#2134: Intermittent file locking bug in installPackage.hs
------------------------+---------------------------------------------------
Reporter: gw | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.8.2 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
------------------------+---------------------------------------------------
After fixing the bug in ticket #1992, I still saw lock occasional
lock errors building ghc. Dumping all of the fd/dev/inode info in
lockFile
and unlockFile showed that the lock hash tables were not inconsistent, as
they
had been in #1992, but that a write lock was being placed on a file before
a read lock was released.
The culprit is the reading of the configuration in
libraries/installPackage.hs
({{{lbi <- getConfig verbosity}}}). This ultimately uses
tryGetConfigStateFile from
libraries/Cabal/Distribution/Simple/Configure.hs. And this function uses
readFile
to read in the configuration file.
The dump of fd/dev/inode info showed that readFile was acquiring a lock
that was
not released before the end of the program. Occasionally, there was an
attempt to
obtain a write lock on the same dev/inode, leading to a lock error. It
looks as if
this is the laziness problem with readFile noted in the IO library
documentation.
Making the read of the configuration file strict and explicitly closing
the handle
fixes the problem for me. Here is the patch:
{{{
--- libraries/Cabal/Distribution/Simple/Configure.hs.sav 2008-03-01
17:01:39.000000000 -0500
+++ libraries/Cabal/Distribution/Simple/Configure.hs 2008-03-01
17:04:25.000000000 -0500
@@ -130,7 +130,7 @@
import qualified System.Info
( os, arch )
import System.IO
- ( hPutStrLn, stderr )
+ ( hPutStrLn, stderr, openFile, IOMode(..), hGetContents, hClose )
import Text.PrettyPrint.HughesPJ
( comma, punctuate, render, nest, sep )
@@ -146,7 +146,9 @@
let dieMsg = "error reading " ++ filename ++
"; run \"setup configure\" command?\n"
if (not e) then return $ Left dieMsg else do
- str <- readFile filename
+ hdl <- openFile filename ReadMode
+ str <- hGetContents hdl >>= \s -> last s `seq` return s
+ hClose hdl
case reads str of
[(bi,_)] -> return $ Right bi
_ -> return $ Left dieMsg
}}}
I'm still not certain why the bug occurs only some of the time, and
whether
it is OS dependent. But readFile's holding of the lock for an
unpredictable
time is just asking for trouble.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2134>
GHC <http://www.haskell.org/ghc/>
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