nixos-rebuild: Add ‘dry-activate’ command

‘nixos-rebuild dry-activate’ builds the new configuration and then
prints what systemd services would be stopped, restarted etc. if the
configuration were actually activated. This could be extended later to
show other activation actions (like uids being deleted).

To prevent confusion, ‘nixos-rebuild dry-run’ has been renamed to
‘nixos-rebuild dry-build’.

3 files changed, 107 insertions, 52 deletions

diff --git a/nixos/doc/manual/man-nixos-rebuild.xml b/nixos/doc/manual/man-nixos-rebuild.xml
index afc159dbd5d7..c529737c3bf3 100644
--- a/nixos/doc/manual/man-nixos-rebuild.xml
+++ b/nixos/doc/manual/man-nixos-rebuild.xml
@@ -1,7 +1,7 @@
 <refentry xmlns="http://docbook.org/ns/docbook"
           xmlns:xlink="http://www.w3.org/1999/xlink"
           xmlns:xi="http://www.w3.org/2001/XInclude">
-  
+
 <refmeta>
   <refentrytitle><command>nixos-rebuild</command></refentrytitle>
   <manvolnum>8</manvolnum>
@@ -22,7 +22,8 @@
       <arg choice='plain'><option>boot</option></arg>
       <arg choice='plain'><option>test</option></arg>
       <arg choice='plain'><option>build</option></arg>
-      <arg choice='plain'><option>dry-run</option></arg>
+      <arg choice='plain'><option>dry-build</option></arg>
+      <arg choice='plain'><option>dry-activate</option></arg>
       <arg choice='plain'><option>build-vm</option></arg>
       <arg choice='plain'><option>build-vm-with-bootloader</option></arg>
     </group>
@@ -114,10 +115,22 @@ $ nix-build /path/to/nixpkgs/nixos -A system
   </varlistentry>
 
   <varlistentry>
-    <term><option>dry-run</option></term>
+    <term><option>dry-build</option></term>
+    <listitem>
+      <para>Show what store paths would be built or downloaded by any
+      of the operations above, but otherwise do nothing.</para>
+    </listitem>
+  </varlistentry>
+
+  <varlistentry>
+    <term><option>dry-activate</option></term>
     <listitem>
-      <para>Simply show what store paths would be built or downloaded
-      by any of the operations above.</para>
+      <para>Build the new configuration, but instead of activating it,
+      show what changes would be performed by the activation (i.e. by
+      <command>nixos-rebuild test</command>). For
+      instance, this command will print which systemd units would be
+      restarted. The list of changes is not guaranteed to be
+      complete.</para>
     </listitem>
   </varlistentry>
 
diff --git a/nixos/modules/installer/tools/nixos-rebuild.sh b/nixos/modules/installer/tools/nixos-rebuild.sh
index 8157f8fc7da5..1d6df8cb3f71 100644
--- a/nixos/modules/installer/tools/nixos-rebuild.sh
+++ b/nixos/modules/installer/tools/nixos-rebuild.sh
@@ -26,7 +26,8 @@ while [ "$#" -gt 0 ]; do
       --help)
         showSyntax
         ;;
-      switch|boot|test|build|dry-run|build-vm|build-vm-with-bootloader)
+      switch|boot|test|build|dry-build|dry-run|dry-activate|build-vm|build-vm-with-bootloader)
+        if [ "$i" = dry-run ]; then i=dry-build; fi
         action="$i"
         ;;
       --install-grub)
@@ -137,7 +138,7 @@ fi
 
 # First build Nix, since NixOS may require a newer version than the
 # current one.
-if [ -n "$rollback" -o "$action" = dry-run ]; then
+if [ -n "$rollback" -o "$action" = dry-build ]; then
     buildNix=
 fi
 
@@ -180,7 +181,7 @@ if [ -n "$canRun" ]; then
 fi
 
 
-if [ "$action" = dry-run ]; then
+if [ "$action" = dry-build ]; then
     extraBuildFlags+=(--dry-run)
 fi
 
@@ -193,7 +194,7 @@ if [ -z "$rollback" ]; then
     if [ "$action" = switch -o "$action" = boot ]; then
         nix-env "${extraBuildFlags[@]}" -p "$profile" -f '<nixpkgs/nixos>' --set -A system
         pathToConfig="$profile"
-    elif [ "$action" = test -o "$action" = build -o "$action" = dry-run ]; then
+    elif [ "$action" = test -o "$action" = build -o "$action" = dry-build -o "$action" = dry-activate ]; then
         nix-build '<nixpkgs/nixos>' -A system -k "${extraBuildFlags[@]}" > /dev/null
         pathToConfig=./result
     elif [ "$action" = build-vm ]; then
@@ -224,7 +225,7 @@ fi
 
 # If we're not just building, then make the new configuration the boot
 # default and/or activate it now.
-if [ "$action" = switch -o "$action" = boot -o "$action" = test ]; then
+if [ "$action" = switch -o "$action" = boot -o "$action" = test -o "$action" = dry-activate ]; then
     if ! $pathToConfig/bin/switch-to-configuration "$action"; then
         echo "warning: error(s) occured while switching to the new configuration" >&2
         exit 1
diff --git a/nixos/modules/system/activation/switch-to-configuration.pl b/nixos/modules/system/activation/switch-to-configuration.pl
index dbe13c022f09..45fb752aaaa6 100644
--- a/nixos/modules/system/activation/switch-to-configuration.pl
+++ b/nixos/modules/system/activation/switch-to-configuration.pl
@@ -9,19 +9,21 @@ use Cwd 'abs_path';
 
 my $out = "@out@";
 
+# To be robust against interruption, record what units need to be started etc.
 my $startListFile = "/run/systemd/start-list";
 my $restartListFile = "/run/systemd/restart-list";
 my $reloadListFile = "/run/systemd/reload-list";
 
 my $action = shift @ARGV;
 
-if (!defined $action || ($action ne "switch" && $action ne "boot" && $action ne "test")) {
+if (!defined $action || ($action ne "switch" && $action ne "boot" && $action ne "test" && $action ne "dry-activate")) {
     print STDERR <<EOF;
 Usage: $0 [switch|boot|test]
 
-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
+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
+dry-activate: show what would be done if this configuration were activated
 EOF
     exit 1;
 }
@@ -56,8 +58,6 @@ EOF
     exit 100;
 }
 
-syslog(LOG_NOTICE, "switching to system configuration $out");
-
 # Ignore SIGHUP so that we're not killed if we're running on (say)
 # virtual console 1 and we restart the "tty1" unit.
 $SIG{PIPE} = "IGNORE";
@@ -116,6 +116,11 @@ sub boolIsTrue {
     return $s eq "yes" || $s eq "true";
 }
 
+sub recordUnit {
+    my ($fn, $unit) = @_;
+    write_file($fn, { append => 1 }, "$unit\n") if $action ne "dry-activate";
+}
+
 # As a fingerprint for determining whether a unit has changed, we use
 # its absolute path. If it has an override file, we append *its*
 # absolute path as well.
@@ -124,9 +129,18 @@ sub fingerprintUnit {
     return abs_path($s) . (-f "${s}.d/overrides.conf" ? " " . abs_path "${s}.d/overrides.conf" : "");
 }
 
-# Stop all services that no longer exist or have changed in the new
-# configuration.
-my (@unitsToStop, @unitsToSkip);
+# Figure out what units need to be stopped, started, restarted or reloaded.
+my (%unitsToStop, %unitsToSkip, %unitsToStart, %unitsToRestart, %unitsToReload);
+
+$unitsToStart{$_} = 1 foreach
+    split('\n', read_file($startListFile, err_mode => 'quiet') // "");
+
+$unitsToRestart{$_} = 1 foreach
+    split('\n', read_file($restartListFile, err_mode => 'quiet') // "");
+
+$unitsToReload{$_} = 1 foreach
+    split '\n', read_file($reloadListFile, err_mode => 'quiet') // "";
+
 my $activePrev = getActiveUnits;
 while (my ($unit, $state) = each %{$activePrev}) {
     my $baseUnit = $unit;
@@ -141,7 +155,7 @@ while (my ($unit, $state) = each %{$activePrev}) {
 
     if (-e $prevUnitFile && ($state->{state} eq "active" || $state->{state} eq "activating")) {
         if (! -e $newUnitFile || abs_path($newUnitFile) eq "/dev/null") {
-            push @unitsToStop, $unit;
+            $unitsToStop{$unit} = 1;
         }
 
         elsif ($unit =~ /\.target$/) {
@@ -155,7 +169,8 @@ while (my ($unit, $state) = each %{$activePrev}) {
             # should not be the case.  Just ignore it.
             if ($unit ne "suspend.target" && $unit ne "hibernate.target" && $unit ne "hybrid-sleep.target") {
                 unless (boolIsTrue($unitInfo->{'RefuseManualStart'} // "no")) {
-                    write_file($startListFile, { append => 1 }, "$unit\n");
+                    $unitsToStart{$unit} = 1;
+                    recordUnit($startListFile, $unit);
                 }
             }
 
@@ -171,7 +186,7 @@ while (my ($unit, $state) = each %{$activePrev}) {
             # (unless there is a PartOf dependency), so this is just a
             # bookkeeping thing to get systemd to do the right thing.
             if (boolIsTrue($unitInfo->{'X-StopOnReconfiguration'} // "no")) {
-                push @unitsToStop, $unit;
+                $unitsToStop{$unit} = 1;
             }
         }
 
@@ -180,16 +195,18 @@ while (my ($unit, $state) = each %{$activePrev}) {
                 # Do nothing.  These cannot be restarted directly.
             } elsif ($unit =~ /\.mount$/) {
                 # Reload the changed mount unit to force a remount.
-                write_file($reloadListFile, { append => 1 }, "$unit\n");
+                $unitsToReload{$unit} = 1;
+                recordUnit($reloadListFile, $unit);
             } elsif ($unit =~ /\.socket$/ || $unit =~ /\.path$/ || $unit =~ /\.slice$/) {
                 # FIXME: do something?
             } else {
                 my $unitInfo = parseUnit($newUnitFile);
                 if (boolIsTrue($unitInfo->{'X-ReloadIfChanged'} // "no")) {
-                    write_file($reloadListFile, { append => 1 }, "$unit\n");
+                    $unitsToReload{$unit} = 1;
+                    recordUnit($reloadListFile, $unit);
                 }
                 elsif (!boolIsTrue($unitInfo->{'X-RestartIfChanged'} // "yes") || boolIsTrue($unitInfo->{'RefuseManualStop'} // "no") ) {
-                    push @unitsToSkip, $unit;
+                    $unitsToSkip{$unit} = 1;
                 } else {
                     # If this unit is socket-activated, then stop the
                     # socket unit(s) as well, and restart the
@@ -202,8 +219,9 @@ while (my ($unit, $state) = each %{$activePrev}) {
                         }
                         foreach my $socket (@sockets) {
                             if (defined $activePrev->{$socket}) {
-                                push @unitsToStop, $socket;
-                                write_file($startListFile, { append => 1 }, "$socket\n");
+                                $unitsToStop{$unit} = 1;
+                                $unitsToStart{$unit} = 1;
+                                recordUnit($startListFile, $socket);
                                 $socketActivated = 1;
                             }
                         }
@@ -213,7 +231,8 @@ while (my ($unit, $state) = each %{$activePrev}) {
 
                         # This unit should be restarted instead of
                         # stopped and started.
-                        write_file($restartListFile, { append => 1 }, "$unit\n");
+                        $unitsToRestart{$unit} = 1;
+                        recordUnit($restartListFile, $unit);
 
                     } else {
 
@@ -222,10 +241,11 @@ while (my ($unit, $state) = each %{$activePrev}) {
                         # We write this to a file to ensure that the
                         # service gets restarted if we're interrupted.
                         if (!$socketActivated) {
-                            write_file($startListFile, { append => 1 }, "$unit\n");
+                            $unitsToStart{$unit} = 1;
+                            recordUnit($startListFile, $unit);
                         }
 
-                        push @unitsToStop, $unit;
+                        $unitsToStop{$unit} = 1;
 
                     }
                 }
@@ -268,14 +288,16 @@ foreach my $mountPoint (keys %$prevFss) {
     my $unit = pathToUnitName($mountPoint) . ".mount";
     if (!defined $new) {
         # Filesystem entry disappeared, so unmount it.
-        push @unitsToStop, $unit;
+        $unitsToStop{$unit} = 1;
     } elsif ($prev->{fsType} ne $new->{fsType} || $prev->{device} ne $new->{device}) {
         # Filesystem type or device changed, so unmount and mount it.
-        write_file($startListFile, { append => 1 }, "$unit\n");
-        push @unitsToStop, $unit;
+        $unitsToStop{$unit} = 1;
+        $unitsToStart{$unit} = 1;
+        recordUnit($startListFile, $unit);
     } elsif ($prev->{options} ne $new->{options}) {
         # Mount options changes, so remount it.
-        write_file($reloadListFile, { append => 1 }, "$unit\n");
+        $unitsToReload{$unit} = 1;
+        recordUnit($reloadListFile, $unit);
     }
 }
 
@@ -294,14 +316,36 @@ foreach my $device (keys %$prevSwaps) {
     # FIXME: update swap options (i.e. its priority).
 }
 
-if (scalar @unitsToStop > 0) {
-    @unitsToStop = unique(@unitsToStop);
-    print STDERR "stopping the following units: ", join(", ", sort(@unitsToStop)), "\n";
-    system("systemctl", "stop", "--", @unitsToStop); # FIXME: ignore errors?
+
+# Should we have systemd re-exec itself?
+my $restartSystemd = abs_path("/proc/1/exe") ne abs_path("@systemd@/lib/systemd/systemd");
+
+
+# Show dry-run actions.
+if ($action eq "dry-activate") {
+    print STDERR "would stop the following units: ", join(", ", sort(keys %unitsToStop)), "\n"
+        if scalar(keys %unitsToStop) > 0;
+    print STDERR "would NOT stop the following changed units: ", join(", ", sort(keys %unitsToSkip)), "\n"
+        if scalar(keys %unitsToSkip) > 0;
+    print STDERR "would restart systemd\n" if $restartSystemd;
+    print STDERR "would restart the following units: ", join(", ", sort(keys %unitsToRestart)), "\n"
+        if scalar(keys %unitsToRestart) > 0;
+    print STDERR "would start the following units: ", join(", ", sort(keys %unitsToStart)), "\n";
+    print STDERR "would reload the following units: ", join(", ", sort(keys %unitsToReload)), "\n"
+        if scalar(keys %unitsToReload) > 0;
+    exit 0;
+}
+
+
+syslog(LOG_NOTICE, "switching to system configuration $out");
+
+if (scalar (keys %unitsToStop) > 0) {
+    print STDERR "stopping the following units: ", join(", ", sort(keys %unitsToStop)), "\n";
+    system("systemctl", "stop", "--", sort(keys %unitsToStop)); # FIXME: ignore errors?
 }
 
-print STDERR "NOT restarting the following units: ", join(", ", sort(@unitsToSkip)), "\n"
-    if scalar @unitsToSkip > 0;
+print STDERR "NOT restarting the following changed units: ", join(", ", sort(keys %unitsToSkip)), "\n"
+    if scalar(keys %unitsToSkip) > 0;
 
 # Activate the new configuration (i.e., update /etc, make accounts,
 # and so on).
@@ -310,7 +354,7 @@ print STDERR "activating the configuration...\n";
 system("$out/activate", "$out") == 0 or $res = 2;
 
 # Restart systemd if necessary.
-if (abs_path("/proc/1/exe") ne abs_path("@systemd@/lib/systemd/systemd")) {
+if ($restartSystemd) {
     print STDERR "restarting systemd...\n";
     system("@systemd@/bin/systemctl", "daemon-reexec") == 0 or $res = 2;
 }
@@ -327,10 +371,9 @@ system("@systemd@/bin/systemctl", "reload-or-restart", "dbus.service");
 
 # Restart changed services (those that have to be restarted rather
 # than stopped and started).
-my @restart = unique(split('\n', read_file($restartListFile, err_mode => 'quiet') // ""));
-if (scalar @restart > 0) {
-    print STDERR "restarting the following units: ", join(", ", sort(@restart)), "\n";
-    system("@systemd@/bin/systemctl", "restart", "--", @restart) == 0 or $res = 4;
+if (scalar(keys %unitsToRestart) > 0) {
+    print STDERR "restarting the following units: ", join(", ", sort(keys %unitsToRestart)), "\n";
+    system("@systemd@/bin/systemctl", "restart", "--", sort(keys %unitsToRestart)) == 0 or $res = 4;
     unlink($restartListFile);
 }
 
@@ -340,17 +383,15 @@ if (scalar @restart > 0) {
 # that are symlinks to other units.  We shouldn't start both at the
 # same time because we'll get a "Failed to add path to set" error from
 # systemd.
-my @start = unique("default.target", "timers.target", "sockets.target", split('\n', read_file($startListFile, err_mode => 'quiet') // ""));
-print STDERR "starting the following units: ", join(", ", sort(@start)), "\n";
-system("@systemd@/bin/systemctl", "start", "--", @start) == 0 or $res = 4;
+print STDERR "starting the following units: ", join(", ", sort(keys %unitsToStart)), "\n";
+system("@systemd@/bin/systemctl", "start", "--", sort(keys %unitsToStart)) == 0 or $res = 4;
 unlink($startListFile);
 
 # Reload units that need it.  This includes remounting changed mount
 # units.
-my @reload = unique(split '\n', read_file($reloadListFile, err_mode => 'quiet') // "");
-if (scalar @reload > 0) {
-    print STDERR "reloading the following units: ", join(", ", sort(@reload)), "\n";
-    system("@systemd@/bin/systemctl", "reload", "--", @reload) == 0 or $res = 4;
+if (scalar(keys %unitsToReload) > 0) {
+    print STDERR "reloading the following units: ", join(", ", sort(keys %unitsToReload)), "\n";
+    system("@systemd@/bin/systemctl", "reload", "--", sort(keys %unitsToReload)) == 0 or $res = 4;
     unlink($reloadListFile);
 }