Rewrites pinentry find code to be more resilient

This change rewrites the custom `pinentry` search code to instead be
a modified form of the standard `/usr/bin/pinetry` fallback script. The
prior behaviour could not handle cases where a `pinentry` executable
existed but was not actually usable. Now it checks using `ldd` for if an
executable is functional or not. Additionally rewritten to be more clear
and easier to extend with newer `pinentry` backends.

`make test` was ran, and it passed the main usability tests for password
entry. As my testing system uses swap it failed during the KDF test from
an error from swap being enabled; I did not attempt further tests with
swap disabled.

This partially fixes Issue dyne#542.

tomb

@@ -487,12 +487,17 @@ ask_password() {
 	local gtkrc
 	local theme
 	local pass_asked
+	local backends
 
 	# Distributions have broken wrappers for pinentry: they do
 	# implement fallback, but they disrupt the output somehow.	We are
-	# better off relying on less intermediaries, so we implement our
-	# own fallback mechanisms. Pinentry supported: curses, gtk-2, qt4, qt5
-	# and x11.
+	# better off relying on less intermediaries, so we copy in the
+	# fallback logic directly here.
+	# Pinentry supported: curses, tty, qt{,4,5}, gtk{,-2}, gnome3, and x11.
+
+	# TODO: Implement a user option to specify a pinentry program and
+	# wrap pinentry with a `_pinentry()` function that implements the
+	# search logic if the user option is not specified: Issue #542
 
 	# make sure LANG is set, default to C
 	LANG=${LANG:-C}
@@ -501,55 +506,39 @@ ask_password() {
 
 	pass_asked=0
 
-	while true; do
-		[[ ! -z $WAYLAND_DISPLAY ]] && {
-			_verbose "wayland display detected"
-			_is_found "pinentry-gnome3" && {
-				_verbose "using pinentry-gnome3 on wayland"
-				output=$(pinentry_assuan_getpass | pinentry-gnome3)
-				break; }
-			# TODO: pinentry on KDE running in wayland?
-		}
-		[[ ! -z $DISPLAY ]] && [[ $pass_asked == 0 ]] && {
-			_verbose "X11 display detected"
-			_is_found "pinentry-gtk-2" && {
-				_verbose "using pinentry-gtk2"
-				output=$(pinentry_assuan_getpass | pinentry-gtk-2)
-				break; }
-			_is_found "pinentry-x11" && {
-				_verbose "using pinentry-x11"
-				output=$(pinentry_assuan_getpass | pinentry-x11)
-				break; }
-			_is_found "pinentry-gnome3" && {
-				_verbose "using pinentry-gnome3 on X11"
-				output=$(pinentry_assuan_getpass | pinentry-gnome3)
-				break; }
-			_is_found "pinentry-qt5" && {
-				_verbose "using pinentry-qt5"
-				output=$(pinentry_assuan_getpass | pinentry-qt5)
-				break; }
-			_is_found "pinentry-qt4" && {
-				_verbose "using pinentry-qt4"
-				output=$(pinentry_assuan_getpass | pinentry-qt4)
-				break; }
-		}
-		_verbose "no display detected"
-		_is_found "pinentry-curses" && {
-			_verbose "using pinentry-curses with no display"
-			output=$(pinentry_assuan_getpass | pinentry-curses)
-			break; }
-		_is_found "pinentry-tty" && {
-			_verbose "using pinentry-tty with no display"
-			output=$(pinentry_assuan_getpass | pinentry-tty)
-			break; }
-		# TODO: fallback using read -s - and beware
-		# using read with or without -r may break
-		# passwords, so this must be covered by a test
-		# for compatibility
-		_failure "Cannot find any pinentry and no DISPLAY detected."
-		exit 1
+	# Guess preferred backend based on environment.
+	backends=(curses tty)
+	if [[ -n "$DISPLAY" || -n "$WAYLAND_DISPLAY" ]]; then
+		_verbose "Graphical display system detected"
+		case "$XDG_CURRENT_DESKTOP" in
+			KDE|LXQT|LXQt)
+				backends=(qt5 qt qt4 gnome3 gtk gtk-2 curses tty)
+				;;
+			*)
+				backends=(gtk gtk-2 x11 gnome3 qt5 qt qt4 curses tty)
+				;;
+		esac
+	fi
+	_verbose "Checking backends '${backends[@]}'"
+
+	for backend in "${backends[@]}"; do
+		local pinentry
+		local lddout
+		pinentry="$(which pinentry-$backend)"
+		lddout=$(ldd "$pinentry" 2>/dev/null) || continue
+		[[ "$lddout" == *'not found'* ]] && continue
+		_verbose "using $pinentry"
+		output=$(pinentry_assuan_getpass | $pinentry)
+		pass_asked=1
+		break
 	done
 
+	# TODO: fallback using read -s - and beware using read with or
+	# without -r may break passwords, so this must be covered by a test
+	# for compatibility
+	[[ $pass_asked == 0 ]] &&
+		_failure "Cannot find any viable pinentry."
+
 	# parse the pinentry output
 	local pinentry_error
 	for i in ${(f)output}; do