#13354: Package database locking patch broke ghc-pkg with non-existent database -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Package system | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by arybczak): Ehh, it's a bit tricky. In theory the following fix should do it: {{{#!diff --- a/utils/ghc-pkg/Main.hs +++ b/utils/ghc-pkg/Main.hs @@ -807,7 +807,10 @@ readParseDatabase :: forall mode t. Verbosity readParseDatabase verbosity mb_user_conf mode use_cache path -- the user database (only) is allowed to be non-existent | Just (user_conf,False) <- mb_user_conf, path == user_conf - = mkPackageDB [] =<< F.mapM (const $ GhcPkg.lockPackageDb path) mode + = do lock <- F.forM mode $ \_ -> do + createDirectoryIfMissing True path + GhcPkg.lockPackageDb cache + mkPackageDB [] lock | otherwise = do e <- tryIO $ getDirectoryContents path case e of @@ -828,7 +831,6 @@ readParseDatabase verbosity mb_user_conf mode use_cache path Right fs | not use_cache -> ignore_cache (const $ return ()) | otherwise -> do - let cache = path cachefilename tdir <- getModificationTime path e_tcache <- tryIO $ getModificationTime cache case e_tcache of @@ -888,6 +890,8 @@ readParseDatabase verbosity mb_user_conf mode use_cache path whenReportCacheErrors = when $ verbosity > Normal || verbosity >= Normal && GhcPkg.isDbOpenReadMode mode where + cache = path cachefilename + recacheAdvice | Just (user_conf, True) <- mb_user_conf, path == user_conf = "Use 'ghc-pkg recache --user' to fix." }}} So if we open user database in read/write mode and it doesn't exist, create the missing directory so that we can create a lock file. However, this clashes with the way we try to open a database that contains a specific package for modification: we open in read write mode and if it doesn't contain given package, we "downgrade" to read only by releasing a lock. So in this case we'll always downgrade, because the database is empty and so an empty directory will be left. In particular, it won't contain `package.cache` file and then compiler/main/Packages.hs:readPackageConfig will fail on attempt to read this database, because it expects `package.cache`to be there. I see two possible solutions: 1. We change the way we open database that contains a package for modification: instead of opening in read write mode and downgrading to read only if necessary, we first open in read only and only after we confirm that the database contains a package, we open in read write mode (and then unfortunately we also need to consider the possibility that the database changed between these two reads and downgrade to read only if the package is not there anymore). Although that still has a potential to create the problem if the database is removed between the first read only read and the subsequent read write read :/ 2. We modify compiler/main/Packages.hs:readPackageConfig to catch "not exists" error on reading `package.cache` file and check if the directory contains any `.conf` file. If so, we propagate the error (and possibly print slightly more user friendy message that cache is not there instead of `openBinaryFile: does not exist (No such file or directory)`), but if not, we consider the database to be empty. I think (2) is both simpler and more sensible. Thoughts? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13354#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler