doc: http_server: document how to specify a default resource

Add documentation showing how to use the _detail parameter when
registering an HTTP service to provide a default resource handling any
unknown path.

Signed-off-by: Matt Rodgers <[email protected]>

doc/connectivity/networking/api/http_server.rst

@@ -121,6 +121,41 @@ Alternatively, an HTTPS service can be defined with
     HTTPS_SERVICE_DEFINE(my_service, "0.0.0.0", &https_service_port, 1, 10,
                          NULL, sec_tag_list, sizeof(sec_tag_list));
 
+The ``_detail`` parameter can be used when defining an HTTP/HTTPS service to
+specify a default resource which will be used if no other resource matches the
+URL. This can be used for example to serve an index page for all unknown paths
+(useful for a single-page app which handles routing in the frontend), or for a
+customised 404 response.
+
+.. code-block:: c
+    static int default_handler(struct http_client_ctx *client, enum http_data_status status,
+		       const struct http_request_ctx *request_ctx,
+		       struct http_response_ctx *response_ctx, void *user_data)
+    {
+        static const char response_404[] = "Oops, page not found!";
+
+        if (status == HTTP_SERVER_DATA_FINAL) {
+            response_ctx->status = 404;
+            response_ctx->body = response_404;
+            response_ctx->body_len = sizeof(response_404) - 1;
+            response_ctx->final_chunk = true;
+        }
+
+        return 0;
+    }
+
+    static struct http_resource_detail_dynamic default_detail = {
+        .common = {
+            .type = HTTP_RESOURCE_TYPE_DYNAMIC,
+            .bitmask_of_supported_http_methods = BIT(HTTP_GET),
+        },
+        .cb = default_handler,
+        .user_data = NULL,
+    };
+
+    /* Register a default resource to handle any unknown path */
+    HTTP_SERVICE_DEFINE(my_service, "0.0.0.0", &http_service_port, 1, 10, &default_detail);
+
 .. note::
 
     HTTPS services rely on TLS credentials being registered in the system.