Better encapsulate MergingRun internals

Provide a couple accessor functions and hide the rest (except the
constructor needed for deriving Generic in the tests).

src/Database/LSMTree/Internal/MergeSchedule.hs

@@ -818,7 +818,7 @@ scaleCreditsForMerge LevelTiering _ (Credits c) =
     -- runs come in).
     MR.Credits (c * (1 + 4))
 
-scaleCreditsForMerge LevelLevelling (DeRef mr) (Credits c) =
+scaleCreditsForMerge LevelLevelling mr (Credits c) =
     -- A levelling merge has 1 input run and one resident run, which is (up
     -- to) 4x bigger than the others. It needs to be completed before
     -- another run comes in.
@@ -828,7 +828,7 @@ scaleCreditsForMerge LevelLevelling (DeRef mr) (Credits c) =
     -- worst-case upper bound by looking at the sizes of the input runs.
     -- As as result, merge work would/could be more evenly distributed over
     -- time when the resident run is smaller than the worst case.
-    let NumRuns n = MR.mergeNumRuns mr
+    let NumRuns n = MR.numRuns mr
        -- same as division rounding up: ceiling (c * n / 4)
     in MR.Credits ((c * n + 3) `div` 4)
 

src/Database/LSMTree/Internal/MergingRun.hs

@@ -8,25 +8,27 @@
 -- | An incremental merge of multiple runs.
 module Database.LSMTree.Internal.MergingRun (
     -- * Merging run
-    MergingRun (..)
+    MergingRun
   , NumRuns (..)
   , new
   , newCompleted
   , duplicateRuns
   , supplyCredits
   , expectCompleted
+  , snapshot
+  , numRuns
 
     -- * Credit tracking
     -- $credittracking
   , Credits (..)
   , CreditThreshold (..)
   , SuppliedCredits (..)
-  , atomicReadSuppliedCredits
 
   -- * Concurrency
   -- $concurrency
 
     -- * Internal state
+  , pattern MergingRun
   , MergingRunState (..)
   , MergeKnownCompleted (..)
   , CreditsVar (..)
@@ -220,6 +222,24 @@ duplicateRuns (DeRef mr) =
       OngoingMerge rs _ -> withActionRegistry $ \reg ->
         V.mapM (\r -> withRollback reg (dupRef r) releaseRef) rs
 
+-- | Take a snapshot of the state of a merging run.
+snapshot ::
+     (PrimMonad m, MonadMVar m)
+  => Ref (MergingRun m h)
+  -> m (MergingRunState m h,
+        SuppliedCredits,
+        NumRuns,
+        NumEntries)
+snapshot (DeRef MergingRun {..}) = do
+    state <- readMVar mergeState
+    (SpentCredits   spent,
+     UnspentCredits unspent) <- atomicReadCredits mergeCreditsVar
+    let supplied = SuppliedCredits (spent + unspent)
+    return (state, supplied, mergeNumRuns, mergeNumEntries)
+
+numRuns :: Ref (MergingRun m h) -> NumRuns
+numRuns (DeRef MergingRun {mergeNumRuns}) = mergeNumRuns
+
 {-------------------------------------------------------------------------------
   Credits
 -------------------------------------------------------------------------------}
@@ -396,19 +416,6 @@ atomicReadCredits ::
 atomicReadCredits (CreditsVar v) =
     unpackCreditsPair <$> atomicReadInt v
 
-{-# INLINE atomicReadSuppliedCredits #-}
-atomicReadSuppliedCredits ::
-     PrimMonad m
-  => CreditsVar (PrimState m)
-  -> m SuppliedCredits
-atomicReadSuppliedCredits (CreditsVar v) = do
-    cp <- atomicReadInt v
-    let !supplied =
-           case cp of
-             CreditsPair (SpentCredits   spent)
-                         (UnspentCredits unspent) -> spent + unspent
-    return (SuppliedCredits supplied)
-
 {-# INLINE atomicModifyInt #-}
 -- | Atomically modify a single mutable integer variable, using a CAS loop.
 atomicModifyInt ::

src/Database/LSMTree/Internal/Snapshot.hs

@@ -193,14 +193,16 @@ toSnapIncomingRun ::
   => IncomingRun m h
   -> m (SnapIncomingRun (Ref (Run m h)))
 toSnapIncomingRun (Single r) = pure (SnapSingleRun r)
-toSnapIncomingRun (Merging mergePolicy (DeRef MR.MergingRun {..})) = do
+toSnapIncomingRun (Merging mergePolicy mergingRun) = do
     -- We need to know how many credits were spend and yet unspent so we can
     -- restore merge work on snapshot load. No need to snapshot the contents
     -- of totalStepsVar here, since we still start counting from 0 again when
     -- loading the snapshot.
-    MR.SuppliedCredits (MR.Credits suppliedCredits)
-         <- MR.atomicReadSuppliedCredits mergeCreditsVar
-    smrs <- toSnapMergingRunState <$> readMVar mergeState
+    (mergingRunState,
+     MR.SuppliedCredits (MR.Credits suppliedCredits),
+     mergeNumRuns,
+     mergeNumEntries) <- MR.snapshot mergingRun
+    let smrs = toSnapMergingRunState mergingRunState
     pure $
       SnapMergingRun
         mergePolicy