sd-id128: export sd_id128_get_app_specific()

We exported sd_id128_get_machine_app_specific() and
sd_id128_get_boot_app_specific(). But those operate on the current machine,
and sometimes the user might want to do the same calculation for a different
machine, or an earlier boot, etc. We expose the underlying implementation
to make this easy.

src/libsystemd/libsystemd.sym

@@ -828,3 +828,8 @@ global:
         sd_journal_step_one;
         sd_session_get_leader;
 } LIBSYSTEMD_253;
+
+LIBSYSTEMD_255 {
+global:
+        sd_id128_get_app_specific;
+} LIBSYSTEMD_254;

src/libsystemd/sd-id128/sd-id128.c

@@ -338,14 +338,14 @@ _public_ int sd_id128_randomize(sd_id128_t *ret) {
         return 0;
 }
 
-static int get_app_specific(sd_id128_t base, sd_id128_t app_id, sd_id128_t *ret) {
+_public_ int sd_id128_get_app_specific(sd_id128_t base, sd_id128_t app_id, sd_id128_t *ret) {
         assert_cc(sizeof(sd_id128_t) < SHA256_DIGEST_SIZE); /* Check that we don't need to pad with zeros. */
         union {
                 uint8_t hmac[SHA256_DIGEST_SIZE];
                 sd_id128_t result;
         } buf;
 
-        assert(ret);
+        assert_return(ret, -EINVAL);
 
         hmac_sha256(&base, sizeof(base), &app_id, sizeof(app_id), buf.hmac);
 
@@ -364,7 +364,7 @@ _public_ int sd_id128_get_machine_app_specific(sd_id128_t app_id, sd_id128_t *re
         if (r < 0)
                 return r;
 
-        return get_app_specific(id, app_id, ret);
+        return sd_id128_get_app_specific(id, app_id, ret);
 }
 
 _public_ int sd_id128_get_boot_app_specific(sd_id128_t app_id, sd_id128_t *ret) {
@@ -377,5 +377,5 @@ _public_ int sd_id128_get_boot_app_specific(sd_id128_t app_id, sd_id128_t *ret)
         if (r < 0)
                 return r;
 
-        return get_app_specific(id, app_id, ret);
+        return sd_id128_get_app_specific(id, app_id, ret);
 }