Search for metadata files in $DATADIR/metadata (#7851)

If files specified with `--metadata-file` are not found in the working
directory, look in `$DATADIR/metadata`.

Expose new `readMetadataFile` function from Text.Pandoc.Class
[API change].

Expose new `PandocCouldNotFindMetadataFileError` constructor for
`PandocError` from Text.Pandoc.Error [API change].

Closes #5876.

3 files changed, 25 insertions, 1 deletions

diff --git a/src/Text/Pandoc/App.hs b/src/Text/Pandoc/App.hs
index 3ddc4c761..b17fdd178 100644
--- a/src/Text/Pandoc/App.hs
+++ b/src/Text/Pandoc/App.hs
@@ -219,7 +219,7 @@ convertWithOpts opts = do
       case optMetadataFiles opts of
         []    -> return mempty
         paths -> mconcat <$>
-           mapM (\path -> do raw <- readFileStrict path
+           mapM (\path -> do raw <- readMetadataFile path
                              yamlToMeta readerOpts (Just path) raw) paths
 
     let transforms = (case optShiftHeadingLevelBy opts of
diff --git a/src/Text/Pandoc/Class/PandocMonad.hs b/src/Text/Pandoc/Class/PandocMonad.hs
index c15ce6444..144a13ba4 100644
--- a/src/Text/Pandoc/Class/PandocMonad.hs
+++ b/src/Text/Pandoc/Class/PandocMonad.hs
@@ -46,6 +46,7 @@ module Text.Pandoc.Class.PandocMonad
   , getResourcePath
   , readDefaultDataFile
   , readDataFile
+  , readMetadataFile
   , fillMediaBag
   , toLang
   , setTranslations
@@ -586,6 +587,25 @@ readDataFile fname = do
             then readFileStrict (userDir </> fname)
             else readDefaultDataFile fname
 
+-- | Read metadata file from the working directory or, if not found there, from
+-- the metadata subdirectory of the user data directory.
+readMetadataFile :: PandocMonad m => FilePath -> m B.ByteString
+readMetadataFile fname = do
+  existsInWorkingDir <- fileExists fname
+  if existsInWorkingDir
+     then readFileStrict fname
+     else do
+       dataDir <- getUserDataDir
+       case dataDir of
+         Nothing ->
+           throwError $ PandocCouldNotFindMetadataFileError $ T.pack fname
+         Just userDir -> do
+           let path = userDir </> "metadata" </> fname
+           existsInUserDir <- fileExists path
+           if existsInUserDir
+              then readFileStrict path
+              else throwError $ PandocCouldNotFindMetadataFileError $ T.pack fname
+
 -- | Read file from from the default data files.
 readDefaultDataFile :: PandocMonad m => FilePath -> m B.ByteString
 readDefaultDataFile "reference.docx" =
diff --git a/src/Text/Pandoc/Error.hs b/src/Text/Pandoc/Error.hs
index 684a6a884..3e2479d61 100644
--- a/src/Text/Pandoc/Error.hs
+++ b/src/Text/Pandoc/Error.hs
@@ -54,6 +54,7 @@ data PandocError = PandocIOError Text IOError
                  | PandocFilterError Text Text
                  | PandocLuaError Text
                  | PandocCouldNotFindDataFileError Text
+                 | PandocCouldNotFindMetadataFileError Text
                  | PandocResourceNotFound Text
                  | PandocTemplateError Text
                  | PandocAppError Text
@@ -118,6 +119,8 @@ renderError e =
     PandocLuaError msg -> "Error running Lua:\n" <> msg
     PandocCouldNotFindDataFileError fn ->
         "Could not find data file " <> fn
+    PandocCouldNotFindMetadataFileError fn ->
+        "Could not find metadata file " <> fn
     PandocResourceNotFound fn ->
         "File " <> fn <> " not found in resource path"
     PandocTemplateError s -> "Error compiling template " <> s
@@ -198,6 +201,7 @@ handleError (Left e) =
       PandocIpynbDecodingError{} -> 93
       PandocUnsupportedCharsetError{} -> 94
       PandocCouldNotFindDataFileError{} -> 97
+      PandocCouldNotFindMetadataFileError{} -> 98
       PandocResourceNotFound{} -> 99
 
 err :: Int -> Text -> IO a