nixos-rebuild: Add list-generations

Add new command `nixos-rebuild list-generations`. It will show an output
like

```
$ nixos-rebuild list-generations
Generation Build-date NixOS version Kernel Configuration Revision Specialisations
52 (current) Fri 2023-08-18 08:17:27 23.11.20230817.0f46300 6.4.10 448160aeccf6a7184bd8a84290d527819f1c552c *
51 Mon 2023-08-07 17:56:41 23.11.20230807.31b1eed 6.4.8 99ef480007ca51e3d440aa4fa6558178d63f9c42 *
```

This also mentions the change in the upcoming release notes

nixos/doc/manual/release-notes/rl-2311.section.md

···
 
 - LXD now supports virtual machine instances to complement the existing container support
 
+- The `nixos-rebuild` command has been given a `list-generations` subcommand. See `man nixos-rebuild` for more details.
+
 ## New Services {#sec-release-23.11-new-services}
 
 - [MCHPRS](https://github.com/MCHPR/MCHPRS), a multithreaded Minecraft server built for redstone. Available as [services.mchprs](#opt-services.mchprs.enable).

pkgs/os-specific/linux/nixos-rebuild/default.nix

···
 , coreutils
 , gnused
 , gnugrep
+, jq
+, util-linux
 , nix
 , lib
 , nixosTests
···
   nix_x86_64_linux = fallback.x86_64-linux;
   nix_i686_linux = fallback.i686-linux;
   nix_aarch64_linux = fallback.aarch64-linux;
-  path = lib.makeBinPath [ coreutils gnused gnugrep ];
+  path = lib.makeBinPath [ coreutils gnused gnugrep jq util-linux ];
   nativeBuildInputs = [
     installShellFiles
   ];

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.8

···
 .Sh SYNOPSIS
 .Nm
 .Bro
-.Cm switch | boot | test | build | dry-build | dry-activate | edit | build-vm | build-vm-with-bootloader
+.Cm switch | boot | test | build | dry-build | dry-activate | edit | build-vm | build-vm-with-bootloader | list-generations Op Fl -json
 .Brc
 .br
 .Op Fl -upgrade | -upgrade-all
···
 containing a
 .Pa /boot
 partition.
+.
+.It Cm list-generations Op Fl -json
+List the available generations in a similar manner to the boot loader
+menu. It shows the generation number, build date and time, NixOS version,
+kernel version and the configuration revision. This is useful to get
+information e.g. for which generation to roll back to with
+.Ic nixos-rebuild switch Fl -generation Ar N
+There is also a json version of output available.
 .El
 .
 .

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

···
 noFlake=
 # comma separated list of vars to preserve when using sudo
 preservedSudoVars=NIXOS_INSTALL_BOOTLOADER
+json=
 
 # log the given argument to stderr
 log() {
···
       --help)
         showSyntax
         ;;
-      switch|boot|test|build|edit|dry-build|dry-run|dry-activate|build-vm|build-vm-with-bootloader)
+      switch|boot|test|build|edit|dry-build|dry-run|dry-activate|build-vm|build-vm-with-bootloader|list-generations)
         if [ "$i" = dry-run ]; then i=dry-build; fi
         # exactly one action mandatory, bail out if multiple are given
         if [ -n "$action" ]; then showSyntax; fi
···
         j="$1"; shift 1
         k="$1"; shift 1
         lockFlags+=("$i" "$j" "$k")
+        ;;
+      --json)
+        json=1
         ;;
       *)
         log "$0: unknown option \`$i'"
···
 
 if [ "$action" = dry-build ]; then
     extraBuildFlags+=(--dry-run)
+fi
+
+if [ "$action" = list-generations ]; then
+    if [ ! -L "$profile" ]; then
+        log "No profile \`$(basename "$profile")' found"
+        exit 1
+    fi
+
+    generation_from_dir() {
+        generation_dir="$1"
+        generation_base="$(basename "$generation_dir")" # Has the format "system-123-link" for generation 123
+        no_link_gen="${generation_base%-link}"  # remove the "-link"
+        echo "${no_link_gen##*-}" # remove everything before the last dash
+    }
+    describe_generation(){
+        generation_dir="$1"
+        generation_number="$(generation_from_dir "$generation_dir")"
+        nixos_version="$(cat "$generation_dir/nixos-version" 2> /dev/null || echo "Unknown")"
+
+        kernel_dir="$(dirname "$(realpath "$generation_dir/kernel")")"
+        kernel_version="$(ls "$kernel_dir/lib/modules" || echo "Unknown")"
+
+        configurationRevision="$("$generation_dir/sw/bin/nixos-version" --configuration-revision 2> /dev/null || true)"
+
+        # Old nixos-version output ignored unknown flags and just printed the version
+        # therefore the following workaround is done not to show the default output
+        nixos_version_default="$("$generation_dir/sw/bin/nixos-version")"
+        if [ "$configurationRevision" == "$nixos_version_default" ]; then
+             configurationRevision=""
+        fi
+
+        # jq automatically quotes the output => don't try to quote it in output!
+        build_date="$(stat "$generation_dir" --format=%W | jq 'todate')"
+
+        pushd "$generation_dir/specialisation/" > /dev/null || :
+        specialisation_list=(*)
+        popd > /dev/null || :
+
+        specialisations="$(jq --compact-output --null-input '$ARGS.positional' --args -- "${specialisation_list[@]}")"
+
+        if [ "$(basename "$generation_dir")" = "$(readlink "$profile")" ]; then
+            current_generation_tag="true"
+        else
+            current_generation_tag="false"
+        fi
+
+        # Escape userdefined strings
+        nixos_version="$(jq -aR <<< "$nixos_version")"
+        kernel_version="$(jq -aR <<< "$kernel_version")"
+        configurationRevision="$(jq -aR <<< "$configurationRevision")"
+        cat << EOF
+{
+  "generation": $generation_number,
+  "date": $build_date,
+  "nixosVersion": $nixos_version,
+  "kernelVersion": $kernel_version,
+  "configurationRevision": $configurationRevision,
+  "specialisations": $specialisations,
+  "current": $current_generation_tag
+}
+EOF
+    }
+
+    find "$(dirname "$profile")" -regex "$profile-[0-9]+-link" |
+        sort -Vr |
+        while read -r generation_dir; do
+            describe_generation "$generation_dir"
+        done |
+        if [ -z "$json" ]; then
+            jq --slurp -r '.[] | [
+                    ([.generation, (if .current == true then "current" else "" end)] | join(" ")),
+                    (.date | fromdate | strflocaltime("%Y-%m-%d %H:%M:%S")),
+                    .nixosVersion, .kernelVersion, .configurationRevision,
+                    (.specialisations | join(" "))
+                ] | @tsv' |
+                column --separator $'\t' --table --table-columns "Generation,Build-date,NixOS version,Kernel,Configuration Revision,Specialisation" |
+                ${PAGER:cat}
+        else
+            jq --slurp .
+        fi
+    exit 0
 fi