Add Graphics.Vty.Config.getTtyEraseChar

This commit adds getTtyEraseChar. This function queries the kernel
for the current terminal's settings to obtain the character assigned
by the "stty erase" command. The "erase" character indicates which
input character the terminal should interpret to mean "backspace" when
the terminal is in canonical input mode. Vty applications run with
canonical mode disabled, but even in those cases some users may want
their "stty erase" setting honored by Vty's input-handling so that
incoming erase characters (according to stty) result in "KBS" key events
being delivered to the application.

cbits/get_tty_erase.c

@@ -0,0 +1,18 @@
+#include <termios.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+// Given a file descriptor for a terminal, get the ERASE character for
+// the terminal. If the terminal info cannot be obtained, this returns
+// zero.
+char vty_get_tty_erase(int fd)
+{
+    struct termios trm;
+
+    if (0 == tcgetattr(fd, &trm)) {
+        return (char) trm.c_cc[VERASE];
+    } else {
+        return (char) 0;
+    }
+}

demos/ModeDemo.hs

@@ -2,6 +2,8 @@ module Main where
 
 import Control.Applicative
 import Data.Monoid
+import Foreign.C.Types (CInt(..), CChar(..))
+import System.Posix.Types (Fd(..))
 
 import Graphics.Vty
 

src/Graphics/Vty/Config.hs

@@ -65,6 +65,7 @@ module Graphics.Vty.Config
   , runParseConfig
   , parseConfigFile
   , defaultConfig
+  , getTtyEraseChar
   )
 where
 
@@ -92,6 +93,7 @@ import System.Directory (getAppUserDataDirectory)
 import System.Environment (lookupEnv)
 import System.Posix.IO (stdInput, stdOutput)
 import System.Posix.Types (Fd(..))
+import Foreign.C.Types (CInt(..), CChar(..))
 
 import Text.Parsec hiding ((<|>))
 import Text.Parsec.Token ( GenLanguageDef(..) )
@@ -306,3 +308,24 @@ instance (GParseAlts f, GParseAlts g) => GParseAlts (f :+: g) where
   gparseAlts con = L1 <$> gparseAlts con <|> R1 <$> gparseAlts con
 
 instance GParseAlts V1 where gparseAlts _ = fail "GParse: V1"
+
+foreign import ccall "vty_get_tty_erase" cGetTtyErase :: Fd -> IO CChar
+
+-- | Get the "erase" character for the terminal attached to the
+-- specified file descriptor. This is the character configured by 'stty
+-- erase'. If the call to 'tcgetattr' fails, this will return 'Nothing'.
+-- Otherwise it will return the character that has been configured to
+-- indicate the canonical mode ERASE behavior. That character can then
+-- be added to the table of strings that we interpret to mean Backspace.
+--
+-- For more details, see:
+--
+-- * https://www.gnu.org/software/libc/manual/html_node/Canonical-or-Not.html
+-- * https://www.gsp.com/cgi-bin/man.cgi?section=1&topic=stty
+-- * https://github.com/matterhorn-chat/matterhorn/issues/565
+getTtyEraseChar :: Fd -> IO (Maybe Char)
+getTtyEraseChar fd = do
+    c <- cGetTtyErase fd
+    if c /= 0
+       then return $ Just $ toEnum $ fromEnum c
+       else return Nothing

vty.cabal

@@ -106,6 +106,7 @@ library
 
   c-sources:           cbits/gwinsz.c
                        cbits/set_term_timing.c
+                       cbits/get_tty_erase.c
                        cbits/mk_wcwidth.c
 
   include-dirs:        cbits