[debug] Use stack buffers with buf_append helper instead of std::string

esphome/components/debug/debug_component.h

@@ -8,6 +8,9 @@
 #include <cstdarg>
 #include <cstdio>
 #include <algorithm>
+#ifdef USE_ESP8266
+#include <pgmspace.h>
+#endif
 
 #ifdef USE_SENSOR
 #include "esphome/components/sensor/sensor.h"
@@ -22,6 +25,24 @@ namespace debug {
 static constexpr size_t DEVICE_INFO_BUFFER_SIZE = 256;
 static constexpr size_t RESET_REASON_BUFFER_SIZE = 128;
 
+#ifdef USE_ESP8266
+// ESP8266: Use vsnprintf_P to keep format strings in flash (PROGMEM)
+// Format strings must be wrapped with PSTR() macro
+inline size_t buf_append_P(char *buf, size_t size, size_t pos, PGM_P fmt, ...) {
+  if (pos >= size) {
+    return size;
+  }
+  va_list args;
+  va_start(args, fmt);
+  int written = vsnprintf_P(buf + pos, size - pos, fmt, args);
+  va_end(args);
+  if (written < 0) {
+    return pos;  // encoding error
+  }
+  return std::min(pos + static_cast<size_t>(written), size);
+}
+#define buf_append(buf, size, pos, fmt, ...) buf_append_P(buf, size, pos, PSTR(fmt), ##__VA_ARGS__)
+#else
 /// Safely append formatted string to buffer, returning new position (capped at size)
 __attribute__((format(printf, 4, 5))) inline size_t buf_append(char *buf, size_t size, size_t pos, const char *fmt,
                                                                ...) {
@@ -37,6 +58,7 @@ __attribute__((format(printf, 4, 5))) inline size_t buf_append(char *buf, size_t
   }
   return std::min(pos + static_cast<size_t>(written), size);
 }
+#endif
 
 class DebugComponent : public PollingComponent {
  public:

esphome/components/debug/debug_esp8266.cpp

@@ -12,7 +12,7 @@ const char *DebugComponent::get_reset_reason_(std::span<char, RESET_REASON_BUFFE
   char *buf = buffer.data();
 #if !defined(CLANG_TIDY)
   String reason = ESP.getResetReason();  // NOLINT
-  snprintf(buf, RESET_REASON_BUFFER_SIZE, "%s", reason.c_str());
+  snprintf_P(buf, RESET_REASON_BUFFER_SIZE, PSTR("%s"), reason.c_str());
   return buf;
 #else
   buf[0] = '\0';