shared/install: resolve specifiers in Also=

Test case:
[Install]
WantedBy= default.target
Also=getty@%p.service

$ ./systemctl --root=/ enable testing@instance.service
Created symlink /etc/systemd/system/default.target.wants/testing@instance.service → /etc/systemd/system/testing@.service.
Created symlink /etc/systemd/system/getty.target.wants/getty@testing.service → /usr/lib/systemd/system/getty@.service.
$ ./systemctl --root=/ disable testing@instance.service
Removed /etc/systemd/system/getty.target.wants/getty@testing.service.
Removed /etc/systemd/system/default.target.wants/testing@instance.service.

Fixes part of #4210.

Resolving specifiers in DefaultInstance seems to work too:
[Install]
WantedBy= default.target
DefaultInstance=%u

$ systemctl --root=/ enable testing3@instance.service
Created symlink /etc/systemd/system/default.target.wants/testing3@instance.service → /etc/systemd/system/testing3@.service.
$ systemctl --root=/ enable testing3@.service
Created symlink /etc/systemd/system/default.target.wants/testing3@zbyszek.service → /etc/systemd/system/testing3@.service.

1 files changed, 11 insertions, 4 deletions

diff --git a/src/shared/install.c b/src/shared/install.c
index 32f7edc8cc..ff1ecbe7ff 100644
--- a/src/shared/install.c
+++ b/src/shared/install.c
@@ -1054,7 +1054,7 @@ static int config_parse_also(
         assert(rvalue);
 
         for (;;) {
-                _cleanup_free_ char *word = NULL;
+                _cleanup_free_ char *word = NULL, *printed = NULL;
 
                 r = extract_first_word(&rvalue, &word, NULL, 0);
                 if (r < 0)
@@ -1062,15 +1062,22 @@ static int config_parse_also(
                 if (r == 0)
                         break;
 
-                r = install_info_add(c, word, NULL, true, &alsoinfo);
+                r = install_full_printf(info, word, &printed);
                 if (r < 0)
                         return r;
 
-                r = strv_push(&info->also, word);
+                if (!unit_name_is_valid(printed, UNIT_NAME_ANY))
+                        return -EINVAL;
+
+                r = install_info_add(c, printed, NULL, true, &alsoinfo);
+                if (r < 0)
+                        return r;
+
+                r = strv_push(&info->also, printed);
                 if (r < 0)
                         return r;
 
-                word = NULL;
+                printed = NULL;
         }
 
         return 0;