From b063b3bab67e54b956b93907a327126a9997b633 Mon Sep 17 00:00:00 2001
From: Erik Fleckstein <erik@tinkerforge.com>
Date: Mon, 21 Mar 2022 14:53:19 +0100
Subject: [PATCH] mqtt: Use string lengths not JSON struct sizes for payload
 limits.

---
 software/src/config.cpp            | 39 ++++++++++++++++++++++++++++++
 software/src/config.h              |  1 +
 software/src/modules/mqtt/mqtt.cpp |  4 +--
 3 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/software/src/config.cpp b/software/src/config.cpp
index 1bf68d5d1..63dc19814 100644
--- a/software/src/config.cpp
+++ b/software/src/config.cpp
@@ -18,6 +18,7 @@
  */
 
 #include "config.h"
+#include "math.h"
 
 struct printer {
   void operator()(const Config::ConfString &x) const { Serial.println("string"); }
@@ -167,6 +168,40 @@ struct to_json {
     const std::vector<String> &keys_to_censor;
 };
 
+struct string_length_visitor {
+    size_t operator()(const Config::ConfString &x) {
+        return x.maxChars + 3; // "" and null terminator
+    }
+    size_t operator()(const Config::ConfFloat &x) {
+        return 42; // Educated guess
+    }
+    size_t operator()(const Config::ConfInt &x) {
+        return 11;
+    }
+    size_t operator()(const Config::ConfUint &x) {
+        return 10;
+    }
+    size_t operator()(const Config::ConfBool &x) {
+        return 5;
+    }
+    size_t operator()(std::nullptr_t x) {
+        return 4;
+    }
+    size_t operator()(const Config::ConfArray &x)
+    {
+        return strict_variant::apply_visitor(string_length_visitor{}, x.prototype->value) * x.maxElements + (x.maxElements + 1); //[,] and n-1 ,
+    }
+    size_t operator()(const Config::ConfObject &x)
+    {
+        size_t sum = 2; // { and }
+        for (size_t i = 0; i < x.value.size(); ++i) {
+            sum += x.value[i].first.length() + 3; // "" and null terminator
+            sum += strict_variant::apply_visitor(string_length_visitor{}, x.value[i].second.value);;
+        }
+        return sum;
+    }
+};
+
 struct json_length_visitor {
     size_t operator()(const Config::ConfString &x) {
         return zero_copy ? 0 : (x.maxChars + 1);
@@ -692,6 +727,10 @@ size_t Config::json_size(bool zero_copy) const {
     return strict_variant::apply_visitor(json_length_visitor{zero_copy}, value);
 }
 
+size_t Config::max_string_length() const {
+    return strict_variant::apply_visitor(string_length_visitor{}, value);
+}
+
 void Config::save_to_file(File file)
 {
     DynamicJsonDocument doc(json_size(false));
diff --git a/software/src/config.h b/software/src/config.h
index 3eeb03c5e..5befbf51f 100644
--- a/software/src/config.h
+++ b/software/src/config.h
@@ -380,6 +380,7 @@ struct Config {
     }
 */
     size_t json_size(bool zero_copy) const;
+    size_t max_string_length() const;
 
     void save_to_file(File file);
 
diff --git a/software/src/modules/mqtt/mqtt.cpp b/software/src/modules/mqtt/mqtt.cpp
index fdb815b7d..72a70d9ed 100644
--- a/software/src/modules/mqtt/mqtt.cpp
+++ b/software/src/modules/mqtt/mqtt.cpp
@@ -73,7 +73,7 @@ void Mqtt::subscribe(String topic_suffix, uint32_t max_payload_length, std::func
 
 void Mqtt::addCommand(size_t commandIdx, const CommandRegistration &reg)
 {
-    auto req_size = reg.config->json_size(true);
+    auto req_size = reg.config->max_string_length();
     if (req_size > MQTT_RECV_BUFFER_SIZE) {
         logger.printfln("MQTT: Recv buf is %u bytes. %s requires %u. Bump MQTT_RECV_BUFFER_SIZE! Not subscribing!", MQTT_RECV_BUFFER_SIZE, reg.path.c_str(), req_size);
         return;
@@ -84,7 +84,7 @@ void Mqtt::addCommand(size_t commandIdx, const CommandRegistration &reg)
     if (mqtt_state.get("connection_state")->asInt() != (int)MqttConnectionState::CONNECTED)
         return;
 
-    subscribe(reg.path, reg.config->json_size(true), [reg, commandIdx](char *payload, size_t payload_len){
+    subscribe(reg.path, reg.config->max_string_length(), [reg, commandIdx](char *payload, size_t payload_len){
         String reason = api.getCommandBlockedReason(commandIdx);
         if (reason != "") {
             logger.printfln("MQTT: Command %s is blocked: %s", reg.path.c_str(), reason.c_str());