login: fix two io.systemd.Login Varlink IDL issues (#40857)

Add missing NoSessionPIDFD error to the interface definition; the error
is emitted by vl_method_create_session() when the session leader process
does not have a pidfd available, but was never declared in the IDL.

Drop SD_JSON_MANDATORY from the ReleaseSession.Id dispatch entry so that
omitting the field (which the IDL allows via SD_VARLINK_NULLABLE)
resolves to the caller's own session via session_is_self(NULL).

src/login/logind-varlink.c

@@ -308,7 +308,7 @@ static int vl_method_release_session(sd_varlink *link, sd_json_variant *paramete
         } p;
 
         static const sd_json_dispatch_field dispatch_table[] = {
-                { "Id", SD_JSON_VARIANT_STRING, sd_json_dispatch_const_string, voffsetof(p, id), SD_JSON_MANDATORY },
+                { "Id", SD_JSON_VARIANT_STRING, sd_json_dispatch_const_string, voffsetof(p, id), 0 },
                 {}
         };
 

src/shared/varlink-io.systemd.Login.c

@@ -94,6 +94,7 @@ static SD_VARLINK_DEFINE_ERROR(AlreadySessionMember);
 static SD_VARLINK_DEFINE_ERROR(VirtualTerminalAlreadyTaken);
 static SD_VARLINK_DEFINE_ERROR(TooManySessions);
 static SD_VARLINK_DEFINE_ERROR(UnitAllocationFailed);
+static SD_VARLINK_DEFINE_ERROR(NoSessionPIDFD);
 
 SD_VARLINK_DEFINE_INTERFACE(
                 io_systemd_Login,
@@ -120,4 +121,6 @@ SD_VARLINK_DEFINE_INTERFACE(
                 SD_VARLINK_SYMBOL_COMMENT("Maximum number of sessions reached"),
                 &vl_error_TooManySessions,
                 SD_VARLINK_SYMBOL_COMMENT("Failed to allocate a unit for the session"),
-                &vl_error_UnitAllocationFailed);
+                &vl_error_UnitAllocationFailed,
+                SD_VARLINK_SYMBOL_COMMENT("The session leader process does not have a pidfd"),
+                &vl_error_NoSessionPIDFD);