Change moreRecentFile test to workaround races

This adds `notLessRecentFile a b` to return true not only when `a`
is younger than `b`, but also when `a` is exactly the same age of `b`, as
that case is subject to race-conditions (if the system time granularity
is too low), and it's better to err on assuming it needs to be regenerated.

This is an attempt to provide a pragmatic workaround for #2311

Cabal/Distribution/Simple/Configure.hs

@@ -100,7 +100,7 @@ import Distribution.Simple.BuildPaths
  ( autogenModulesDir )
 import Distribution.Simple.Utils
  ( die, warn, info, setupMessage
- , createDirectoryIfMissingVerbose, moreRecentFile
+ , createDirectoryIfMissingVerbose, notLessRecentFile
  , intercalate, cabalVersion
  , writeFileAtomic
  , withTempFile )
@@ -281,7 +281,7 @@ showHeader pkgId = BLC8.unwords
 -- .cabal file.
 checkPersistBuildConfigOutdated :: FilePath -> FilePath -> IO Bool
 checkPersistBuildConfigOutdated distPref pkg_descr_file = do
- pkg_descr_file `moreRecentFile` (localBuildInfoFile distPref)
+ pkg_descr_file `notLessRecentFile` (localBuildInfoFile distPref)
 
 -- |@dist\/setup-config@
 localBuildInfoFile :: FilePath -> FilePath

Cabal/Distribution/Simple/PreProcess.hs

@@ -51,7 +51,7 @@ import Distribution.Simple.LocalBuildInfo
 import Distribution.Simple.BuildPaths (autogenModulesDir,cppHeaderName)
 import Distribution.Simple.Utils
  ( createDirectoryIfMissingVerbose, withUTF8FileContents, writeUTF8File
- , die, setupMessage, intercalate, copyFileVerbose, moreRecentFile
+ , die, setupMessage, intercalate, copyFileVerbose, notLessRecentFile
  , findFileWithExtension, findFileWithExtension' )
 import Distribution.Simple.Program
  ( Program(..), ConfiguredProgram(..), programPath
@@ -269,7 +269,7 @@ preprocessFile searchLoc buildLoc forSDist baseFile verbosity builtinSuffixes ha
  recomp <- case ppsrcFiles of
  Nothing -> return True
  Just ppsrcFile ->
- psrcFile `moreRecentFile` ppsrcFile
+ psrcFile `notLessRecentFile` ppsrcFile
  when recomp $ do
  let destDir = buildLoc </> dirName srcStem
  createDirectoryIfMissingVerbose verbosity True destDir

Cabal/Distribution/Simple/Utils.hs

@@ -87,6 +87,8 @@ module Distribution.Simple.Utils (
  -- * modification time
  moreRecentFile,
  existsAndIsMoreRecentThan,
+ notLessRecentFile,
+ existsAndIsNotLessRecentThan,
 
  -- * temp files and dirs
  TempFileOptions(..), defaultTempFileOptions,
@@ -785,6 +787,27 @@ existsAndIsMoreRecentThan a b = do
  then return False
  else a `moreRecentFile` b
 
+-- | Variant of 'moreRecentFile' comparing times using '>='
+-- comparision instead of '>'. This reduces race-conditions for
+-- generated files when the system time granularity is too low.
+--
+notLessRecentFile :: FilePath -> FilePath -> IO Bool
+notLessRecentFile a b = do
+ exists <- doesFileExist b
+ if not exists
+ then return True
+ else do tb <- getModificationTime b
+ ta <- getModificationTime a
+ return (ta >= tb)
+
+-- | Like 'notLessRecentFile', but also checks that the first file exists.
+existsAndIsNotLessRecentThan :: FilePath -> FilePath -> IO Bool
+existsAndIsNotLessRecentThan a b = do
+ exists <- doesFileExist a
+ if not exists
+ then return False
+ else a `notLessRecentFile` b
+
 ----------------------------------------
 -- Copying and installing files and dirs