Payment rejected by the peer (LSP) with an "UNKNOWN_NEXT_PEER" message

[madelinevibes]

Reported by @nepet.
Seeing the following Valgrind errors testing the LSP implementation. Errors are appearing on a node that was trying to make a payment that got rejected by the peer (LSP) with an "UNKNOWN_NEXT_PEER".
(unsure if related to the 25.09 rc's)

------------------------------- Valgrind errors --------------------------------
Valgrind error file: valgrind-errors.3343609
==3343609== Use of uninitialised value of size 8
==3343609==    at 0x4A5CF1B: _itoa_word (_itoa.c:183)
==3343609==    by 0x4A6814C: __printf_buffer (vfprintf-process-arg.c:155)
==3343609==    by 0x4A8FED2: __vsnprintf_internal (vsnprintf.c:96)
==3343609==    by 0x1E197E: vsnprintf (stdio2.h:100)
==3343609==    by 0x1E197E: do_vfmt (str.c:59)
==3343609==    by 0x1E1AE4: tal_vfmt_ (str.c:82)
==3343609==    by 0x1E1BB7: tal_fmt_ (str.c:37)
==3343609==    by 0x12AF10: fmt_amount_msat (amount.c:64)
==3343609==    by 0x11D893: channel_hints_update (libplugin-pay.c:446)
==3343609==    by 0x120153: handle_intermediate_failure (libplugin-pay.c:1436)
==3343609==    by 0x1202F8: payment_addgossip_success (libplugin-pay.c:1612)
==3343609==    by 0x12067B: payment_waitsendpay_finished (libplugin-pay.c:1694)
==3343609==    by 0x118041: handle_rpc_reply (libplugin.c:1123)
==3343609==
{
   <insert_a_suppression_name_here>
   Memcheck:Value8
   fun:_itoa_word
   fun:__printf_buffer
   fun:__vsnprintf_internal
   fun:vsnprintf
   fun:do_vfmt
   fun:tal_vfmt_
   fun:tal_fmt_
   fun:fmt_amount_msat
   fun:channel_hints_update
   fun:handle_intermediate_failure
   fun:payment_addgossip_success
   fun:payment_waitsendpay_finished
   fun:handle_rpc_reply
}
==3343609== Conditional jump or move depends on uninitialised value(s)
==3343609==    at 0x4A5CF2C: _itoa_word (_itoa.c:183)
==3343609==    by 0x4A6814C: __printf_buffer (vfprintf-process-arg.c:155)
==3343609==    by 0x4A8FED2: __vsnprintf_internal (vsnprintf.c:96)
==3343609==    by 0x1E197E: vsnprintf (stdio2.h:100)
==3343609==    by 0x1E197E: do_vfmt (str.c:59)
==3343609==    by 0x1E1AE4: tal_vfmt_ (str.c:82)
==3343609==    by 0x1E1BB7: tal_fmt_ (str.c:37)
==3343609==    by 0x12AF10: fmt_amount_msat (amount.c:64)
==3343609==    by 0x11D893: channel_hints_update (libplugin-pay.c:446)
==3343609==    by 0x120153: handle_intermediate_failure (libplugin-pay.c:1436)
==3343609==    by 0x1202F8: payment_addgossip_success (libplugin-pay.c:1612)
==3343609==    by 0x12067B: payment_waitsendpay_finished (libplugin-pay.c:1694)
==3343609==    by 0x118041: handle_rpc_reply (libplugin.c:1123)
==3343609==
{
   <insert_a_suppression_name_here>
   Memcheck:Cond
   fun:_itoa_word
   fun:__printf_buffer
   fun:__vsnprintf_internal
   fun:vsnprintf
   fun:do_vfmt
   fun:tal_vfmt_
   fun:tal_fmt_
   fun:fmt_amount_msat
   fun:channel_hints_update
   fun:handle_intermediate_failure
   fun:payment_addgossip_success
   fun:payment_waitsendpay_finished
   fun:handle_rpc_reply
}
==3343609== Conditional jump or move depends on uninitialised value(s)
==3343609==    at 0x4A68F2C: __printf_buffer (vfprintf-process-arg.c:186)
==3343609==    by 0x4A8FED2: __vsnprintf_internal (vsnprintf.c:96)
==3343609==    by 0x1E197E: vsnprintf (stdio2.h:100)
==3343609==    by 0x1E197E: do_vfmt (str.c:59)
==3343609==    by 0x1E1AE4: tal_vfmt_ (str.c:82)
==3343609==    by 0x1E1BB7: tal_fmt_ (str.c:37)
==3343609==    by 0x12AF10: fmt_amount_msat (amount.c:64)
==3343609==    by 0x11D893: channel_hints_update (libplugin-pay.c:446)
==3343609==    by 0x120153: handle_intermediate_failure (libplugin-pay.c:1436)
==3343609==    by 0x1202F8: payment_addgossip_success (libplugin-pay.c:1612)
==3343609==    by 0x12067B: payment_waitsendpay_finished (libplugin-pay.c:1694)
==3343609==    by 0x118041: handle_rpc_reply (libplugin.c:1123)
==3343609==    by 0x118334: rpc_read_response_one (libplugin.c:1438)
==3343609==
{
   <insert_a_suppression_name_here>
   Memcheck:Cond
   fun:__printf_buffer
   fun:__vsnprintf_internal
   fun:vsnprintf
   fun:do_vfmt
   fun:tal_vfmt_
   fun:tal_fmt_
   fun:fmt_amount_msat
   fun:channel_hints_update
   fun:handle_intermediate_failure
   fun:payment_addgossip_success
   fun:payment_waitsendpay_finished
   fun:handle_rpc_reply
   fun:rpc_read_response_one
}
==3343609== Use of uninitialised value of size 8
==3343609==    at 0x4A5CF1B: _itoa_word (_itoa.c:183)
==3343609==    by 0x4A6814C: __printf_buffer (vfprintf-process-arg.c:155)
==3343609==    by 0x4A8FED2: __vsnprintf_internal (vsnprintf.c:96)
==3343609==    by 0x1DEB3F: vsnprintf (stdio2.h:100)
==3343609==    by 0x1DEB3F: json_out_addv (json_out.c:239)
==3343609==    by 0x137A6B: json_add_primitive_fmt (json_stream.c:180)
==3343609==    by 0x137E9D: json_add_u64 (json_stream.c:292)
==3343609==    by 0x13884C: json_add_amount_msat (json_stream.c:634)
==3343609==    by 0x119C80: channel_hint_to_json (channel_hint.c:38)
==3343609==    by 0x11BA21: channel_hint_notify_core (libplugin-pay.c:392)
==3343609==    by 0x11BA7C: channel_hint_notify (libplugin-pay.c:410)
==3343609==    by 0x11D8E5: channel_hints_update (libplugin-pay.c:453)
==3343609==    by 0x120153: handle_intermediate_failure (libplugin-pay.c:1436)
==3343609==
{
   <insert_a_suppression_name_here>
   Memcheck:Value8
   fun:_itoa_word
   fun:__printf_buffer
   fun:__vsnprintf_internal
   fun:vsnprintf
   fun:json_out_addv
   fun:json_add_primitive_fmt
   fun:json_add_u64
   fun:json_add_amount_msat
   fun:channel_hint_to_json
   fun:channel_hint_notify_core
   fun:channel_hint_notify
   fun:channel_hints_update
   fun:handle_intermediate_failure
}
==3343609== Conditional jump or move depends on uninitialised value(s)
==3343609==    at 0x4A5CF2C: _itoa_word (_itoa.c:183)
==3343609==    by 0x4A6814C: __printf_buffer (vfprintf-process-arg.c:155)
==3343609==    by 0x4A8FED2: __vsnprintf_internal (vsnprintf.c:96)
==3343609==    by 0x1DEB3F: vsnprintf (stdio2.h:100)
==3343609==    by 0x1DEB3F: json_out_addv (json_out.c:239)
==3343609==    by 0x137A6B: json_add_primitive_fmt (json_stream.c:180)
==3343609==    by 0x137E9D: json_add_u64 (json_stream.c:292)
==3343609==    by 0x13884C: json_add_amount_msat (json_stream.c:634)
==3343609==    by 0x119C80: channel_hint_to_json (channel_hint.c:38)
==3343609==    by 0x11BA21: channel_hint_notify_core (libplugin-pay.c:392)
==3343609==    by 0x11BA7C: channel_hint_notify (libplugin-pay.c:410)
==3343609==    by 0x11D8E5: channel_hints_update (libplugin-pay.c:453)
==3343609==    by 0x120153: handle_intermediate_failure (libplugin-pay.c:1436)
==3343609==
{
   <insert_a_suppression_name_here>
   Memcheck:Cond
   fun:_itoa_word
   fun:__printf_buffer
   fun:__vsnprintf_internal
   fun:vsnprintf
   fun:json_out_addv
   fun:json_add_primitive_fmt
   fun:json_add_u64
   fun:json_add_amount_msat
   fun:channel_hint_to_json
   fun:channel_hint_notify_core
   fun:channel_hint_notify
   fun:channel_hints_update
   fun:handle_intermediate_failure
}
==3343609== Conditional jump or move depends on uninitialised value(s)
==3343609==    at 0x4A68F2C: __printf_buffer (vfprintf-process-arg.c:186)
==3343609==    by 0x4A8FED2: __vsnprintf_internal (vsnprintf.c:96)
==3343609==    by 0x1DEB3F: vsnprintf (stdio2.h:100)
==3343609==    by 0x1DEB3F: json_out_addv (json_out.c:239)
==3343609==    by 0x137A6B: json_add_primitive_fmt (json_stream.c:180)
==3343609==    by 0x137E9D: json_add_u64 (json_stream.c:292)
==3343609==    by 0x13884C: json_add_amount_msat (json_stream.c:634)
==3343609==    by 0x119C80: channel_hint_to_json (channel_hint.c:38)
==3343609==    by 0x11BA21: channel_hint_notify_core (libplugin-pay.c:392)
==3343609==    by 0x11BA7C: channel_hint_notify (libplugin-pay.c:410)
==3343609==    by 0x11D8E5: channel_hints_update (libplugin-pay.c:453)
==3343609==    by 0x120153: handle_intermediate_failure (libplugin-pay.c:1436)
==3343609==    by 0x1202F8: payment_addgossip_success (libplugin-pay.c:1612)
==3343609==
{
   <insert_a_suppression_name_here>
   Memcheck:Cond
   fun:__printf_buffer
   fun:__vsnprintf_internal
   fun:vsnprintf
   fun:json_out_addv
   fun:json_add_primitive_fmt
   fun:json_add_u64
   fun:json_add_amount_msat
   fun:channel_hint_to_json
   fun:channel_hint_notify_core
   fun:channel_hint_notify
   fun:channel_hints_update
   fun:handle_intermediate_failure
   fun:payment_addgossip_success
}

[vincenzopalazzo]

is it possible that this is related to #8225 ? But probably this is just a forward payment