From 35ffbac5897293bad66bcdae5818da55958950f7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Roman=20Smr=C5=BE?= <roman.smrz@seznam.cz>
Date: Sat, 4 Apr 2026 12:31:43 +0200
Subject: Simplify task scheduling logic with fewer variables

Fixes a race happening when exception is receved during the "acquire"
phase, which would leave a stale value for "next task".
---
 src/Job.hs | 42 +++++++++++++-----------------------------
 1 file changed, 13 insertions(+), 29 deletions(-)

(limited to 'src')

diff --git a/src/Job.hs b/src/Job.hs
index 740a9f8..9286839 100644
--- a/src/Job.hs
+++ b/src/Job.hs
@@ -135,11 +135,10 @@ textJobStatusDetails = \case
 
 
 data JobManager = JobManager
-    { jmSemaphore :: TVar Int
+    { jmMaxRunningTasks :: Int
     , jmDataDir :: FilePath
     , jmJobs :: TVar (Map JobId (TVar (JobStatus JobOutput)))
     , jmNextTaskId :: TVar TaskId
-    , jmNextTask :: TVar (Maybe TaskId)
     , jmReadyTasks :: TVar (Set TaskId)
     , jmRunningTasks :: TVar (Map TaskId ThreadId)
     , jmCancelled :: TVar Bool
@@ -163,11 +162,9 @@ instance Exception JobCancelledException
 
 
 newJobManager :: FilePath -> Int -> IO JobManager
-newJobManager jmDataDir queueLen = do
-    jmSemaphore <- newTVarIO queueLen
+newJobManager jmDataDir jmMaxRunningTasks = do
     jmJobs <- newTVarIO M.empty
     jmNextTaskId <- newTVarIO (TaskId 0)
-    jmNextTask <- newTVarIO Nothing
     jmReadyTasks <- newTVarIO S.empty
     jmRunningTasks <- newTVarIO M.empty
     jmCancelled <- newTVarIO False
@@ -196,41 +193,28 @@ runManagedJob JobManager {..} tid cancel job = bracket acquire release $ \case
     True -> cancel
     False -> job
   where
-    acquire = liftIO $ do
+    acquire = (`onException` release False) $ liftIO $ do
         atomically $ do
             writeTVar jmReadyTasks . S.insert tid =<< readTVar jmReadyTasks
-            trySelectNext
         threadId <- myThreadId
         atomically $ do
             readTVar jmCancelled >>= \case
                 True -> return True
-                False -> readTVar jmNextTask >>= \case
-                    Just tid' | tid' == tid -> do
-                        writeTVar jmNextTask Nothing
-                        writeTVar jmRunningTasks . M.insert tid threadId =<< readTVar jmRunningTasks
-                        return False
+                False -> readTVar jmRunningTasks >>= \case
+                    running | M.size running < jmMaxRunningTasks -> do
+                        (S.minView <$> readTVar jmReadyTasks) >>= \case
+                            Just ( tid', ready ) | tid' == tid -> do
+                                writeTVar jmReadyTasks ready
+                                writeTVar jmRunningTasks $ M.insert tid threadId running
+                                return False
+                            _ -> retry
                     _ -> retry
 
     release False = liftIO $ atomically $ do
-        free <- readTVar jmSemaphore
-        writeTVar jmSemaphore $ free + 1
-        trySelectNext
+        writeTVar jmReadyTasks . S.delete tid =<< readTVar jmReadyTasks
+        writeTVar jmRunningTasks . M.delete tid =<< readTVar jmRunningTasks
     release True = return ()
 
-    trySelectNext = do
-        readTVar jmNextTask >>= \case
-            Just _ -> return ()
-            Nothing -> do
-                readTVar jmSemaphore >>= \case
-                    0   -> return ()
-                    sem -> (S.minView <$> readTVar jmReadyTasks) >>= \case
-                        Nothing -> return ()
-                        Just ( tid', ready ) -> do
-                            writeTVar jmReadyTasks ready
-                            writeTVar jmSemaphore (sem - 1)
-                            writeTVar jmNextTask (Just tid')
-                            writeTVar jmRunningTasks . M.delete tid =<< readTVar jmRunningTasks
-
 
 runJobs :: JobManager -> Output -> [ Job ]
         -> (JobId -> JobStatus JobOutput -> Bool) -- ^ Rerun condition
-- 
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