Allow IOG's contra-tracer

[jasagredo]

Description

A first attempt at integrating this into ouroboros-consensus showed a misalign in the contra-tracer version. Changes were sufficiently simple that I think it is reasonable to put some CPPs here and there and be done while someone else figures out how to reconcile our stack with the Hackage contra-tracer.

Checklist

• Read our contribution guidelines at CONTRIBUTING.md, and make sure that this PR complies with the guidelines.

[jorisdral]

It's not yet clear to me why we need the cabal flag. The core library's code works with 0.2 from Hackage, 0.1 from Hackage, and 0.1 from CHaP, right? What would happen if we just put contra-tracer ^>=0.1 || ^>=0.2 here, without the cabal flag?

[jasagredo]

It might work, indeed.

[jorisdral]

My suggestion would be to do the following, which I tried locally and seemed to work:

diff --git a/lsm-tree.cabal b/lsm-tree.cabal
index 6ba33cfd..579b6cc8 100644
--- a/lsm-tree.cabal
+++ b/lsm-tree.cabal
@@ -582,7 +582,7 @@ library
     , bytestring              ^>=0.11.4.0 || ^>=0.12.1.0
     , cborg                   ^>=0.2.10.0
     , containers              ^>=0.6      || ^>=0.7
-    , contra-tracer           ^>=0.2
+    , contra-tracer           ^>=0.1      || ^>=0.2
     , crc32c                  ^>=0.2.1
     , deepseq                 ^>=1.4      || ^>=1.5
     , filepath
diff --git a/src/Database/LSMTree/Internal/Unsafe.hs b/src/Database/LSMTree/Internal/Unsafe.hs
index 4fd75489..4ad7b644 100644
--- a/src/Database/LSMTree/Internal/Unsafe.hs
+++ b/src/Database/LSMTree/Internal/Unsafe.hs
@@ -202,12 +202,15 @@ data TableTrace =
   | TraceSupplyUnionCredits UnionCredits
   deriving stock Show

-contramapTraceMerge :: Monad m => Tracer m TableTrace -> Tracer m (AtLevel MergeTrace)
+contramapTraceMerge :: forall m. Monad m => Tracer m TableTrace -> Tracer m (AtLevel MergeTrace)
 #ifdef DEBUG_TRACES
 contramapTraceMerge t = TraceMerge `contramap` t
 #else
-contramapTraceMerge t = traceMaybe (const Nothing) t
+contramapTraceMerge _t = nullTracer
 #endif
+  where
+    -- See #766
+    _unused = pure @m ()

 data CursorTrace =
     TraceCreateCursor TableId

[jorisdral]

The _unused is there purely to fulfill the Monad m constraint, which is obsolete for nullTracer, but it's nicer if we have the same signature regardless of the DEBUG_TRACES flag

[jasagredo]

I think this looks good. I will push this diff on top of my branch tomorrow.

[jorisdral]

I think there may have been some misunderstanding, I meant: make only the changes in the diff above, and remove all the other changes that were already there

That would make the core library compatible with the IOG contra-tracer. The tests and benchmarks don't have to change, since we don't run them with the IOG contra-tracer anyway

[jorisdral]

Huh, apparently the PR was closed because the base branch disappeared. Could you reopen it @jasagredo ?

The PR history says I closed it but I did no such thing manually 😛