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nixos/modules/system/boot/loader/generic-extlinux-compatible/default.nix

@@ -0,0 +1,137 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+with lib;
+
+let
+  blCfg = config.boot.loader;
+  dtCfg = config.hardware.deviceTree;
+  cfg = blCfg.generic-extlinux-compatible;
+
+  timeoutStr = if blCfg.timeout == null then "-1" else toString blCfg.timeout;
+
+  # The builder used to write during system activation
+  builder = import ./extlinux-conf-builder.nix { inherit lib pkgs; };
+  # The builder exposed in populateCmd, which runs on the build architecture
+  populateBuilder = import ./extlinux-conf-builder.nix {
+    inherit lib;
+    pkgs = pkgs.buildPackages;
+  };
+in
+{
+  options = {
+    boot.loader.generic-extlinux-compatible = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Whether to generate an extlinux-compatible configuration file
+          under `/boot/extlinux.conf`.  For instance,
+          U-Boot's generic distro boot support uses this file format.
+
+          See [U-boot's documentation](https://u-boot.readthedocs.io/en/latest/develop/distro.html)
+          for more information.
+        '';
+      };
+
+      useGenerationDeviceTree = mkOption {
+        default = true;
+        type = types.bool;
+        description = ''
+          Whether to generate Device Tree-related directives in the
+          extlinux configuration.
+
+          When enabled, the bootloader will attempt to load the device
+          tree binaries from the generation's kernel.
+
+          Note that this affects all generations, regardless of the
+          setting value used in their configurations.
+        '';
+      };
+
+      configurationLimit = mkOption {
+        default = 20;
+        example = 10;
+        type = types.int;
+        description = ''
+          Maximum number of configurations in the boot menu.
+        '';
+      };
+
+      mirroredBoots = mkOption {
+        default = [ { path = "/boot"; } ];
+        example = [
+          { path = "/boot1"; }
+          { path = "/boot2"; }
+        ];
+        description = ''
+          Mirror the boot configuration to multiple paths.
+        '';
+
+        type =
+          with types;
+          listOf (submodule {
+            options = {
+              path = mkOption {
+                example = "/boot1";
+                type = types.str;
+                description = ''
+                  The path to the boot directory where the extlinux-compatible
+                  configuration files will be written.
+                '';
+              };
+            };
+          });
+      };
+
+      populateCmd = mkOption {
+        type = types.str;
+        readOnly = true;
+        description = ''
+          Contains the builder command used to populate an image,
+          honoring all options except the `-c <path-to-default-configuration>`
+          argument.
+          Useful to have for sdImage.populateRootCommands
+        '';
+      };
+
+    };
+  };
+
+  config =
+    let
+      builderArgs =
+        "-g ${toString cfg.configurationLimit} -t ${timeoutStr}"
+        + lib.optionalString (dtCfg.name != null) " -n ${dtCfg.name}"
+        + lib.optionalString (!cfg.useGenerationDeviceTree) " -r";
+      installBootLoader = pkgs.writeScript "install-extlinux-conf.sh" (
+        ''
+          #!${pkgs.runtimeShell}
+          set -e
+        ''
+        + flip concatMapStrings cfg.mirroredBoots (args: ''
+          ${builder} ${builderArgs} -d '${args.path}' -c "$@"
+        '')
+      );
+    in
+    mkIf cfg.enable {
+      system.build.installBootLoader = installBootLoader;
+      system.boot.loader.id = "generic-extlinux-compatible";
+
+      boot.loader.generic-extlinux-compatible.populateCmd = "${populateBuilder} ${builderArgs}";
+
+      assertions = [
+        {
+          assertion = cfg.mirroredBoots != [ ];
+          message = ''
+            You must not remove all elements from option 'boot.loader.generic-extlinux-compatible.mirroredBoots',
+            otherwise the system will not be bootable.
+          '';
+        }
+      ];
+    };
+}

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.nix

@@ -0,0 +1,14 @@
+{ lib, pkgs }:
+
+pkgs.replaceVarsWith {
+  src = ./extlinux-conf-builder.sh;
+  isExecutable = true;
+  replacements = {
+    path = lib.makeBinPath [
+      pkgs.coreutils
+      pkgs.gnused
+      pkgs.gnugrep
+    ];
+    inherit (pkgs) bash;
+  };
+}

nixos/modules/system/boot/loader/generic-extlinux-compatible/extlinux-conf-builder.sh

@@ -0,0 +1,170 @@
+#! @bash@/bin/sh -e
+
+shopt -s nullglob
+
+export PATH=/empty:@path@
+
+usage() {
+    echo "usage: $0 -t <timeout> -c <path-to-default-configuration> [-d <boot-dir>] [-g <num-generations>] [-n <dtbName>] [-r]" >&2
+    exit 1
+}
+
+timeout=                # Timeout in centiseconds
+menu=1                  # Enable menu by default
+default=                # Default configuration
+target=/boot            # Target directory
+numGenerations=0        # Number of other generations to include in the menu
+
+while getopts "t:c:d:g:n:r" opt; do
+    case "$opt" in
+        t) # U-Boot interprets '0' as infinite
+            if [ "$OPTARG" -lt 0 ]; then
+                # When negative (or null coerced to -1), disable timeout which means that we wait forever for input
+                timeout=0
+            elif [ "$OPTARG" = 0 ]; then
+                # When zero, which means disabled in Nix module, disable menu which results in instant boot of the default item
+                # .. timeout is actually ignored by u-Boot but set here for the rest of the script
+                timeout=1
+                menu=0
+            else
+                # Positive results in centi-seconds of timeout, which when passed with no input results in boot of the default item
+                timeout=$((OPTARG * 10))
+            fi
+            ;;
+        c) default="$OPTARG" ;;
+        d) target="$OPTARG" ;;
+        g) numGenerations="$OPTARG" ;;
+        n) dtbName="$OPTARG" ;;
+        r) noDeviceTree=1 ;;
+        \?) usage ;;
+    esac
+done
+
+[ "$timeout" = "" -o "$default" = "" ] && usage
+
+mkdir -p $target/nixos
+mkdir -p $target/extlinux
+
+# Convert a path to a file in the Nix store such as
+# /nix/store/<hash>-<name>/file to <hash>-<name>-<file>.
+cleanName() {
+    local path="$1"
+    echo "$path" | sed 's|^/nix/store/||' | sed 's|/|-|g'
+}
+
+# Copy a file from the Nix store to $target/nixos.
+declare -A filesCopied
+
+copyToKernelsDir() {
+    local src=$(readlink -f "$1")
+    local dst="$target/nixos/$(cleanName $src)"
+    # Don't copy the file if $dst already exists.  This means that we
+    # have to create $dst atomically to prevent partially copied
+    # kernels or initrd if this script is ever interrupted.
+    if ! test -e $dst; then
+        local dstTmp=$dst.tmp.$$
+        cp -r $src $dstTmp
+        mv $dstTmp $dst
+    fi
+    filesCopied[$dst]=1
+    result=$dst
+}
+
+# Copy its kernel, initrd and dtbs to $target/nixos, and echo out an
+# extlinux menu entry
+addEntry() {
+    local path=$(readlink -f "$1")
+    local tag="$2" # Generation number or 'default'
+
+    if ! test -e $path/kernel -a -e $path/initrd; then
+        return
+    fi
+
+    copyToKernelsDir "$path/kernel"; kernel=$result
+    copyToKernelsDir "$path/initrd"; initrd=$result
+    dtbDir=$(readlink -m "$path/dtbs")
+    if [ -e "$dtbDir" ]; then
+        copyToKernelsDir "$dtbDir"; dtbs=$result
+    fi
+
+    timestampEpoch=$(stat -L -c '%Z' $path)
+
+    timestamp=$(date "+%Y-%m-%d %H:%M" -d @$timestampEpoch)
+    nixosLabel="$(cat $path/nixos-version)"
+    extraParams="$(cat $path/kernel-params)"
+
+    echo
+    echo "LABEL nixos-$tag"
+    if [ "$tag" = "default" ]; then
+        echo "  MENU LABEL NixOS - Default"
+    else
+        echo "  MENU LABEL NixOS - Configuration $tag ($timestamp - $nixosLabel)"
+    fi
+    echo "  LINUX ../nixos/$(basename $kernel)"
+    echo "  INITRD ../nixos/$(basename $initrd)"
+    echo "  APPEND init=$path/init $extraParams"
+
+    if [ -n "$noDeviceTree" ]; then
+        return
+    fi
+
+    if [ -d "$dtbDir" ]; then
+        # if a dtbName was specified explicitly, use that, else use FDTDIR
+        if [ -n "$dtbName" ]; then
+            echo "  FDT ../nixos/$(basename $dtbs)/${dtbName}"
+        else
+            echo "  FDTDIR ../nixos/$(basename $dtbs)"
+        fi
+    else
+        if [ -n "$dtbName" ]; then
+            echo "Explicitly requested dtbName $dtbName, but there's no FDTDIR - bailing out." >&2
+            exit 1
+        fi
+    fi
+}
+
+tmpFile="$target/extlinux/extlinux.conf.tmp.$$"
+
+cat > $tmpFile <<EOF
+# Generated file, all changes will be lost on nixos-rebuild!
+
+# Change this to e.g. nixos-42 to temporarily boot to an older configuration.
+DEFAULT nixos-default
+
+TIMEOUT $timeout
+EOF
+
+[ "$menu" == "1" ] \
+  && echo "MENU TITLE ------------------------------------------------------------" >> $tmpFile
+
+addEntry $default default >> $tmpFile
+
+if [ "$numGenerations" -gt 0 ]; then
+    # Add up to $numGenerations generations of the system profile to the menu,
+    # in reverse (most recent to least recent) order.
+    for generation in $(
+            (cd /nix/var/nix/profiles && ls -d system-*-link) \
+            | sed 's/system-\([0-9]\+\)-link/\1/' \
+            | sort -n -r \
+            | head -n $numGenerations); do
+        link=/nix/var/nix/profiles/system-$generation-link
+        addEntry $link "${generation}-default"
+        for specialisation in $(
+            ls /nix/var/nix/profiles/system-$generation-link/specialisation \
+            | sort -n -r); do
+            link=/nix/var/nix/profiles/system-$generation-link/specialisation/$specialisation
+            addEntry $link "${generation}-${specialisation}"
+        done
+    done >> $tmpFile
+fi
+
+mv -f $tmpFile $target/extlinux/extlinux.conf
+
+# Remove obsolete files from $target/nixos.
+for fn in $target/nixos/*; do
+    if ! test "${filesCopied[$fn]}" = 1; then
+        echo "Removing no longer needed boot file: $fn"
+        chmod +w -- "$fn"
+        rm -rf -- "$fn"
+    fi
+done