check for font open errors

CHANGES.md

@@ -1,5 +1,11 @@
 # Change Log / Release Notes
 
+## _unreleased_
+
+  * `xftOpenFont` and `xftOpenFontXlfd` now throw exceptions if opening a
+    font fails. (They used to return a null pointer, usually leading to
+    crashes.)
+
 ## 0.3.4 (2021-12-11)
 
   * Dropped support for GHC 7.10.

Graphics/X11/Xft.hsc

@@ -58,6 +58,7 @@ import Codec.Binary.UTF8.String as UTF8
 import Control.Arrow ((&&&))
 import Control.Monad (void)
 import Data.Char (ord)
+import Data.Coerce (coerce)
 import Data.Function (on)
 import Data.List (foldl')
 import Data.List.NonEmpty (NonEmpty)
@@ -171,15 +172,16 @@ foreign import ccall "XftFontOpenName"
 
 xftFontOpen :: Display -> Screen -> String -> IO XftFont
 xftFontOpen dpy screen fontname =
-    withCAString fontname $
-      \cfontname -> cXftFontOpen dpy (fi (screenNumberOfScreen screen)) cfontname
+    withCAString fontname $ \cfontname -> coerce (throwIfNull "xftFontOpen") $
+      cXftFontOpen dpy (fi (screenNumberOfScreen screen)) cfontname
 
 foreign import ccall "XftFontOpenXlfd"
   cXftFontOpenXlfd :: Display -> CInt -> CString -> IO XftFont
 
 xftFontOpenXlfd :: Display -> Screen -> String -> IO XftFont
 xftFontOpenXlfd dpy screen fontname =
-    withCAString fontname $ \cfontname -> cXftFontOpenXlfd dpy (fi (screenNumberOfScreen screen)) cfontname
+    withCAString fontname $ \cfontname -> coerce (throwIfNull "xftFontOpenXlfd") $
+      cXftFontOpenXlfd dpy (fi (screenNumberOfScreen screen)) cfontname
 
 foreign import ccall "XftLockFace"
   xftLockFace :: XftFont -> IO ()                  -- FIXME XftLockFace returns FT_face not void

X11-xft.cabal

@@ -1,6 +1,6 @@
 cabal-version: 1.24
 name:          X11-xft
-version:       0.3.4
+version:       0.3.5
 license:       BSD3
 license-file:  LICENSE
 author:        Clemens Fruhwirth