Improve error message when unable to read a sqlite database due to permissions problem

Old message was:

sqlite query crashed: thread blocked indefinitely in an MVar operation

New message is eg:

sqlite worker thread crashed: SQLite3 returned ErrorCan'tOpen while attempting to perform open ".git/annex/keysdb/db".

The worker thread used to throw an exception. But before that
exception was seen by anything waiting on the worker thread to
finish, the takeMVar in queryDb would have crashed with
BlockedIndefinitelyOnMVar.

Sponsored-by: k0ld on Patreon

CHANGELOG

@@ -8,6 +8,8 @@ git-annex (10.20230215) UNRELEASED; urgency=medium
     at the end.
   * git-annex.cabal: Move webapp build deps under the Assistant build flag
     so git-annex can be built again without yesod etc installed.
+  * Improve error message when unable to read a sqlite database due to
+    permissions problem.
 
  -- Joey Hess <id@joeyh.name>  Tue, 14 Feb 2023 14:11:11 -0400
 

Database/Handle.hs

@@ -1,6 +1,6 @@
 {- Persistent sqlite database handles.
  -
- - Copyright 2015-2022 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2023 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -38,8 +38,9 @@ import Control.Monad.Logger (runNoLoggingT)
 import System.IO
 
 {- A DbHandle is a reference to a worker thread that communicates with
- - the database. It has a MVar which Jobs are submitted to. -}
-data DbHandle = DbHandle RawFilePath (Async ()) (MVar Job)
+ - the database. It has a MVar which Jobs are submitted to. 
+ - There is also an MVar which it will fill when there is a fatal error-}
+data DbHandle = DbHandle RawFilePath (Async ()) (MVar Job) (MVar String)
 
 {- Name of a table that should exist once the database is initialized. -}
 type TableName = String
@@ -49,17 +50,18 @@ type TableName = String
 openDb :: RawFilePath -> TableName -> IO DbHandle
 openDb db tablename = do
 	jobs <- newEmptyMVar
-	worker <- async (workerThread db tablename jobs)
+	errvar <- newEmptyMVar
+	worker <- async (workerThread db tablename jobs errvar)
 	
 	-- work around https://github.com/yesodweb/persistent/issues/474
 	liftIO $ fileEncoding stderr
 
-	return $ DbHandle db worker jobs
+	return $ DbHandle db worker jobs errvar
 
 {- This is optional; when the DbHandle gets garbage collected it will
  - auto-close. -}
 closeDb :: DbHandle -> IO ()
-closeDb (DbHandle _db worker jobs) = do
+closeDb (DbHandle _db worker jobs _) = do
 	debugLocks $ putMVar jobs CloseJob
 	wait worker
 
@@ -74,12 +76,15 @@ closeDb (DbHandle _db worker jobs) = do
  - it is able to run.
  -}
 queryDb :: DbHandle -> SqlPersistM a -> IO a
-queryDb (DbHandle _db _ jobs) a = do
+queryDb (DbHandle _db _ jobs errvar) a = do
 	res <- newEmptyMVar
 	putMVar jobs $ QueryJob $
 		debugLocks $ liftIO . putMVar res =<< tryNonAsync a
-	debugLocks $ (either throwIO return =<< takeMVar res)
-		`catchNonAsync` (\e -> error $ "sqlite query crashed: " ++ show e)
+	debugLocks $ takeMVarSafe res >>= \case
+		Right r -> either throwIO return r
+		Left BlockedIndefinitelyOnMVar -> do
+			err <- takeMVar errvar
+			error $ "sqlite worker thread crashed: " ++ err
 
 {- Writes a change to the database.
  -
@@ -91,7 +96,7 @@ queryDb (DbHandle _db _ jobs) a = do
  - process at least once each 30 seconds.
  -}
 commitDb :: DbHandle -> SqlPersistM () -> IO ()
-commitDb h@(DbHandle db _ _) wa = 
+commitDb h@(DbHandle db _ _ _) wa = 
 	robustly (commitDb' h wa) maxretries emptyDatabaseInodeCache
   where
 	robustly a retries ic = do
@@ -108,7 +113,7 @@ commitDb h@(DbHandle db _ _) wa =
 	maxretries = 300 :: Int -- 30 seconds of briefdelay
 
 commitDb' :: DbHandle -> SqlPersistM () -> IO (Either SomeException ())
-commitDb' (DbHandle _ _ jobs) a = do
+commitDb' (DbHandle _ _ jobs _) a = do
 	debug "Database.Handle" "commitDb start"
 	res <- newEmptyMVar
 	putMVar jobs $ ChangeJob $
@@ -125,18 +130,17 @@ data Job
 	| ChangeJob (SqlPersistM ())
 	| CloseJob
 
-workerThread :: RawFilePath -> TableName -> MVar Job -> IO ()
-workerThread db tablename jobs = newconn
+workerThread :: RawFilePath -> TableName -> MVar Job -> MVar String -> IO ()
+workerThread db tablename jobs errvar = newconn
   where
 	newconn = do
 		v <- tryNonAsync (runSqliteRobustly tablename db loop)
 		case v of
-			Left e -> giveup $
-				"sqlite worker thread crashed: " ++ show e
+			Left e -> putMVar errvar (show e)
 			Right cont -> cont
 	
 	loop = do
-		job <- liftIO getjob
+		job <- liftIO (takeMVarSafe jobs)
 		case job of
 			-- Exception is thrown when the MVar is garbage
 			-- collected, which means the whole DbHandle
@@ -149,9 +153,6 @@ workerThread db tablename jobs = newconn
 				-- Exit the sqlite connection so the
 				-- database gets updated on disk.
 				return newconn
-	
-	getjob :: IO (Either BlockedIndefinitelyOnMVar Job)
-	getjob = try $ takeMVar jobs
 
 {- Like runSqlite, but more robust.
  -
@@ -325,3 +326,7 @@ isDatabaseModified (DatabaseInodeCache a1 b1) (DatabaseInodeCache a2 b2) =
 	ismodified (Just a) (Just b) = not (compareStrong a b)
 	ismodified Nothing Nothing = False
 	ismodified _ _ = True
+
+takeMVarSafe :: MVar a -> IO (Either BlockedIndefinitelyOnMVar a)
+takeMVarSafe = try . takeMVar
+