Fix skipAbsoluteFeeCheck in liquidity policy (#509)

There were two separate issues:
- the bypass wasn't working at all because we were using a max of 0.msat
- the bypass should only apply to off-chain payments

src/commonMain/kotlin/fr/acinq/lightning/payment/LiquidityPolicy.kt

@@ -16,7 +16,7 @@ sealed class LiquidityPolicy {
      * Allow automated liquidity managements, within relative and absolute fee limits. Both conditions must be met.
      * @param maxAbsoluteFee max absolute fee
      * @param maxRelativeFeeBasisPoints max relative fee (all included: service fee and mining fee) (1_000 bips = 10 %)
-     * @param skipAbsoluteFeeCheck useful for pay-to-open, being more lax may make sense when the sender doesn't retry payments
+     * @param skipAbsoluteFeeCheck only applies for off-chain payments, being more lax may make sense when the sender doesn't retry payments
      */
     data class Auto(val maxAbsoluteFee: Satoshi, val maxRelativeFeeBasisPoints: Int, val skipAbsoluteFeeCheck: Boolean) : LiquidityPolicy()
 
@@ -25,7 +25,7 @@ sealed class LiquidityPolicy {
         return when (this) {
             is Disable -> LiquidityEvents.Rejected.Reason.PolicySetToDisabled
             is Auto -> {
-                val maxAbsoluteFee = if (skipAbsoluteFeeCheck) 0.msat else this.maxAbsoluteFee.toMilliSatoshi()
+                val maxAbsoluteFee = if (skipAbsoluteFeeCheck && source == LiquidityEvents.Source.OffChainPayment) Long.MAX_VALUE.msat else this.maxAbsoluteFee.toMilliSatoshi()
                 val maxRelativeFee = amount * maxRelativeFeeBasisPoints / 10_000
                 logger.info { "liquidity policy check: fee=$fee maxAbsoluteFee=$maxAbsoluteFee maxRelativeFee=$maxRelativeFee policy=$this" }
                 if (fee > maxRelativeFee) {

src/commonTest/kotlin/fr/acinq/lightning/payment/LiquidityPolicyTestsCommon.kt

@@ -41,4 +41,20 @@ class LiquidityPolicyTestsCommon : LightningTestSuite() {
         assertNull(policy.maybeReject(amount = 10_000_000.msat, fee = 2_000_000.msat, source = LiquidityEvents.Source.OffChainPayment, logger))
 
     }
+
+    @Test
+    fun `policy rejection skip absolute check`() {
+
+        val policy = LiquidityPolicy.Auto(maxAbsoluteFee = 1_000.sat, maxRelativeFeeBasisPoints = 5_000 /* 3000 = 30 % */, skipAbsoluteFeeCheck = true)
+
+        // fee is over absolute, and it's an offchain payment so the check passes
+        assertNull(policy.maybeReject(amount = 4_000_000.msat, fee = 2_000_000.msat, source = LiquidityEvents.Source.OffChainPayment, logger))
+
+        // fee is over absolute, but it's an on-chain payment so the check fails
+        assertEquals(
+            expected = LiquidityEvents.Rejected.Reason.TooExpensive.OverAbsoluteFee(policy.maxAbsoluteFee),
+            actual = policy.maybeReject(amount = 4_000_000.msat, fee = 2_000_000.msat, source = LiquidityEvents.Source.OnChainWallet, logger)?.reason
+        )
+
+    }
 }