Introduce RET_GATHER and use it in src/shared/

The idea is to make it easier to implement the common pattern of
accumulating errors (negative values) in an accumulator to return
the first error.

src/shared/bus-util.c

@@ -516,14 +516,8 @@ int bus_track_add_name_many(sd_bus_track *t, char **l) {
 
         /* Continues adding after failure, and returns the first failure. */
 
-        STRV_FOREACH(i, l) {
-                int k;
-
-                k = sd_bus_track_add_name(t, *i);
-                if (k < 0 && r >= 0)
-                        r = k;
-        }
-
+        STRV_FOREACH(i, l)
+                RET_GATHER(r, sd_bus_track_add_name(t, *i));
         return r;
 }
 

src/shared/devnode-acl.c

@@ -218,8 +218,8 @@ int devnode_acl_all(const char *seat,
                 k = devnode_acl(n, flush, del, old_uid, add, new_uid);
                 if (k == -ENOENT)
                         log_debug("Device %s disappeared while setting ACLs", n);
-                else if (k < 0 && r == 0)
-                        r = k;
+                else
+                        RET_GATHER(r, k);
         }
 
         return r;

src/shared/nscd-flush.c

@@ -128,21 +128,15 @@ static int nscd_flush_cache_one(const char *database, usec_t end) {
 }
 
 int nscd_flush_cache(char **databases) {
-        usec_t end;
         int r = 0;
 
-        /* Tries to invalidate the specified database in nscd. We do this carefully, with a 5s timeout, so that we
-         * don't block indefinitely on another service. */
+        /* Tries to invalidate the specified database in nscd. We do this carefully, with a 5s timeout,
+         * so that we don't block indefinitely on another service. */
 
-        end = usec_add(now(CLOCK_MONOTONIC), NSCD_FLUSH_CACHE_TIMEOUT_USEC);
+        usec_t end = usec_add(now(CLOCK_MONOTONIC), NSCD_FLUSH_CACHE_TIMEOUT_USEC);
 
-        STRV_FOREACH(i, databases) {
-                int k;
-
-                k = nscd_flush_cache_one(*i, end);
-                if (k < 0 && r >= 0)
-                        r = k;
-        }
+        STRV_FOREACH(i, databases)
+                RET_GATHER(r, nscd_flush_cache_one(*i, end));
 
         return r;
 }