core: imply SetLoginEnvironment= if PAMName= is set

This geneally makes sense as setting up a PAM session pretty much
defines what a login session is.

In context of #30547 this has the benefit that we can take benefit of
the SetLoginEnvironment= effect without having to set it explicitly,
thus retaining some compat of the uid0 client towards older systemd
service managers.

man/systemd.exec.xml

@@ -703,12 +703,15 @@
       <varlistentry>
         <term><varname>SetLoginEnvironment=</varname></term>
 
-        <listitem><para>Takes a boolean parameter that controls whether to set <varname>$HOME</varname>,
-        <varname>$LOGNAME</varname>, and <varname>$SHELL</varname> environment variables. If unset, this is
-        controlled by whether <varname>User=</varname> is set. If true, they will always be set for system services,
-        i.e. even when the default user <literal>root</literal> is used. If false, the mentioned variables are not set
-        by systemd, no matter whether <varname>User=</varname> is used or not. This option normally has no effect
-        on user services, since these variables are typically inherited from user manager's own environment anyway.</para>
+        <listitem><para>Takes a boolean parameter that controls whether to set the <varname>$HOME</varname>,
+        <varname>$LOGNAME</varname>, and <varname>$SHELL</varname> environment variables. If not set, this
+        defaults to true if <varname>User=</varname>, <varname>DynamicUser=</varname> or
+        <varname>PAMName=</varname> are set, false otherwise. If set to true, the variables will always be
+        set for system services, i.e. even when the default user <literal>root</literal> is used. If set to
+        false, the mentioned variables are not set by the service manager, no matter whether
+        <varname>User=</varname>, <varname>DynamicUser=</varname>, or <varname>PAMName=</varname> are used or
+        not. This option normally has no effect on services of the per-user service manager, since in that
+        case these variables are typically inherited from user manager's own environment anyway.</para>
 
         <xi:include href="version-info.xml" xpointer="v255"/></listitem>
       </varlistentry>

src/core/exec-invoke.c

@@ -1883,7 +1883,7 @@ static int build_environment(
                                                     "Failed to determine user credentials for root: %m");
         }
 
-        bool set_user_login_env = c->set_login_environment >= 0 ? c->set_login_environment : (c->user || c->dynamic_user);
+        bool set_user_login_env = exec_context_get_set_login_environment(c);
 
         if (username) {
                 x = strjoin("USER=", username);

src/core/execute.c

@@ -1661,6 +1661,15 @@ uint64_t exec_context_get_timer_slack_nsec(const ExecContext *c) {
         return (uint64_t) MAX(r, 0);
 }
 
+bool exec_context_get_set_login_environment(const ExecContext *c) {
+        assert(c);
+
+        if (c->set_login_environment >= 0)
+                return c->set_login_environment;
+
+        return c->user || c->dynamic_user || c->pam_name;
+}
+
 char** exec_context_get_syscall_filter(const ExecContext *c) {
         _cleanup_strv_free_ char **l = NULL;
 

src/core/execute.h

@@ -527,6 +527,7 @@ int exec_context_get_nice(const ExecContext *c);
 int exec_context_get_cpu_sched_policy(const ExecContext *c);
 int exec_context_get_cpu_sched_priority(const ExecContext *c);
 uint64_t exec_context_get_timer_slack_nsec(const ExecContext *c);
+bool exec_context_get_set_login_environment(const ExecContext *c);
 char** exec_context_get_syscall_filter(const ExecContext *c);
 char** exec_context_get_syscall_archs(const ExecContext *c);
 char** exec_context_get_syscall_log(const ExecContext *c);