error: static assertion failed: Insufficient buffer size for CoAP processing!

[Tristen]

I have an OpenThread application that worked on NCS v1.9.1 and after updating to NCS v2.0.0 (nothing in the source code or kconfig was changed), the application fails to build with a handful of the same error: error: static assertion failed: Insufficient buffer size for CoAP processing!. Here is one of the error's (see build_log.txt attached for the complete build):

[60/823] Building CXX object modules/openthread/build/src/cli/CMakeFiles/openthread-cli-ftd.dir/cli_joiner.cpp.obj
FAILED: modules/openthread/build/src/cli/CMakeFiles/openthread-cli-ftd.dir/cli_joiner.cpp.obj 
C:\ncs\toolchains\v2.0.0\opt\zephyr-sdk\arm-zephyr-eabi\bin\arm-zephyr-eabi-g++.exe -DKERNEL -DMBEDTLS_CONFIG_FILE=\"nrf-config.h\" -DMBEDTLS_USER_CONFIG_FILE=\"nrf-config-user.h\" -DNRF5340_XXAA_APPLICATION -DNRF_802154_ACK_TIMEOUT_ENABLED=1 -DNRF_802154_ECB_PRIORITY=-1 -DNRF_802154_ENCRYPTION_ENABLED=1 -DNRF_802154_IE_WRITER_ENABLED=1 -DNRF_802154_INTERNAL_RADIO_IRQ_HANDLING=0 -DNRF_802154_PENDING_EXTENDED_ADDRESSES="" -DNRF_802154_PENDING_SHORT_ADDRESSES="" -DNRF_802154_RX_BUFFERS=16 -DNRF_802154_SECURITY_WRITER_ENABLED=1 -DNRF_802154_SWI_PRIORITY=1 -DNRF_802154_TX_STARTED_NOTIFY_ENABLED=1 -DNRF_802154_USE_RAW_API=1 -DNRF_SKIP_FICR_NS_COPY_TO_RAM -DOPENTHREAD_CONFIG_ASSERT_ENABLE=0 -DOPENTHREAD_CONFIG_BACKBONE_ROUTER_DUA_NDPROXYING_ENABLE=0 -DOPENTHREAD_CONFIG_BACKBONE_ROUTER_MULTICAST_ROUTING_ENABLE=0 -DOPENTHREAD_CONFIG_COMMISSIONER_ENABLE=1 -DOPENTHREAD_CONFIG_ENABLE_BUILTIN_MBEDTLS=0 -DOPENTHREAD_CONFIG_ENABLE_BUILTIN_MBEDTLS_MANAGEMENT=0 -DOPENTHREAD_CONFIG_LOG_LEVEL=2 -DOPENTHREAD_CONFIG_MAC_CSL_AUTO_SYNC_ENABLE=0 -DOPENTHREAD_CONFIG_NCP_HDLC_ENABLE=1 -DOPENTHREAD_CONFIG_PING_SENDER_ENABLE=1 -DOPENTHREAD_CONFIG_THREAD_VERSION=OT_THREAD_VERSION_1_2 -DOPENTHREAD_FTD=1 -DOPENTHREAD_PROJECT_CORE_CONFIG_FILE=\"openthread-core-zephyr-config.h\" -DOPENTHREAD_SPINEL_CONFIG_RCP_RESTORATION_MAX_COUNT=0 -DPACKAGE_NAME=\"OPENTHREAD\" -DUSE_PARTITION_MANAGER=1 -D_FORTIFY_SOURCE=2 -D__LINUX_ERRNO_EXTENSIONS__ -D__PROGRAM_START -D__ZEPHYR__=1 -DPACKAGE_VERSION=\"e2a765599\" -Imodules/openthread/build/etc/cmake -IC:/ncs/v2.0.0/modules/lib/openthread/etc/cmake -IC:/ncs/v2.0.0/modules/lib/openthread/src -IC:/ncs/v2.0.0/modules/lib/openthread/src/core -Imodules/nrfxlib/nrfxlib/nrf_security/src/include/generated -IC:/ncs/v2.0.0/nrfxlib/nrf_security/include -IC:/ncs/v2.0.0/nrfxlib/nrf_security/include/mbedtls -IC:/ncs/v2.0.0/nrfxlib/nrf_security/include/psa -IC:/ncs/v2.0.0/mbedtls/include -IC:/ncs/v2.0.0/mbedtls/include/mbedtls -IC:/ncs/v2.0.0/mbedtls/include/psa -IC:/ncs/v2.0.0/mbedtls/library -IC:/ncs/v2.0.0/nrfxlib/nrf_security/../crypto/nrf_oberon/include -IC:/ncs/v2.0.0/nrfxlib/nrf_security/../crypto/nrf_oberon/include/mbedtls -IC:/ncs/v2.0.0/nrfxlib/nrf_security/../crypto/nrf_oberon/include/psa -IC:/ncs/v2.0.0/nrfxlib/crypto/nrf_cc312_platform/include -IC:/ncs/v2.0.0/nrfxlib/crypto/nrf_oberon/include/mbedtls -IC:/ncs/v2.0.0/nrfxlib/crypto/nrf_oberon/include -IC:/ncs/v2.0.0/modules/lib/openthread/examples/platforms/zephyr -IC:/ncs/v2.0.0/zephyr/include/zephyr -IC:/ncs/v2.0.0/zephyr/include -Izephyr/include/generated -IC:/ncs/v2.0.0/zephyr/soc/arm/nordic_nrf/nrf53 -IC:/ncs/v2.0.0/zephyr/lib/libc/newlib/include -IC:/ncs/v2.0.0/zephyr/lib/util/fnmatch/. -IC:/ncs/v2.0.0/zephyr/soc/arm/nordic_nrf/common/. -IC:/ncs/v2.0.0/zephyr/subsys/net/lib/sockets/. -IC:/ncs/v2.0.0/zephyr/subsys/net/lib/openthread/platform/. -IC:/ncs/v2.0.0/zephyr/subsys/settings/include -IC:/ncs/v2.0.0/nrf/include -IC:/ncs/v2.0.0/modules/hal/cmsis/CMSIS/Core/Include -IC:/ncs/v2.0.0/modules/hal/nordic/nrfx -IC:/ncs/v2.0.0/modules/hal/nordic/nrfx/drivers/include -IC:/ncs/v2.0.0/modules/hal/nordic/nrfx/mdk -IC:/ncs/v2.0.0/zephyr/modules/hal_nordic/nrfx/. -Imodules/libmetal/libmetal/lib/include -IC:/ncs/v2.0.0/modules/lib/open-amp/open-amp/lib/include -IC:/ncs/v2.0.0/nrfxlib/nrf_802154/serialization/include -IC:/ncs/v2.0.0/nrfxlib/nrf_802154/serialization/include/platform -IC:/ncs/v2.0.0/nrfxlib/nrf_802154/serialization/include/serialization -IC:/ncs/v2.0.0/nrfxlib/nrf_802154/serialization/include/host -isystem C:/ncs/v2.0.0/modules/lib/openthread/include -isystem C:/ncs/v2.0.0/zephyr/subsys/cpp/include -isystem C:/ncs/v2.0.0/modules/lib/openthread/examples/platforms -Wall -Wextra -Wshadow -Wno-c++14-compat -fno-exceptions -Os -fcheck-new -std=c++11 -Wno-register -fno-rtti -imacros C:/Users/dev/Development/thread-dev/hub-thread-fw/build-nRF5340-DK/zephyr/include/generated/autoconf.h -ffreestanding -fno-common -g -gdwarf-4 -fdiagnostics-color=always -mcpu=cortex-m33 -mthumb -mabi=aapcs -mfpu=fpv5-sp-d16 -mfloat-abi=hard -mfp16-format=ieee -imacros C:/ncs/v2.0.0/zephyr/include/zephyr/toolchain/zephyr_stdint.h -Wformat -Wformat-security -Wno-format-zero-length -Wno-main -Wpointer-arith -Wexpansion-to-defined -Wno-unused-but-set-variable -fno-asynchronous-unwind-tables -fno-pie -fno-pic -fno-reorder-functions -fno-defer-pop -fmacro-prefix-map=C:/Users/dev/Development/thread-dev/hub-thread-fw=CMAKE_SOURCE_DIR -fmacro-prefix-map=C:/ncs/v2.0.0/zephyr=ZEPHYR_BASE -fmacro-prefix-map=C:/ncs/v2.0.0=WEST_TOPDIR -ffunction-sections -fdata-sections -specs=nano.specs -nostdinc++ -fno-builtin -imacros C:/Users/dev/Development/thread-dev/hub-thread-fw/build-nRF5340-DK/zephyr/include/generated/autoconf.h -std=c++11 -MD -MT modules/openthread/build/src/cli/CMakeFiles/openthread-cli-ftd.dir/cli_joiner.cpp.obj -MF modules\openthread\build\src\cli\CMakeFiles\openthread-cli-ftd.dir\cli_joiner.cpp.obj.d -o modules/openthread/build/src/cli/CMakeFiles/openthread-cli-ftd.dir/cli_joiner.cpp.obj -c C:/ncs/v2.0.0/modules/lib/openthread/src/cli/cli_joiner.cpp
In file included from C:\ncs\v2.0.0\modules\lib\openthread\src\core\coap\coap.hpp:36,
                 from C:\ncs\v2.0.0\modules\lib\openthread\src\core\backbone_router\bbr_leader.hpp:44,
                 from C:\ncs\v2.0.0\modules\lib\openthread\src\core\common\instance.hpp:65,
                 from C:\ncs\v2.0.0\modules\lib\openthread\src\cli\cli.hpp:78,
                 from C:\ncs\v2.0.0\modules\lib\openthread\src\cli\cli_joiner.cpp:38:
C:/ncs/v2.0.0/modules/lib/openthread/src/core/coap/coap_message.hpp: In member function 'const ot::Coap::Message::HelpData& ot::Coap::Message::GetHelpData() const':
C:\ncs\v2.0.0\modules\lib\openthread\src\core\coap\coap_message.hpp:953:61: error: static assertion failed: Insufficient buffer size for CoAP processing!
  953 |         static_assert(sizeof(HelpData) + kHelpDataAlignment <= kHeadBufferDataSize,
      |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

Note that I was able to build the Thread CLI example, so this issue seems to be only with my application.

 2500.thread.zip
4064.build_log.txt

[Jørgen Holmefjord]

Hi,

As far as I can see, kHeadBufferDataSize is primarily set by the size of OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE. Can you check if you find any references to this and what this is set to in your project/SDK installation?

Best regards,
Jørgen

[Tristen]

I checked and saw the size is defined as (sizeof(void *) * 32). 

[Tristen]

My issue seems to be due to changes in NCS v2.0.0. As pointed out by a Nordic colleague:

Anyway, I see that due to additional fields in Message metadata, there 4-bytes less to store CoAP metadata compared to NCS v1.9.1.
What is unusual in your config, is a huge number of supporting children - 511 (maximum by the spec) - is this a requirement?

Since in Message metadata, we have a bitmap for each child, supporting 511 of them adds 64-bytes. In order to fit now, you can for example reduce the number of supported children to 480 (64B - 4B = 60B * 8 = 480 children).

Then building passes: `west build -p -b nrf52840dk_nrf52840 -- -DCONFIG_OPENTHREAD_MAX_CHILDREN=480`. Still there is a unrelated linker error in his application.

After changing to 480 max children, the project now builds. Yes, my project does require at least 500 children due to product requirements. So eventually this needs to be properly resolved. 

Still, this indicates a problem with NCS v2.0.0. Either the kconfig should not allow selecting over 480 children, or the buffer size should be increased in lieu of the newly added additional fields in Message metadata.

[Tristen]

I found a better solution to remove the max children restriction. All we need to do is add two extra bytes to the CoAP Message Buffer by adding the following line to the Kconfig:

CONFIG_OPENTHREAD_CUSTOM_PARAMETERS="OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE=130"

However, if Block-wise operations are enabled, the buffer will need to be larger and should be set to 138. 

I would argue that the default value set in misc.h for the definition of OPENTHREAD_CONFIG_MESSAGE_BUFFER_SIZE ought to be changed in a future NCS release to accommodate the worst case Thread/CoAP kconfig options.