Improve Purchase api verification

[Pururun]

1. Add exponential back off for purchase verification
2. Add purchase verification after a purchase flow has failed

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This change is 

[linear]

DROID-534 Add exponential back off to payment verification

If we get failed verification we should try again with exponential back off.

[sabercodic]

Reviewed 1 of 6 files at r1, all commit messages.
Reviewable status: 1 of 6 files reviewed, 1 unresolved discussion (waiting on @Pururun)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/usecase/PaymentUseCase.kt line 57 at r1 (raw file):

                backOffDelayFactor = VERIFICATION_BACK_OFF_FACTOR
            ) {
                it is VerificationResult.Error

Does it work on both VerificationError and BillingError?

[Pururun]

Reviewable status: 1 of 6 files reviewed, 1 unresolved discussion (waiting on @sabercodic)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/usecase/PaymentUseCase.kt line 57 at r1 (raw file):

Previously, sabercodic wrote…

Does it work on both VerificationError and BillingError?

The idea is that it will retry on both billing error (unable to retrieve any purchase from Google) or VerificationError (unable to verify with our api).

[sabercodic]

Reviewable status: 1 of 6 files reviewed, 1 unresolved discussion (waiting on @Pururun)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 138 at r1 (raw file):

@Suppress("UNCHECKED_CAST")
suspend inline fun <T> Flow<T>.retryWhen(

Can we go with better naming here?

[Pururun]

Reviewable status: 1 of 6 files reviewed, 1 unresolved discussion (waiting on @sabercodic)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 138 at r1 (raw file):

Previously, sabercodic wrote…

Can we go with better naming here?

Done.

[Rawa]

Reviewable status: 1 of 6 files reviewed, 3 unresolved discussions (waiting on @Pururun and @sabercodic)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 152 at r2 (raw file):

        }
        .retryWhen { cause, attempt ->
            if (attempt < maxAttempts) {

Small detail that I think makes it a bit more readable, we should early terminate if it is possible. E.g in this case:

if(attempt >= maxAttemps) {
   return false
}

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 154 at r2 (raw file):

            if (attempt < maxAttempts) {
                delay(backOffDelay)
                backOffDelay *= backOffDelayFactor

Suggestion: We simply calculate this value every single time, then we don't have to bother modifying the var.

fun Long.pow(exponent: Int): Long = toDouble().pow(exponent).toLong()
initialBackOffDelay * backOffDelayFactor.pow(attempt)

Start of the function would be more clear as well:

): Flow<T> = map {

[sabercodic]

Reviewed 3 of 6 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @Pururun)

[sabercodic]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @Pururun)

[Rawa]

Reviewable status: 5 of 7 files reviewed, 3 unresolved discussions (waiting on @Pururun and @sabercodic)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 154 at r2 (raw file):

Previously, Rawa (David Göransson) wrote…

Suggestion: We simply calculate this value every single time, then we don't have to bother modifying the var.

fun Long.pow(exponent: Int): Long = toDouble().pow(exponent).toLong()
initialBackOffDelay * backOffDelayFactor.pow(attempt)

Start of the function would be more clear as well:

): Flow<T> = map {

nice 👍

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 144 at r3 (raw file):

    crossinline predicate: (T) -> Boolean,
): Flow<T> =
    this.map {

Do we need this? Shouldn't it be possible to remove?

[Pururun]

Reviewable status: 5 of 7 files reviewed, 1 unresolved discussion (waiting on @Rawa and @sabercodic)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 144 at r3 (raw file):

Previously, Rawa (David Göransson) wrote…

Do we need this? Shouldn't it be possible to remove?

Not sure I follow what should be removed?

[Rawa]

Reviewable status: 5 of 7 files reviewed, 1 unresolved discussion (waiting on @Pururun and @sabercodic)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 144 at r3 (raw file):

Previously, Pururun (Jonatan Rhodin) wrote…

Not sure I follow what should be removed?

this.map -> map, sorry it wasn't very clear that i meant the keyword this ^^

[Pururun]

Reviewable status: 5 of 7 files reviewed, 1 unresolved discussion (waiting on @Rawa and @sabercodic)

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android/app/src/main/kotlin/net/mullvad/mullvadvpn/util/FlowUtils.kt line 144 at r3 (raw file):

Previously, Rawa (David Göransson) wrote…

this.map -> map, sorry it wasn't very clear that i meant the keyword this ^^

Ah yes, that could be removed. Done

[Rawa]

Reviewable status: 5 of 7 files reviewed, all discussions resolved (waiting on @sabercodic)