systemctl: warn if trying to disable a unit with no install info

Trying to disable a unit with no install info is mostly useless, so
adding a warning like we do for enable (with the new dbus method
'DisableUnitFilesWithFlagsAndInstallInfo()'). Note that it would
still find and remove symlinks to the unit in /etc, regardless of
whether it has install info or not, just like before. And if there are
actually files to remove, we suppress the warning.

Fixes #17689

man/org.freedesktop.systemd1.xml

@@ -209,6 +209,10 @@ node /org/freedesktop/systemd1 {
       DisableUnitFilesWithFlags(in  as files,
                                 in  t flags,
                                 out a(sss) changes);
+      DisableUnitFilesWithFlagsAndInstallInfo(in  as files,
+                                              in  t flags,
+                                              out b carries_install_info,
+                                              out a(sss) changes);
       ReenableUnitFiles(in  as files,
                         in  b runtime,
                         in  b force,
@@ -916,6 +920,8 @@ node /org/freedesktop/systemd1 {
 
     <variablelist class="dbus-method" generated="True" extra-ref="DisableUnitFilesWithFlags()"/>
 
+    <variablelist class="dbus-method" generated="True" extra-ref="DisableUnitFilesWithFlagsAndInstallInfo()"/>
+
     <variablelist class="dbus-method" generated="True" extra-ref="ReenableUnitFiles()"/>
 
     <variablelist class="dbus-method" generated="True" extra-ref="LinkUnitFiles()"/>
@@ -1417,7 +1423,7 @@ node /org/freedesktop/systemd1 {
       enabled for runtime only (true, <filename>/run/</filename>), or persistently (false,
       <filename>/etc/</filename>). The second one controls whether symlinks pointing to other units shall be
       replaced if necessary. This method returns one boolean and an array of the changes made. The boolean
-      signals whether the unit files contained any enablement information (i.e. an [Install]) section. The
+      signals whether the unit files contained any enablement information (i.e. an [Install] section). The
       changes array consists of structures with three strings: the type of the change (one of
       <literal>symlink</literal> or <literal>unlink</literal>), the file name of the symlink and the
       destination of the symlink. Note that most of the following calls return a changes list in the same
@@ -1440,6 +1446,11 @@ node /org/freedesktop/systemd1 {
       replaced if necessary. <varname>SD_SYSTEMD_UNIT_PORTABLE</varname> will add or remove the symlinks in
       <filename>/etc/systemd/system.attached</filename> and <filename>/run/systemd/system.attached</filename>.</para>
 
+      <para><function>DisableUnitFilesWithFlagsAndInstallInfo()</function> is similar to
+      <function>DisableUnitFilesWithFlags()</function> and takes the same arguments, but returns
+      a boolean to indicate whether the unit files contain any enablement information, like
+      <function>EnableUnitFiles()</function>. The changes made are still returned in an array.</para>
+
       <para>Similarly, <function>ReenableUnitFiles()</function> applies the changes to one or more units that
       would result from disabling and enabling the unit quickly one after the other in an atomic
       fashion. This is useful to apply updated [Install] information contained in unit files.</para>

src/core/dbus-manager.c

@@ -2425,6 +2425,7 @@ static int method_disable_unit_files_generic(
                 sd_bus_message *message,
                 Manager *m,
                 int (*call)(LookupScope scope, UnitFileFlags flags, const char *root_dir, char *files[], InstallChange **changes, size_t *n_changes),
+                bool carries_install_info,
                 sd_bus_error *error) {
 
         _cleanup_strv_free_ char **l = NULL;
@@ -2440,7 +2441,8 @@ static int method_disable_unit_files_generic(
         if (r < 0)
                 return r;
 
-        if (sd_bus_message_is_method_call(message, NULL, "DisableUnitFilesWithFlags")) {
+        if (sd_bus_message_is_method_call(message, NULL, "DisableUnitFilesWithFlags") ||
+            sd_bus_message_is_method_call(message, NULL, "DisableUnitFilesWithFlagsAndInstallInfo")) {
                 uint64_t raw_flags;
 
                 r = sd_bus_message_read(message, "t", &raw_flags);
@@ -2469,19 +2471,23 @@ static int method_disable_unit_files_generic(
         if (r < 0)
                 return install_error(error, r, changes, n_changes);
 
-        return reply_install_changes_and_free(m, message, -1, changes, n_changes, error);
+        return reply_install_changes_and_free(m, message, carries_install_info ? r : -1, changes, n_changes, error);
 }
 
 static int method_disable_unit_files_with_flags(sd_bus_message *message, void *userdata, sd_bus_error *error) {
-        return method_disable_unit_files_generic(message, userdata, unit_file_disable, error);
+        return method_disable_unit_files_generic(message, userdata, unit_file_disable, /* carries_install_info = */ false, error);
+}
+
+static int method_disable_unit_files_with_flags_and_install_info(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        return method_disable_unit_files_generic(message, userdata, unit_file_disable, /* carries_install_info = */ true, error);
 }
 
 static int method_disable_unit_files(sd_bus_message *message, void *userdata, sd_bus_error *error) {
-        return method_disable_unit_files_generic(message, userdata, unit_file_disable, error);
+        return method_disable_unit_files_generic(message, userdata, unit_file_disable, /* carries_install_info = */ false, error);
 }
 
 static int method_unmask_unit_files(sd_bus_message *message, void *userdata, sd_bus_error *error) {
-        return method_disable_unit_files_generic(message, userdata, unit_file_unmask, error);
+        return method_disable_unit_files_generic(message, userdata, unit_file_unmask, /* carries_install_info = */ false, error);
 }
 
 static int method_revert_unit_files(sd_bus_message *message, void *userdata, sd_bus_error *error) {
@@ -3194,6 +3200,11 @@ const sd_bus_vtable bus_manager_vtable[] = {
                                 SD_BUS_RESULT("a(sss)", changes),
                                 method_disable_unit_files_with_flags,
                                 SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD_WITH_ARGS("DisableUnitFilesWithFlagsAndInstallInfo",
+                                SD_BUS_ARGS("as", files, "t", flags),
+                                SD_BUS_RESULT("b", carries_install_info, "a(sss)", changes),
+                                method_disable_unit_files_with_flags_and_install_info,
+                                SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD_WITH_ARGS("ReenableUnitFiles",
                                 SD_BUS_ARGS("as", files, "b", runtime, "b", force),
                                 SD_BUS_RESULT("b", carries_install_info, "a(sss)", changes),

src/shared/install.c

@@ -2790,25 +2790,38 @@ static int do_unit_file_disable(
 
         _cleanup_(install_context_done) InstallContext ctx = { .scope = scope };
         _cleanup_set_free_free_ Set *remove_symlinks_to = NULL;
+        InstallInfo *info;
+        bool has_install_info = false;
         int r;
 
         STRV_FOREACH(name, names) {
                 if (!unit_name_is_valid(*name, UNIT_NAME_ANY))
                         return install_changes_add(changes, n_changes, -EUCLEAN, *name, NULL);
 
-                r = install_info_add(&ctx, *name, NULL, lp->root_dir, /* auxiliary= */ false, NULL);
+                r = install_info_add(&ctx, *name, NULL, lp->root_dir, /* auxiliary= */ false, &info);
+                if (r >= 0)
+                        r = install_info_traverse(&ctx, lp, info, SEARCH_LOAD|SEARCH_FOLLOW_CONFIG_SYMLINKS, NULL);
+
                 if (r < 0)
-                        return r;
+                        return install_changes_add(changes, n_changes, r, *name, NULL);
+
+                /* If we enable multiple units, some with install info and others without,
+                 * the "empty [Install] section" warning is not shown. Let's make the behavior
+                 * of disable align with that. */
+                has_install_info = has_install_info || install_info_has_rules(info) || install_info_has_also(info);
         }
 
         r = install_context_mark_for_removal(&ctx, lp, &remove_symlinks_to, config_path, changes, n_changes);
+        if (r >= 0)
+                r = remove_marked_symlinks(remove_symlinks_to, config_path, lp, flags & UNIT_FILE_DRY_RUN, changes, n_changes);
+
         if (r < 0)
                 return r;
 
-        return remove_marked_symlinks(remove_symlinks_to, config_path, lp, flags & UNIT_FILE_DRY_RUN, changes, n_changes);
+        /* The warning is shown only if it's a no-op */
+        return install_changes_have_modification(*changes, *n_changes) || has_install_info;
 }
 
-
 int unit_file_disable(
                 LookupScope scope,
                 UnitFileFlags flags,

src/systemctl/systemctl-enable.c

@@ -109,9 +109,10 @@ int verb_enable(int argc, char *argv[], void *userdata) {
                 if (streq(verb, "enable")) {
                         r = unit_file_enable(arg_scope, flags, arg_root, names, &changes, &n_changes);
                         carries_install_info = r;
-                } else if (streq(verb, "disable"))
+                } else if (streq(verb, "disable")) {
                         r = unit_file_disable(arg_scope, flags, arg_root, names, &changes, &n_changes);
-                else if (streq(verb, "reenable")) {
+                        carries_install_info = r;
+                } else if (streq(verb, "reenable")) {
                         r = unit_file_reenable(arg_scope, flags, arg_root, names, &changes, &n_changes);
                         carries_install_info = r;
                 } else if (streq(verb, "link"))
@@ -165,7 +166,8 @@ int verb_enable(int argc, char *argv[], void *userdata) {
                         method = "EnableUnitFiles";
                         expect_carries_install_info = true;
                 } else if (streq(verb, "disable")) {
-                        method = "DisableUnitFiles";
+                        method = "DisableUnitFilesWithFlagsAndInstallInfo";
+                        expect_carries_install_info = true;
                         send_force = false;
                 } else if (streq(verb, "reenable")) {
                         method = "ReenableUnitFiles";
@@ -208,7 +210,10 @@ int verb_enable(int argc, char *argv[], void *userdata) {
                 }
 
                 if (send_runtime) {
-                        r = sd_bus_message_append(m, "b", arg_runtime);
+                        if (streq(method, "DisableUnitFilesWithFlagsAndInstallInfo"))
+                                r = sd_bus_message_append(m, "t", arg_runtime ? UNIT_FILE_RUNTIME : 0);
+                        else
+                                r = sd_bus_message_append(m, "b", arg_runtime);
                         if (r < 0)
                                 return bus_log_create_error(r);
                 }
@@ -245,7 +250,7 @@ int verb_enable(int argc, char *argv[], void *userdata) {
         if (carries_install_info == 0 && !ignore_carries_install_info)
                 log_notice("The unit files have no installation config (WantedBy=, RequiredBy=, Also=,\n"
                            "Alias= settings in the [Install] section, and DefaultInstance= for template\n"
-                           "units). This means they are not meant to be enabled using systemctl.\n"
+                           "units). This means they are not meant to be enabled or disabled using systemctl.\n"
                            " \n" /* trick: the space is needed so that the line does not get stripped from output */
                            "Possible reasons for having this kind of units are:\n"
                            "%1$s A unit may be statically enabled by being symlinked from another unit's\n"