From fedeeea9cb91f627d15a34779daed58fb01d2e7c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andr=C3=A9=20Fiedler?= <mail@andrefiedler.de>
Date: Sat, 7 Mar 2026 18:43:17 +0100
Subject: [PATCH] Refactor Config_Web to use heap-allocated JSON document and
 add stack usage check script

---
 scripts/check_config_web_stack_usage.sh | 12 ++++++++++++
 src/Config_Web.cpp                      | 17 +++++++++++++----
 2 files changed, 25 insertions(+), 4 deletions(-)
 create mode 100755 scripts/check_config_web_stack_usage.sh

diff --git a/scripts/check_config_web_stack_usage.sh b/scripts/check_config_web_stack_usage.sh
new file mode 100755
index 0000000..896b630
--- /dev/null
+++ b/scripts/check_config_web_stack_usage.sh
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+if rg -n '^[[:space:]]+StaticJsonDocument<kConfigJsonCapacity>[[:space:]]+[A-Za-z_][A-Za-z0-9_]*;' src/Config_Web.cpp; then
+	echo "Config_Web.cpp still allocates kConfigJsonCapacity-sized StaticJsonDocument on the stack" >&2
+	exit 1
+fi
+
+echo "Config_Web.cpp avoids stack-local kConfigJsonCapacity StaticJsonDocument allocations"
diff --git a/src/Config_Web.cpp b/src/Config_Web.cpp
index 6b1f7b1..181cb38 100644
--- a/src/Config_Web.cpp
+++ b/src/Config_Web.cpp
@@ -9,6 +9,7 @@ namespace
 	const size_t kConfigJsonCapacity = 4096;
 	const byte kDnsPort = 53;
 	const unsigned long kWifiReconnectIntervalMs = 10000;
+	using ConfigJsonDocument = StaticJsonDocument<kConfigJsonCapacity>;
 
 	WebServer server(80);
 	DNSServer dnsServer;
@@ -17,6 +18,14 @@ namespace
 	bool staReconnectEnabled = false;
 	bool staConnectionKnown = false;
 	unsigned long lastStaReconnectAttemptMs = 0;
+	ConfigJsonDocument g_configJsonScratch;
+
+	ConfigJsonDocument &configJsonScratch()
+	{
+		// Keep the large config JSON buffer off ESP32 Arduino's 8 KB loopTask stack.
+		g_configJsonScratch.clear();
+		return g_configJsonScratch;
+	}
 
 	void redirectToRoot()
 	{
@@ -371,7 +380,7 @@ bool loadConfigFromFile()
 		return saveConfigToFile();
 	}
 
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	DeserializationError err = deserializeJson(doc, file);
 	file.close();
 	if (err)
@@ -424,7 +433,7 @@ bool saveConfigToFile()
 		return false;
 	}
 
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	JsonArray params = doc.createNestedArray("parameters");
 	for (size_t i = 0; i < configParameterCount; ++i)
 	{
@@ -461,7 +470,7 @@ bool saveConfigToFile()
 
 String buildConfigJson()
 {
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	JsonArray params = doc.createNestedArray("parameters");
 	for (size_t i = 0; i < configParameterCount; ++i)
 	{
@@ -497,7 +506,7 @@ String buildConfigJson()
 
 bool applyConfigJson(const String &payload, String &errorMessage)
 {
-	StaticJsonDocument<kConfigJsonCapacity> doc;
+	ConfigJsonDocument &doc = configJsonScratch();
 	DeserializationError err = deserializeJson(doc, payload);
 	if (err)
 	{