shared/dropin: support -.service.d/ top level drop-in for service units

Closes #12830

man/systemd.service.xml

@@ -63,6 +63,19 @@
     <para>The <citerefentry><refentrytitle>systemd-run</refentrytitle><manvolnum>1</manvolnum></citerefentry>
     command allows creating <filename>.service</filename> and <filename>.scope</filename> units dynamically
     and transiently from the command line.</para>
+
+    <para>In addition to the various drop-in behaviors described in
+    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+    services also support a top-level drop-in with <filename>-.service.d/</filename> that allows
+    altering or adding to the settings of all services on the system.
+    The formatting and precedence of applying drop-in configurations follow what is defined in
+    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+    However, configurations in <filename>-.service.d/</filename> have the lowest precedence compared to settings
+    in the service specific override directories. For example, for <filename>foo-bar-baz.service</filename>,
+    drop-ins in <filename>foo-bar-baz.service.d/</filename> override the ones in
+    <filename>foo-bar-.service.d/</filename>, which override the ones <filename>foo-.service.d/</filename>,
+    which override the ones in <filename>-.service.d/</filename>.
+    </para>
   </refsect1>
 
   <refsect1>

man/systemd.special.xml

@@ -119,6 +119,15 @@
           </listitem>
         </varlistentry>
 
+        <varlistentry>
+          <term><filename>-.service</filename></term>
+          <listitem>
+            <para>This is a reserved unit name used to support top-level drop-ins for services. See
+            <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+            for details.</para>
+          </listitem>
+        </varlistentry>
+
         <varlistentry>
           <term><filename>basic.target</filename></term>
           <listitem>

man/systemd.unit.xml

@@ -192,6 +192,10 @@
     over unit files wherever located. Multiple drop-in files with different names are applied in
     lexicographic order, regardless of which of the directories they reside in.</para>
 
+    <para>Service units also support a top-level drop-in directory for modifying the settings of all service units. See
+    <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for details.</para>
+
     <!-- Note that we do not document .include here, as we consider it mostly obsolete, and want
          people to use .d/ drop-ins instead. -->
 

src/basic/unit-name.c

@@ -678,8 +678,13 @@ bool service_unit_name_is_valid(const char *name) {
 
         /* If it's a template or instance, get the prefix as a service name. */
         if (unit_name_is_valid(name, UNIT_NAME_INSTANCE|UNIT_NAME_TEMPLATE)) {
-                assert_se(unit_name_to_prefix(name, &prefix) == 0);
-                assert_se(s = strjoin(prefix, ".service"));
+                if (unit_name_to_prefix(name, &prefix) < 0)
+                        return false;
+
+                s = strjoin(prefix, ".service");
+                if (!s)
+                        return false;
+
                 service_name = s;
         }
 

src/core/service.c

@@ -554,7 +554,7 @@ static int service_verify(Service *s) {
 
         if (!service_unit_name_is_valid(UNIT(s)->id)) {
                 log_unit_error(UNIT(s), "Service name is invalid or reserved. Refusing.");
-                return -ENOEXEC;
+                return -EINVAL;
         }
 
         if (!s->exec_command[SERVICE_EXEC_START] && !s->exec_command[SERVICE_EXEC_STOP]

src/shared/dropin.c

@@ -19,6 +19,7 @@
 #include "mkdir.h"
 #include "path-util.h"
 #include "set.h"
+#include "special.h"
 #include "string-util.h"
 #include "strv.h"
 #include "unit-name.h"
@@ -226,12 +227,34 @@ int unit_file_find_dropin_paths(
                 char ***ret) {
 
         _cleanup_strv_free_ char **dirs = NULL;
+        UnitType type = _UNIT_TYPE_INVALID;
         char *name, **p;
         Iterator i;
         int r;
 
         assert(ret);
 
+        /* All the names in the unit are of the same type so just grab one. */
+        name = (char*) set_first(names);
+        if (name) {
+                type = unit_name_to_type(name);
+                if (type < 0)
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                               "Failed to to derive unit type from unit name: %s",
+                                               name);
+        }
+
+        /* Special drop in for -.service. Add this first as it's the most generic
+         * and should be able to be overridden by more specific drop-ins. */
+        if (type == UNIT_SERVICE)
+                STRV_FOREACH(p, lookup_path)
+                        (void) unit_file_find_dirs(original_root,
+                                                   unit_path_cache,
+                                                   *p,
+                                                   SPECIAL_ROOT_SERVICE,
+                                                   dir_suffix,
+                                                   &dirs);
+
         SET_FOREACH(name, names, i)
                 STRV_FOREACH(p, lookup_path)
                         (void) unit_file_find_dirs(original_root, unit_path_cache, *p, name, dir_suffix, &dirs);

test/TEST-15-DROPIN/test-dropin.sh

@@ -101,6 +101,19 @@ test_basic_dropins () {
     check_ok b Wants c.service
     systemctl stop a c
 
+    echo "*** test -.service.d/ top level drop-in"
+    create_services a b
+    check_ko a ExecCondition "/bin/echo a"
+    check_ko b ExecCondition "/bin/echo b"
+    mkdir -p /usr/lib/systemd/system/-.service.d
+    cat >/usr/lib/systemd/system/-.service.d/override.conf <<EOF
+[Service]
+ExecCondition=/bin/echo %n
+EOF
+    check_ok a ExecCondition "/bin/echo a"
+    check_ok b ExecCondition "/bin/echo b"
+    rm -rf /usr/lib/systemd/system/-.service.d
+
     clear_services a b c
 }