[commit: ghc] wip/reapply-atomic-writes: compiler: Refactor: extract `withAtomicRename` (ced9f3c)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : wip/reapply-atomic-writes
Link       : http://ghc.haskell.org/trac/ghc/changeset/ced9f3cf781e62d5becdbaf4fc57d65a9eaf1876/ghc

>---------------------------------------------------------------

commit ced9f3cf781e62d5becdbaf4fc57d65a9eaf1876
Author: Niklas Hambüchen <mail at nh2.me>
Date:   Sun Feb 17 21:09:29 2019 +0100

    compiler: Refactor: extract `withAtomicRename`


>---------------------------------------------------------------

ced9f3cf781e62d5becdbaf4fc57d65a9eaf1876
 compiler/main/DriverPipeline.hs | 13 ++++++-------
 compiler/utils/Util.hs          | 24 +++++++++++++++++++++++-
 2 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/compiler/main/DriverPipeline.hs b/compiler/main/DriverPipeline.hs
index 3f59ed3..f1ef637 100644
--- a/compiler/main/DriverPipeline.hs
+++ b/compiler/main/DriverPipeline.hs
@@ -1341,7 +1341,10 @@ runPhase (RealPhase (As with_cpp)) input_fn dflags
         let local_includes = [ SysTools.Option ("-iquote" ++ p)
                              | p <- includePathsQuote cmdline_include_paths ]
         let runAssembler inputFilename outputFilename
-                = liftIO $ as_prog dflags
+              = liftIO $ do
+                  withAtomicRename outputFilename $ \temp_outputFilename -> do
+                    as_prog
+                       dflags
                        (local_includes ++ global_includes
                        -- See Note [-fPIC for assembler]
                        ++ map SysTools.Option pic_c_flags
@@ -1371,15 +1374,11 @@ runPhase (RealPhase (As with_cpp)) input_fn dflags
                           , SysTools.Option "-c"
                           , SysTools.FileOption "" inputFilename
                           , SysTools.Option "-o"
-                          , SysTools.FileOption "" outputFilename
+                          , SysTools.FileOption "" temp_outputFilename
                           ])
 
         liftIO $ debugTraceMsg dflags 4 (text "Running the assembler")
-
-        -- Atomic write by writing to temp file and then renaming
-        let temp_output_fn = output_fn <.> "tmp"
-        runAssembler input_fn temp_output_fn
-        liftIO $ renameFile temp_output_fn output_fn
+        runAssembler input_fn output_fn
 
         return (RealPhase next_phase, output_fn)
 
diff --git a/compiler/utils/Util.hs b/compiler/utils/Util.hs
index 16864fe..41f63f2 100644
--- a/compiler/utils/Util.hs
+++ b/compiler/utils/Util.hs
@@ -99,6 +99,7 @@ module Util (
         doesDirNameExist,
         getModificationUTCTime,
         modificationTimeIfExists,
+        withAtomicRename,
 
         global, consIORef, globalM,
         sharedGlobal, sharedGlobalM,
@@ -145,9 +146,10 @@ import GHC.Stack (HasCallStack)
 
 import Control.Applicative ( liftA2 )
 import Control.Monad    ( liftM, guard )
+import Control.Monad.IO.Class ( MonadIO, liftIO )
 import GHC.Conc.Sync ( sharedCAF )
 import System.IO.Error as IO ( isDoesNotExistError )
-import System.Directory ( doesDirectoryExist, getModificationTime )
+import System.Directory ( doesDirectoryExist, getModificationTime, renameFile )
 import System.FilePath
 
 import Data.Char        ( isUpper, isAlphaNum, isSpace, chr, ord, isDigit, toUpper
@@ -1304,6 +1306,26 @@ modificationTimeIfExists f = do
                         else ioError e
 
 -- --------------------------------------------------------------
+-- atomic file writing by writing to a temporary file first (see #14533)
+--
+-- This should be used in all cases where GHC writes files to disk
+-- and uses their modification time to skip work later,
+-- as otherwise a partially written file (e.g. due to crash or Ctrl+C)
+-- also results in a skip.
+
+withAtomicRename :: (MonadIO m) => FilePath -> (FilePath -> m a) -> m a
+withAtomicRename targetFile f = do
+  -- The temp file must be on the same file system (mount) as the target file
+  -- to result in an atomic move on most platforms.
+  -- The standard way to ensure that is to place it into the same directory.
+  -- This can still be fooled when somebody mounts a different file system
+  -- at just the right time, but that is not a case we aim to cover here.
+  let temp = targetFile <.> "tmp"
+  res <- f temp
+  liftIO $ renameFile temp targetFile
+  return res
+
+-- --------------------------------------------------------------
 -- split a string at the last character where 'pred' is True,
 -- returning a pair of strings. The first component holds the string
 -- up (but not including) the last character for which 'pred' returned