TOSQUASH fixup overflow workarounds and their explanations

Author: nfrisby

ouroboros-consensus-diffusion/test/consensus-test/Test/Consensus/HardFork/Combinator/A.hs

@@ -330,7 +330,8 @@ instance LedgerSupportsMempool BlockA where
 
 instance TxLimits BlockA where
   type TxMeasure BlockA = ByteSize
-  blockCapacityTxMeasure _cfg _st     = ByteSize 1
+  -- default mempool capacity is two blocks, so maxBound/2 avoids overflow
+  blockCapacityTxMeasure _cfg _st     = ByteSize $ maxBound `div` 2
   txMeasure              _cfg _st _tx = ByteSize 0
 
 newtype instance TxId (GenTx BlockA) = TxIdA Int

ouroboros-consensus-diffusion/test/consensus-test/Test/Consensus/HardFork/Combinator/B.hs

@@ -266,7 +266,8 @@ instance LedgerSupportsMempool BlockB where
 
 instance TxLimits BlockB where
   type TxMeasure BlockB = ByteSize
-  blockCapacityTxMeasure _cfg _st     = ByteSize 1
+  -- default mempool capacity is two blocks, so maxBound/2 avoids overflow
+  blockCapacityTxMeasure _cfg _st     = ByteSize $ maxBound `div` 2
   txMeasure              _cfg _st _tx = ByteSize 0
 
 data instance TxId (GenTx BlockB)

ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Mempool/Capacity.hs

@@ -63,6 +63,8 @@ computeMempoolCapacity cfg st override =
     blockCount = case override of
       NoMempoolCapacityBytesOverride            -> 2
       MempoolCapacityBytesOverride (ByteSize x) ->
+        -- This calculation is happening at Word32. Thus overflow is silently
+        -- accepted.
         max 1 $ (x + oneBlockBytes - 1) `div` oneBlockBytes
 
     SemigroupViaMeasure capacity =

ouroboros-consensus/test/consensus-test/Test/Consensus/Mempool.hs

@@ -1029,8 +1029,11 @@ prop_Mempool_idx_consistency (Actions actions) =
       { testLedgerState        = testInitLedger
       , testInitialTxs         = []
       , testMempoolCapOverride =
-          MempoolCapacityBytesOverride $ ByteSize $ maxBound - 100 * 1024 * 1024
-          --- can't use maxBound, because then the check in addTx overflows
+            MempoolCapacityBytesOverride
+          $ ByteSize
+          $ maxBound - unByteSize simpleBlockCapacity
+          --- can't use maxBound, because then 'computeMempoolCapacity'
+          --- calculation overflows, resulting in a capacity of just one block
       }
 
     lastOfMempoolRemoved txsInMempool = \case