Fix gitRev TH splice

Previously we had a TH quotation calling a TH splce, but that meant that
the TH splice was, under certain curcumstances, being cached in the object
file in `~/.cabal/store`.

Solution is to pull the TH splice into the TH quotation. This should force
the running of the `git rev-parse` command whenever the TH quotation is
evaluated.

cardano-git-rev/cardano-git-rev.cabal

@@ -34,7 +34,6 @@ library
   c-sources:            cbits/rev.c
 
   exposed-modules:      Cardano.Git.Rev
-                        Cardano.Git.RevFromGit
 
   build-depends:        process
                       , template-haskell

cardano-git-rev/src/Cardano/Git/Rev.hs

@@ -9,34 +9,39 @@ module Cardano.Git.Rev
   ) where
 
 import           Data.Text (Text)
-import qualified Data.Text as T
+import qualified Data.Text as Text
 
-#if !defined(arm_HOST_ARCH)
-import           Cardano.Git.RevFromGit (gitRevFromGit)
-#endif
 import           Foreign.C.String (CString)
 import           GHC.Foreign (peekCStringLen)
 import           Language.Haskell.TH (Exp, Q)
 import           System.IO (utf8)
 import           System.IO.Unsafe (unsafeDupablePerformIO)
 
+#if !defined(arm_HOST_ARCH)
+import           Control.Exception (catch)
+import           System.Exit (ExitCode (..))
+import qualified System.IO as IO
+import           System.IO.Error (isDoesNotExistError)
+import           System.Process (readProcessWithExitCode)
+#endif
+
 foreign import ccall "&_cardano_git_rev" c_gitrev :: CString
 
 -- This must be a TH splice to ensure the git commit is captured at build time.
 -- ie called as `$(gitRev)`.
 gitRev :: Q Exp
 gitRev =
     [| if
-         | gitRevEmbed /= zeroRev -> gitRevEmbed
-         | T.null fromGit         -> zeroRev
+         | gitRevEmbed /= zeroRev -> fromGit
+         | Text.null fromGit      -> zeroRev
          | otherwise              -> fromGit
     |]
 
 -- Git revision embedded after compilation using
 -- Data.FileEmbed.injectWith. If nothing has been injected,
 -- this will be filled with 0 characters.
 gitRevEmbed :: Text
-gitRevEmbed = T.pack $ drop 28 $ unsafeDupablePerformIO (peekCStringLen utf8 (c_gitrev, 68))
+gitRevEmbed = Text.pack $ drop 28 $ unsafeDupablePerformIO (peekCStringLen utf8 (c_gitrev, 68))
 
 -- Git revision found during compilation by running git. If
 -- git could not be run, then this will be empty.
@@ -45,8 +50,26 @@ fromGit :: Text
   -- cross compiling to arm fails; due to a linker bug
 fromGit = ""
 #else
-fromGit = T.strip (T.pack $(gitRevFromGit))
+fromGit = Text.strip $ Text.pack (unsafeDupablePerformIO runGitRevParse)
 #endif
 
 zeroRev :: Text
 zeroRev = "0000000000000000000000000000000000000000"
+
+#if !defined(arm_HOST_ARCH)
+runGitRevParse :: IO String
+runGitRevParse = do
+  (exitCode, output, errorMessage) <- readProcessWithExitCode_ "git" ["rev-parse", "--verify", "HEAD"] ""
+  case exitCode of
+    ExitSuccess -> pure output
+    ExitFailure _ -> do
+      IO.hPutStrLn IO.stderr $ "WARNING: " ++ errorMessage
+      pure ""
+  where
+    readProcessWithExitCode_ :: FilePath -> [String] -> String -> IO (ExitCode, String, String)
+    readProcessWithExitCode_ cmd args input =
+      catch (readProcessWithExitCode cmd args input) $ \e ->
+      if isDoesNotExistError e
+        then pure (ExitFailure 127, "", show e)
+        else pure (ExitFailure 999, "", show e)
+#endif