SCS cache

Summary:
A really basic cache. We're expecting tends of thousands of revisions,
so cache size is not really an issue. I went with a bounded cache that
has approximate LRU by keeping two semi-spaces, kind of like a 2-space
GC. It's suboptimal in space and time but scores highly in simplicity.

Reviewed By: nhawkes

Differential Revision: D55800094

fbshipit-source-id: d2655a6e5d3b5d886e159d200fe22024f4098af1

glean/glass/Glean/Glass/Env.hs

@@ -53,7 +53,7 @@ data Config = Config
       -- ^ whether to trust listDatabases and how often to wait to retry N times
   , numWorkerThreads :: Maybe Int
   , snapshotBackend :: EventBaseDataplane -> Some SnapshotBackend
-  , sourceControl :: EventBaseDataplane -> Some SourceControl
+  , sourceControl :: EventBaseDataplane -> IO (Some SourceControl)
   , tracer :: GlassTracer
   , welcomeMessage :: EventBaseDataplane -> Config -> IO Text
   }
@@ -64,7 +64,7 @@ setSnapshotBackend snapshotBackend config =
   config { snapshotBackend = snapshotBackend }
 
 setSourceControl
-  :: (EventBaseDataplane -> Some SourceControl)
+  :: (EventBaseDataplane -> IO (Some SourceControl))
   -> Config -> Config
 setSourceControl sourceControl config =
   config { sourceControl = sourceControl }

glean/glass/Glean/Glass/Main.hs

@@ -87,8 +87,9 @@ withEnv Glass.Config{..} gleanDB f =
   withLogger cfgapi $ \logger ->
   withFb303 serviceName $ \fb303 ->
   withBackendWithDefaultOptions evp cfgapi gleanService (Just schema_id)
-    $ \backend ->
-  withLatestRepos backend (sourceControl evp) (Just logger)
+    $ \backend -> do
+  scm <- sourceControl evp
+  withLatestRepos backend scm (Just logger)
     (if isRemote gleanService then listDatabasesRetry else Nothing) refreshFreq
     $ \latestGleanRepos -> do
       repoMapping <- getRepoMapping
@@ -97,7 +98,7 @@ withEnv Glass.Config{..} gleanDB f =
         , gleanIndexBackend = indexBackend backend
         , gleanDB = gleanDB
         , snapshotBackend = snapshotBackend evp
-        , sourceControl = sourceControl evp
+        , sourceControl = scm
         , ..
         }
 

glean/glass/Glean/Glass/Options.hs

@@ -38,7 +38,7 @@ configParser = do
   listDatabasesRetry <- listDatabasesRetryParser
   numWorkerThreads <- workerThreadsParser
   snapshotBackend <- pure (const $ Some NilSnapshotBackend)
-  sourceControl <- pure (const (Some NilSourceControl))
+  sourceControl <- pure (const (return (Some NilSourceControl)))
   tracer <- pure mempty
   welcomeMessage <- pure (pure (pure . defaultWelcomeMessage))
   return Glass.Config{configKey = Glass.defaultConfigKey, ..}