lws_dll2: add helper for typed object name from owner list

There's a good pattern that's encouraged by using lws_struct pieces, that
we have an lws_dll2 owner with an array of objects listed in it that exist
in an lwsac.  And because it came from JSON, there is tending to be a
logical name for the objects.

This adds a typed helper and wrapper to scan the owner list looking for
a specific name (of a specified length, not NUL terminated) in a specific
member of the listed objects, which must be a NUL-terminated const char *.
Again this is a good pattern that's encouraged by use of lws_tokenize
to recover the name we're looking for.

So it leads to the helper that can cleanly search for a listed object of the
right name from an owner, and return the typed object pointer or NULL, from a
length-specified string.

include/libwebsockets/lws-dll2.h

@@ -245,6 +245,21 @@ LWS_VISIBLE LWS_EXTERN void
 lws_dll2_add_sorted(lws_dll2_t *d, lws_dll2_owner_t *own,
 		    int (*compare)(const lws_dll2_t *d, const lws_dll2_t *i));
 
+LWS_VISIBLE LWS_EXTERN void *
+_lws_dll2_search_sz_pl(lws_dll2_owner_t *own, const char *name, size_t namelen,
+		      size_t dll2_ofs, size_t ptr_ofs);
+
+/*
+ * Searches objects in an owner list linearly and returns one with a given
+ * member C-string matching a supplied length-provided string if it exists, else
+ * NULL.
+ */
+
+#define lws_dll2_search_sz_pl(own, name, namelen, type, membd2list, membptr) \
+		((type *)_lws_dll2_search_sz_pl(own, name, namelen, \
+				       offsetof(type, membd2list), \
+				       offsetof(type, membptr)))
+
 #if defined(_DEBUG)
 void
 lws_dll2_describe(struct lws_dll2_owner *owner, const char *desc);

lib/core/lws_dll2.c

@@ -209,6 +209,25 @@ lws_dll2_add_sorted(lws_dll2_t *d, lws_dll2_owner_t *own,
 	lws_dll2_add_tail(d, own);
 }
 
+void *
+_lws_dll2_search_sz_pl(lws_dll2_owner_t *own, const char *name, size_t namelen,
+		       size_t dll2_ofs, size_t ptr_ofs)
+{
+	lws_start_foreach_dll(struct lws_dll2 *, p, lws_dll2_get_head(own)) {
+		uint8_t *ref = ((uint8_t *)p) - dll2_ofs;
+		/*
+		 * We have to read the const char * at the computed place and
+		 * the string is where that points
+		 */
+		const char *str = *((const char **)(ref + ptr_ofs));
+
+		if (str && !strncmp(str, name, namelen) && !str[namelen])
+			return (void *)ref;
+	} lws_end_foreach_dll(p);
+
+	return NULL;
+}
+
 #if defined(_DEBUG)
 
 void

lib/misc/lws-struct-lejp.c

@@ -256,7 +256,7 @@ lws_struct_default_lejp_cb(struct lejp_ctx *ctx, char reason)
 
 			return 1;
 		}
-		lwsl_notice("%s: created '%s' object size %d\n", __func__,
+		lwsl_info("%s: created '%s' object size %d\n", __func__,
 				pmap->colname, (int)pmap->aux);
 
 		if (pmap->type == LSMT_LIST) {

minimal-examples/api-tests/api-test-lws_struct-json/main.c

@@ -597,6 +597,38 @@ done:
 		free(buf);
 	}
 
+	{
+		struct x { lws_dll2_t list; const char *sz; };
+		struct x x1, x2, *xp;
+		lws_dll2_owner_t o;
+
+		lws_dll2_owner_clear(&o);
+		memset(&x1, 0, sizeof(x1));
+		memset(&x2, 0, sizeof(x2));
+
+		x1.sz = "nope";
+		x2.sz = "yes";
+
+		lws_dll2_add_tail(&x1.list, &o);
+		lws_dll2_add_tail(&x2.list, &o);
+
+		xp = lws_dll2_search_sz_pl(&o, "yes", 3, struct x, list, sz);
+		if (xp != &x2) {
+			lwsl_err("%s: 1 xp %p\n", __func__, xp);
+			goto bail;
+		}
+		xp = lws_dll2_search_sz_pl(&o, "nope", 4, struct x, list, sz);
+		if (xp != &x1) {
+			lwsl_err("%s: 2 xp %p\n", __func__, xp);
+			goto bail;
+		}
+		xp = lws_dll2_search_sz_pl(&o, "wrong", 4, struct x, list, sz);
+		if (xp) {
+			lwsl_err("%s: 3 xp %p\n", __func__, xp);
+			goto bail;
+		}
+	}
+
 
 	lwsl_user("Completed: PASS\n");