boot/init: Use the new evdev library

It still uses evdev internally, but first let's change the interface,
*then* change the implementation details.

boot/init/default.nix

@@ -40,6 +40,7 @@ stdenv.mkDerivation {
 
     # This is the "script" that will be loaded.
     mrbc -o init.mrb \
+      $(find ${../lib} -type f | sort) \
       $(find lib -type f | sort) \
       $(get_tasks) \
       init.rb

boot/init/tasks/switch_root.rb

@@ -129,11 +129,7 @@ class Tasks::SwitchRoot < SingletonTask
       "KEY_ESC", # QEMU doesn't pass through CTRL and SHIFT as expected here...
     ]
 
-    # Do *not* use System.run as it would fail the boot on return value != 0
-    system(LOADER, "/applets/key-held.mrb", *keys)
-
-    # It returns `0` on key being held.
-    $?.exitstatus == 0
+    Evdev.keys_held(keys)
   end
 
   # Checks if the user wants to select a generation.

boot/lib/evdev.rb

@@ -0,0 +1,26 @@
+module Evdev
+  # TODO: Remove dependency on `evtest` and rather rely on direct evdev bindings.
+  def self.keys_held(keys)
+    # See:
+    #  * https://github.com/torvalds/linux/blob/master/include/uapi/linux/input-event-codes.h
+
+    # `evdev` works on `/dev/input/event*` nodes.
+    # Check them *all*. We can't know which is a keyboard.
+    Dir.glob("/dev/input/event*").each do |ev|
+      # `evtest` only tests one key at a time with `--query`...
+      keys.each do |key|
+        system("evtest", "--query", ev, "EV_KEY", key)
+        # One of the keys desired is held
+        if $?.exitstatus == 10
+          puts "#{key} is being held"
+          return true
+        end
+
+        # Failed for other reason
+        puts "Failed to run evtest, #{$?.exitstatus}" unless $?.exitstatus == 0
+      end
+    end
+
+    return false
+  end
+end