw ExceptionWrapper(it as Any)
+            }
+            it
+        }
+        .retryWhen { _, attempt ->
+            if (attempt >= maxAttempts) {
+                return@retryWhen false
+            }
+            val backOffDelay = initialBackOffDelay * backOffDelayFactor.pow(attempt.toInt())
+            delay(backOffDelay)
+            true
+        }
+        .catch {
+            if (it is ExceptionWrapper) {
+                this.emit(it.item as T)
+            } else {
+                throw it
+            }
+        }
+
+class ExceptionWrapper(val item: Any) : Throwable()
diff --git a/android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/LongExtensions.kt b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/LongExtensions.kt
new file mode 100644
index 000000000000..850776ca7631
--- /dev/null
+++ b/android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/LongExtensions.kt
@@ -0,0 +1,5 @@
+package net.mullvad.mullvadvpn.util
+
+import kotlin.math.pow
+
+fun Long.pow(exponent: Int): Long = toDouble().pow(exponent).toLong()