nixos/activation: Add pre-switch checks

Add an option for shell script fragments that are ran before switching
to a new NixOS system configuration (pre installation of bootloader or
system activation). Also add a new subcommand for
switch-to-configuration called "check" that will cause the program to
always exit after checks are ran.

nixos/modules/module-list.nix

@@ -1612,6 +1612,7 @@
   ./services/x11/xserver.nix
   ./system/activation/activatable-system.nix
   ./system/activation/activation-script.nix
+  ./system/activation/pre-switch-check.nix
   ./system/activation/specialisation.nix
   ./system/activation/switchable-system.nix
   ./system/activation/bootspec.nix

nixos/modules/system/activation/pre-switch-check.nix

@@ -0,0 +1,44 @@
+{ lib, pkgs, ... }:
+let
+  preSwitchCheckScript =
+    set:
+    lib.concatLines (
+      lib.mapAttrsToList (name: text: ''
+        # pre-switch check ${name}
+        (
+          ${text}
+        )
+        if [[ $? != 0 ]]; then
+          echo "Pre-switch check '${name}' failed"
+          exit 1
+        fi
+      '') set
+    );
+in
+{
+  options.system.preSwitchChecks = lib.mkOption {
+    default = { };
+    example = lib.literalExpression ''
+      { failsEveryTime =
+        '''
+          false
+        ''';
+      }
+    '';
+
+    description = ''
+      A set of shell script fragments that are executed before the switch to a
+      new NixOS system configuration. A failure in any of these fragments will
+      cause the switch to fail and exit early.
+    '';
+
+    type = lib.types.attrsOf lib.types.str;
+
+    apply =
+      set:
+      set
+      // {
+        script = pkgs.writeShellScript "pre-switch-checks" (preSwitchCheckScript set);
+      };
+  };
+}

nixos/modules/system/activation/switch-to-configuration.pl

@@ -78,10 +78,11 @@ if ("@localeArchive@" ne "") {
     $ENV{LOCALE_ARCHIVE} = "@localeArchive@";
 }
 
-if (!defined($action) || ($action ne "switch" && $action ne "boot" && $action ne "test" && $action ne "dry-activate")) {
+if (!defined($action) || ($action ne "switch" && $action ne "boot" && $action ne "test" && $action ne "dry-activate" && $action ne "check")) {
     print STDERR <<"EOF";
-Usage: $0 [switch|boot|test|dry-activate]
+Usage: $0 [check|switch|boot|test|dry-activate]
 
+check:        run pre-switch checks and exit
 switch:       make the configuration the boot default and activate now
 boot:         make the configuration the boot default
 test:         activate the configuration, but don\'t make it the boot default
@@ -101,6 +102,17 @@ open(my $stc_lock, '>>', '/run/nixos/switch-to-configuration.lock') or die "Coul
 flock($stc_lock, LOCK_EX) or die "Could not acquire lock - $!";
 openlog("nixos", "", LOG_USER);
 
+# run pre-switch checks
+if (($ENV{"NIXOS_NO_CHECK"} // "") ne "1") {
+    chomp(my $pre_switch_checks = <<'EOFCHECKS');
+@preSwitchCheck@
+EOFCHECKS
+    system("$pre_switch_checks $out") == 0 or exit 1;
+    if ($action eq "check") {
+        exit 0;
+    }
+}
+
 # Install or update the bootloader.
 if ($action eq "switch" || $action eq "boot") {
     chomp(my $install_boot_loader = <<'EOFBOOTLOADER');

nixos/modules/system/activation/switchable-system.nix

@@ -61,6 +61,7 @@ in
           --subst-var-by coreutils "${pkgs.coreutils}" \
           --subst-var-by distroId ${lib.escapeShellArg config.system.nixos.distroId} \
           --subst-var-by installBootLoader ${lib.escapeShellArg config.system.build.installBootLoader} \
+          --subst-var-by preSwitchCheck ${lib.escapeShellArg config.system.preSwitchChecks.script} \
           --subst-var-by localeArchive "${config.i18n.glibcLocales}/lib/locale/locale-archive" \
           --subst-var-by perl "${perlWrapped}" \
           --subst-var-by shell "${pkgs.bash}/bin/sh" \
@@ -93,6 +94,7 @@ in
             --set TOPLEVEL ''${!toplevelVar} \
             --set DISTRO_ID ${lib.escapeShellArg config.system.nixos.distroId} \
             --set INSTALL_BOOTLOADER ${lib.escapeShellArg config.system.build.installBootLoader} \
+            --set PRE_SWITCH_CHECK ${lib.escapeShellArg config.system.preSwitchChecks.script} \
             --set LOCALE_ARCHIVE ${config.i18n.glibcLocales}/lib/locale/locale-archive \
             --set SYSTEMD ${config.systemd.package}
         )

nixos/modules/system/activation/top-level.nix

@@ -342,6 +342,7 @@ in
       perl = pkgs.perl.withPackages (p: with p; [ ConfigIniFiles FileSlurp ]);
       # End if legacy environment variables
 
+      preSwitchCheck = config.system.preSwitchChecks.script;
 
       # Not actually used in the builder. `passedChecks` is just here to create
       # the build dependencies. Checks are similar to build dependencies in the

nixos/tests/switch-test.nix

@@ -612,6 +612,10 @@ in {
     other = {
       system.switch.enable = true;
       users.mutableUsers = true;
+      specialisation.failingCheck.configuration.system.preSwitchChecks.failEveryTime = ''
+        echo this will fail
+        false
+      '';
     };
   };
 
@@ -684,6 +688,11 @@ in {
 
     boot_loader_text = "Warning: do not know how to make this configuration bootable; please enable a boot loader."
 
+    with subtest("pre-switch checks"):
+        machine.succeed("${stderrRunner} ${otherSystem}/bin/switch-to-configuration check")
+        out = switch_to_specialisation("${otherSystem}", "failingCheck", action="check", fail=True)
+        assert_contains(out, "this will fail")
+
     with subtest("actions"):
         # boot action
         out = switch_to_specialisation("${machine}", "simpleService", action="boot")

pkgs/by-name/sw/switch-to-configuration-ng/src/src/main.rs

@@ -79,6 +79,7 @@ const DRY_RELOAD_BY_ACTIVATION_LIST_FILE: &str = "/run/nixos/dry-activation-relo
 #[derive(Debug, Clone, PartialEq)]
 enum Action {
     Switch,
+    Check,
     Boot,
     Test,
     DryActivate,
@@ -93,6 +94,7 @@ impl std::str::FromStr for Action {
             "boot" => Self::Boot,
             "test" => Self::Test,
             "dry-activate" => Self::DryActivate,
+            "check" => Self::Check,
             _ => bail!("invalid action {s}"),
         })
     }
@@ -105,6 +107,7 @@ impl Into<&'static str> for &Action {
             Action::Boot => "boot",
             Action::Test => "test",
             Action::DryActivate => "dry-activate",
+            Action::Check => "check",
         }
     }
 }
@@ -129,6 +132,28 @@ fn parse_os_release() -> Result<HashMap<String, String>> {
         }))
 }
 
+fn do_pre_switch_check(command: &str, toplevel: &Path) -> Result<()> {
+    let mut cmd_split = command.split_whitespace();
+    let Some(argv0) = cmd_split.next() else {
+        bail!("missing first argument in install bootloader commands");
+    };
+
+    match std::process::Command::new(argv0)
+        .args(cmd_split.collect::<Vec<&str>>())
+        .arg(toplevel)
+        .spawn()
+        .map(|mut child| child.wait())
+    {
+        Ok(Ok(status)) if status.success() => {}
+        _ => {
+            eprintln!("Pre-switch checks failed");
+            die()
+        }
+    }
+
+    Ok(())
+}
+
 fn do_install_bootloader(command: &str, toplevel: &Path) -> Result<()> {
     let mut cmd_split = command.split_whitespace();
     let Some(argv0) = cmd_split.next() else {
@@ -939,7 +964,8 @@ fn do_user_switch(parent_exe: String) -> anyhow::Result<()> {
 
 fn usage(argv0: &str) -> ! {
     eprintln!(
-        r#"Usage: {} [switch|boot|test|dry-activate]
+        r#"Usage: {} [check|switch|boot|test|dry-activate]
+check:        run pre-switch checks and exit
 switch:       make the configuration the boot default and activate now
 boot:         make the configuration the boot default
 test:         activate the configuration, but don't make it the boot default
@@ -955,6 +981,7 @@ fn do_system_switch(action: Action) -> anyhow::Result<()> {
     let out = PathBuf::from(required_env("OUT")?);
     let toplevel = PathBuf::from(required_env("TOPLEVEL")?);
     let distro_id = required_env("DISTRO_ID")?;
+    let pre_switch_check = required_env("PRE_SWITCH_CHECK")?;
     let install_bootloader = required_env("INSTALL_BOOTLOADER")?;
     let locale_archive = required_env("LOCALE_ARCHIVE")?;
     let new_systemd = PathBuf::from(required_env("SYSTEMD")?);
@@ -1013,6 +1040,18 @@ fn do_system_switch(action: Action) -> anyhow::Result<()> {
         bail!("Failed to initialize logger");
     }
 
+    if std::env::var("NIXOS_NO_CHECK")
+        .as_deref()
+        .unwrap_or_default()
+        != "1"
+    {
+        do_pre_switch_check(&pre_switch_check, &toplevel)?;
+    }
+
+    if *action == Action::Check {
+        return Ok(());
+    }
+
     // Install or update the bootloader.
     if matches!(action, Action::Switch | Action::Boot) {
         do_install_bootloader(&install_bootloader, &toplevel)?;