add: log pool flushes

Emit a dedicated PoolFlushed observation when the DB pool is released during schema cache reload.

src/PostgREST/AppState.hs

@@ -219,10 +219,14 @@ usePool AppState{stateObserver=observer, stateMainThreadId=mainThreadId, ..} ses
 
 -- | Flush the connection pool so that any future use of the pool will
 -- use connections freshly established after this call.
+-- | Emits PoolFlushed observation
 flushPool :: AppState -> IO ()
-flushPool AppState{..} = SQL.release statePool
+flushPool AppState{..} = do
+  SQL.release statePool
+  stateObserver PoolFlushed
 
 -- | Destroy the pool on shutdown.
+-- | Differs from flushPool in not emiting PoolFlushed observation.
 destroyPool :: AppState -> IO ()
 destroyPool AppState{..} = SQL.release statePool
 

src/PostgREST/Logger.hs

@@ -110,6 +110,9 @@ observationLogger loggerState logLevel obs = case obs of
   o@PoolRequestFullfilled ->
     when (logLevel >= LogDebug) $ do
       logWithZTime loggerState $ observationMessage o
+  o@PoolFlushed ->
+    when (logLevel >= LogDebug) $ do
+      logWithZTime loggerState $ observationMessage o
   o@JwtCacheEviction ->
     when (logLevel >= LogDebug) $ do
       logWithZTime loggerState $ observationMessage o

src/PostgREST/Observation.hs

@@ -64,6 +64,7 @@ data Observation
   | HasqlPoolObs SQL.Observation
   | PoolRequest
   | PoolRequestFullfilled
+  | PoolFlushed
   | JwtCacheLookup Bool
   | JwtCacheEviction
   | TerminationUnixSignalObs Text
@@ -161,6 +162,8 @@ observationMessage = \case
     "Trying to borrow a connection from pool"
   PoolRequestFullfilled ->
     "Borrowed a connection from the pool"
+  PoolFlushed ->
+    "Database connection pool flushed"
   JwtCacheLookup _ ->
     "Looked up a JWT in JWT cache"
   JwtCacheEviction ->