From eff8d3bdb114bdd48bc84589d876f31fe3acc83b Mon Sep 17 00:00:00 2001
From: SrTobi <code.databyte@gmail.com>
Date: Wed, 13 Jan 2021 18:55:30 +0100
Subject: [PATCH] nixos/grub: implements GRUB_SAVEDEFAULT feature

Grub will remember the configuration that was booted and
select it as default menu entry the next time (#108206).
---
 .../modules/system/boot/loader/grub/grub.nix  |  2 ++
 .../system/boot/loader/grub/install-grub.pl   | 31 +++++++++++++++++--
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/nixos/modules/system/boot/loader/grub/grub.nix b/nixos/modules/system/boot/loader/grub/grub.nix
index e183bc3648ce..1be663670384 100644
--- a/nixos/modules/system/boot/loader/grub/grub.nix
+++ b/nixos/modules/system/boot/loader/grub/grub.nix
@@ -553,6 +553,8 @@ in
         apply = toString;
         description = ''
           Index of the default menu item to be booted.
+          Can also be set to "saved", which will make GRUB select
+          the menu item that was used at the last boot.
         '';
       };
 
diff --git a/nixos/modules/system/boot/loader/grub/install-grub.pl b/nixos/modules/system/boot/loader/grub/install-grub.pl
index e0167654748e..4d8537d4c327 100644
--- a/nixos/modules/system/boot/loader/grub/install-grub.pl
+++ b/nixos/modules/system/boot/loader/grub/install-grub.pl
@@ -85,6 +85,7 @@ my $bootloaderId = get("bootloaderId");
 my $forceInstall = get("forceInstall");
 my $font = get("font");
 my $theme = get("theme");
+my $saveDefault = $defaultEntry eq "saved";
 $ENV{'PATH'} = get("path");
 
 die "unsupported GRUB version\n" if $grubVersion != 1 && $grubVersion != 2;
@@ -250,6 +251,8 @@ if ($copyKernels == 0) {
 my $conf .= "# Automatically generated.  DO NOT EDIT THIS FILE!\n";
 
 if ($grubVersion == 1) {
+    # $defaultEntry might be "saved", indicating that we want to use the last selected configuration as default.
+    # Incidentally this is already the correct value for the grub 1 config to achieve this behaviour.
     $conf .= "
     default $defaultEntry
     timeout $timeout
@@ -305,6 +308,10 @@ else {
         " . $grubStore->search;
     }
     # FIXME: should use grub-mkconfig.
+    my $defaultEntryText = $defaultEntry;
+    if ($saveDefault) {
+        $defaultEntryText = "\"\${saved_entry}\"";
+    }
     $conf .= "
     " . $grubBoot->search . "
     if [ -s \$prefix/grubenv ]; then
@@ -318,11 +325,19 @@ else {
     set next_entry=
     save_env next_entry
     set timeout=1
+    set boot_once=true
     else
-    set default=$defaultEntry
+    set default=$defaultEntryText
     set timeout=$timeout
     fi
 
+    function savedefault {
+        if [ -z \"\${boot_once}\"]; then
+        saved_entry=\"\${chosen}\"
+        save_env saved_entry
+        fi
+    }
+
     # Setup the graphics stack for bios and efi systems
     if [ \"\${grub_platform}\" = \"efi\" ]; then
     insmod efi_gop
@@ -468,9 +483,16 @@ sub addEntry {
         $conf .= "  $extraPerEntryConfig\n" if $extraPerEntryConfig;
         $conf .= "  kernel $xen $xenParams\n" if $xen;
         $conf .= "  " . ($xen ? "module" : "kernel") . " $kernel $kernelParams\n";
-        $conf .= "  " . ($xen ? "module" : "initrd") . " $initrd\n\n";
+        $conf .= "  " . ($xen ? "module" : "initrd") . " $initrd\n";
+        if ($saveDefault) {
+            $conf .= "  savedefault\n";
+        }
+        $conf .= "\n";
     } else {
         $conf .= "menuentry \"$name\" " . ($options||"") . " {\n";
+        if ($saveDefault) {
+            $conf .= "  savedefault\n";
+        }
         $conf .= $grubBoot->search . "\n";
         if ($copyKernels == 0) {
             $conf .= $grubStore->search . "\n";
@@ -605,6 +627,11 @@ my $efiTarget = getEfiTarget();
 
 # Append entries detected by os-prober
 if (get("useOSProber") eq "true") {
+    if ($saveDefault) {
+        # os-prober will read this to determine if "savedefault" should be added to generated entries
+        $ENV{'GRUB_SAVEDEFAULT'} = "true";
+    }
+
     my $targetpackage = ($efiTarget eq "no") ? $grub : $grubEfi;
     system(get("shell"), "-c", "pkgdatadir=$targetpackage/share/grub $targetpackage/etc/grub.d/30_os-prober >> $tmpFile");
 }