From fd902a646ac2b0cfa5650edc1b2d16740135f009 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Tue, 20 Oct 2026 01:09:13 -1000
Subject: [PATCH] [ota] Accept an unknown image size in the RP2040 backend so
 web_server OTA works (#21178)

---
 esphome/components/ota/ota_backend.h              |  5 +++--
 .../components/ota/ota_backend_arduino_rp2.cpp    | 15 ++++++++++++---
 tests/components/web_server/test.rp2040-ard.yaml  |  3 +++
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/esphome/components/ota/ota_backend.h b/esphome/components/ota/ota_backend.h
index 840d1ac11c7..3e8b5954906 100644
--- a/esphome/components/ota/ota_backend.h
+++ b/esphome/components/ota/ota_backend.h
@@ -122,8 +122,9 @@ class UpdateLock {
 // The OTA backend method surface. Exactly one backend exists per build,
 // selected in ota_backend_factory.h where this concept is asserted on
 // make_ota_backend()'s return type. Semantics beyond the signatures:
-// - begin: prepare for an image of the given size; ota_type defaults to an
-//   app update, so both call forms must be accepted.
+// - begin: prepare for an image of the given size; 0 means the size is unknown
+//   (web_server upload) and the backend allows the largest image it can take.
+//   ota_type defaults to an app update, so both call forms must be accepted.
 // - set_update_md5: expected digest of the incoming image, hex string.
 // - write: consume the next chunk; end: finalize and mark bootable.
 // - abort: safe to call in any state, including after end().
diff --git a/esphome/components/ota/ota_backend_arduino_rp2.cpp b/esphome/components/ota/ota_backend_arduino_rp2.cpp
index d4d94be2363..6c56819c281 100644
--- a/esphome/components/ota/ota_backend_arduino_rp2.cpp
+++ b/esphome/components/ota/ota_backend_arduino_rp2.cpp
@@ -9,6 +9,12 @@
 
 #include <Updater.h>
 
+// LittleFS region bounds, provided by the arduino-pico linker script.
+// NOLINTNEXTLINE(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp)
+extern "C" uint8_t _FS_start;
+// NOLINTNEXTLINE(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp)
+extern "C" uint8_t _FS_end;
+
 namespace esphome::ota {
 
 ESPHOME_LOG_TAG(TAG, "ota");
@@ -21,8 +27,11 @@ OTAResponseTypes ArduinoRP2OTABackend::begin(size_t image_size, OTAType ota_type
   if (ota_type != OTA_TYPE_UPDATE_APP) {
     return OTA_RESPONSE_ERROR_UNSUPPORTED_OTA_TYPE;
   }
-  // OTA size of 0 is not currently handled, but
-  // web_server is not supported for RP2040, so this is not an issue.
+  // 0 means the size is unknown (web_server upload). The updater only checks the size
+  // against the LittleFS region it stages the image in, so ask for all of it.
+  if (image_size == 0) {
+    image_size = reinterpret_cast<uintptr_t>(&_FS_end) - reinterpret_cast<uintptr_t>(&_FS_start);
+  }
   bool ret = Update.begin(image_size, U_FLASH);
   if (ret) {
     rp2::preferences_prevent_write(true);
@@ -36,7 +45,7 @@ OTAResponseTypes ArduinoRP2OTABackend::begin(size_t image_size, OTAType ota_type
     return OTA_RESPONSE_ERROR_WRONG_NEW_FLASH_CONFIG;
   if (error == UPDATE_ERROR_FLASH_CONFIG)
     return OTA_RESPONSE_ERROR_WRONG_CURRENT_FLASH_CONFIG;
-  if (error == UPDATE_ERROR_SPACE)
+  if (error == UPDATE_ERROR_SPACE || error == UPDATE_ERROR_SIZE)
     return OTA_RESPONSE_ERROR_RP2040_NOT_ENOUGH_SPACE;
 
   ESP_LOGE(TAG, "Begin error: %d", error);
diff --git a/tests/components/web_server/test.rp2040-ard.yaml b/tests/components/web_server/test.rp2040-ard.yaml
index 6a01180892a..56bd155717f 100644
--- a/tests/components/web_server/test.rp2040-ard.yaml
+++ b/tests/components/web_server/test.rp2040-ard.yaml
@@ -6,3 +6,6 @@ web_server:
     username: admin
     password: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
     type: basic
+
+ota:
+  - platform: web_server