shared/install: skip unnecessary chasing of symlinks in disable

We use the symlink source name and destination names to decide whether to remove
the symlink. But if the source name is enough to decide to remove the symlink,
we'd still look up the destination for no good reason. This is a slow operation,
let's skip it.

src/shared/install.c

@@ -598,8 +598,7 @@ static int remove_marked_symlinks_fd(
                                 r = q;
 
                 } else if (de->d_type == DT_LNK) {
-                        _cleanup_free_ char *p = NULL, *dest = NULL;
-                        const char *rp;
+                        _cleanup_free_ char *p = NULL;
                         bool found;
                         int q;
 
@@ -611,24 +610,32 @@ static int remove_marked_symlinks_fd(
                                 return -ENOMEM;
                         path_simplify(p);
 
-                        q = chase_symlinks(p, lp->root_dir, CHASE_NONEXISTENT, &dest, NULL);
-                        if (q == -ENOENT)
-                                continue;
-                        if (q < 0) {
-                                log_debug_errno(q, "Failed to resolve symlink \"%s\": %m", p);
-                                unit_file_changes_add(changes, n_changes, q, p, NULL);
+                        /* We remove all links pointing to a file or path that is marked, as well as all
+                         * files sharing the same name as a file that is marked. Do path chasing only if
+                         * we don't already know that we want to remove the symlink. */
+                        found = set_contains(remove_symlinks_to, de->d_name);
 
-                                if (r == 0)
-                                        r = q;
-                                continue;
-                        }
+                        if (!found) {
+                                _cleanup_free_ char *dest = NULL;
+
+
+                                q = chase_symlinks(p, lp->root_dir, CHASE_NONEXISTENT, &dest, NULL);
+                                if (q == -ENOENT)
+                                        continue;
+                                if (q < 0) {
+                                        log_debug_errno(q, "Failed to resolve symlink \"%s\": %m", p);
+                                        unit_file_changes_add(changes, n_changes, q, p, NULL);
 
-                        /* We remove all links pointing to a file or path that is marked, as well as all files sharing
-                         * the same name as a file that is marked. */
+                                        if (r == 0)
+                                                r = q;
+                                        continue;
+                                }
+
+                                found = set_contains(remove_symlinks_to, dest) ||
+                                        set_contains(remove_symlinks_to, basename(dest));
+
+                        }
 
-                        found = set_contains(remove_symlinks_to, dest) ||
-                                set_contains(remove_symlinks_to, basename(dest)) ||
-                                set_contains(remove_symlinks_to, de->d_name);
 
                         if (!found)
                                 continue;
@@ -649,7 +656,7 @@ static int remove_marked_symlinks_fd(
                         /* Now, remember the full path (but with the root prefix removed) of
                          * the symlink we just removed, and remove any symlinks to it, too. */
 
-                        rp = skip_root(lp->root_dir, p);
+                        const char *rp = skip_root(lp->root_dir, p);
                         q = mark_symlink_for_removal(&remove_symlinks_to, rp ?: p);
                         if (q < 0)
                                 return q;