Text.Pandoc.Templates: Fix bug in getTemplate.

When calling `fetchItem` in `getTemplate`, we temporarily
reset the `stSourceURL` in CommonState so that the template
is sought locally. Previously, an exception in `fetchItem`
would prevent `stSourceURL` from being set back to its original
value. This could result in a local file being fetched instead
of a remote one. Note that an exception is triggered when one
uses e.g. `--template default.html5`; in that case `getTemplate`
handles the exception by looking for the file in the user
data directory.

This patch fixes the bug so that the `stSourceURL` is reset
regardless of whether `fetchItem` raises an exception.

Thanks to Yingjie Su for identifying the problem.

Author: jgm

pandoc.cabal

@@ -544,7 +544,7 @@ library
                  ipynb                 >= 0.2      && < 0.3,
                  jira-wiki-markup      >= 1.5.1    && < 1.6,
                  mime-types            >= 0.1.1    && < 0.2,
-                 mtl                   >= 2.2      && < 2.4,
+                 mtl                   >= 2.3      && < 2.4,
                  network-uri           >= 2.6      && < 2.8,
                  pandoc-types          >= 1.23.1.2 && < 1.24,
                  parsec                >= 3.1      && < 3.2,

src/Text/Pandoc/Templates.hs

@@ -47,7 +47,7 @@ import Text.Pandoc.Class.PandocMonad (PandocMonad, fetchItem,
                                       getCommonState, modifyCommonState,
                                       toTextM)
 import Text.Pandoc.Data (readDataFile)
-import Control.Monad.Except (catchError, throwError)
+import Control.Monad.Except (throwError, tryError)
 import Data.Text (Text)
 import qualified Data.Text as T
 import Text.Pandoc.Error
@@ -74,24 +74,22 @@ instance PandocMonad m => TemplateMonad (WithPartials m) where
 -- | Retrieve text for a template.
 getTemplate :: PandocMonad m => FilePath -> m Text
 getTemplate tp =
-  ((do surl <- stSourceURL <$> getCommonState
+   (do surl <- stSourceURL <$> getCommonState
        -- we don't want to look for templates remotely
        -- unless the full URL is specified:
-       modifyCommonState $ \st -> st{
-         stSourceURL = Nothing }
-       (bs, _) <- fetchItem $ T.pack tp
-       modifyCommonState $ \st -> st{
-         stSourceURL = surl }
-       return bs)
-   `catchError`
-   (\e -> case e of
+       modifyCommonState $ \st -> st{ stSourceURL = Nothing }
+       res <- tryError $ fetchItem $ T.pack tp
+       modifyCommonState $ \st -> st{ stSourceURL = surl }
+       case res of
+         Right (bs, _) -> return bs
+         Left e -> case e of
              PandocResourceNotFound _ ->
                 -- see #5987 on reason for takeFileName
                 readDataFile ("templates" </> takeFileName tp)
              PandocIOError _ ioe | isDoesNotExistError ioe ->
                 -- see #5987 on reason for takeFileName
                 readDataFile ("templates" </> takeFileName tp)
-             _ -> throwError e)) >>= toTextM tp
+             _ -> throwError e) >>= toTextM tp
 
 -- | Get default template for the specified writer.
 getDefaultTemplate :: PandocMonad m