luksroot: Wait for the header (device) to appear

The LUKS header can be on another device (e.g. a USB stick). In my case
it can take up to two seconds until the partition on my USB stick is
available (i.e. the decryption fails without this patch). This will also
remove some redundancy by providing the shell function `wait_target` and
slightly improve the output (one "." per second and a success/failure
indication after 10 seconds instead of always printing "ok").

nixos/modules/system/boot/luksroot.nix

@@ -6,29 +6,38 @@ let
   luks = config.boot.initrd.luks;
 
   openCommand = name': { name, device, header, keyFile, keyFileSize, allowDiscards, yubikey, ... }: assert name' == name; ''
-    # Wait for luksRoot to appear, e.g. if on a usb drive.
-    # XXX: copied and adapted from stage-1-init.sh - should be
-    # available as a function.
-    if ! test -e ${device}; then
-        echo -n "waiting 10 seconds for device ${device} to appear..."
-        for try in $(seq 10); do
-            sleep 1
-            if test -e ${device}; then break; fi
-            echo -n .
-        done
-        echo "ok"
-    fi
+
+    # Wait for a target (e.g. device, keyFile, header, ...) to appear.
+    wait_target() {
+        local name="$1"
+        local target="$2"
+
+        if [ ! -e $target ]; then
+            echo -n "Waiting 10 seconds for $name $target to appear"
+            local success=false;
+            for try in $(seq 10); do
+                echo -n "."
+                sleep 1
+                if [ -e $target ]; then success=true break; fi
+            done
+            if [ $success = true ]; then
+                echo " - success";
+            else
+                echo " - failure";
+            fi
+        fi
+    }
+
+    # Wait for luksRoot (and optionally keyFile and/or header) to appear, e.g.
+    # if on a USB drive.
+    wait_target "device" ${device}
 
     ${optionalString (keyFile != null) ''
-    if ! test -e ${keyFile}; then
-        echo -n "waiting 10 seconds for key file ${keyFile} to appear..."
-        for try in $(seq 10); do
-            sleep 1
-            if test -e ${keyFile}; then break; fi
-            echo -n .
-        done
-        echo "ok"
-    fi
+      wait_target "key file" ${keyFile}
+    ''}
+
+    ${optionalString (header != null) ''
+      wait_target "header" ${header}
     ''}
 
     open_normally() {