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

Author: hvr

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