Greenfield API: Unhandled error on "preview proposed store on-chain payment method"

Describe the bug On setting the accountKeyPath data of the preview proposed store on-chain payment methods BTCPay Server shows an unhandled Exception error in the logs.

To Reproduce the bug On the mentioned endpoint when you set the derivationScheme with a legacy tpub key and the accountKeyPath to m/84'/0'/0' it throws an internal server error.

Expected behavior Should return other http status code with meaningful error message.

Your BTCPay Environment (please complete the following information):

• BTCPay Server Version: v1.2.1.0
• Deployment Method: Docker

Logs (if applicable)

2021-08-19 21:41:08.868 +00:00 [ERR] Connection id "0HMAUJTM9RLGR", Request id "0HMAUJTM9RLGR:00000001": An unhandled exception was thrown by the application.
NBitcoin.JsonConverters.JsonObjectException: Invalid rooted key path
   at NBitcoin.JsonConverters.KeyPathJsonConverter.ReadJson(JsonReader reader, Type objectType, Object existingValue, JsonSerializer serializer)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.SetPropertyValue(JsonProperty property, JsonConverter propertyConverter, JsonContainerContract containerContract, JsonProperty containerProperty, JsonReader reader, Object target)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.PopulateObject(Object newObject, JsonReader reader, JsonObjectContract contract, JsonProperty member, String id)
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Formatters.NewtonsoftJsonInputFormatter.ReadRequestBodyAsync(InputFormatterContext context, Encoding encoding)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Binders.BodyModelBinder.BindModelAsync(ModelBindingContext bindingContext)
   at Microsoft.AspNetCore.Mvc.ModelBinding.ParameterBinder.BindModelAsync(ActionContext actionContext, IModelBinder modelBinder, IValueProvider valueProvider, ParameterDescriptor parameter, ModelMetadata metadata, Object value)
   at Microsoft.AspNetCore.Mvc.Controllers.ControllerBinderDelegateProvider.<>c__DisplayClass0_0.<<CreateBinderDelegate>g__Bind|0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeInnerFilterAsync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at BTCPayServer.Hosting.BTCPayMiddleware.Invoke(HttpContext httpContext) in /source/BTCPayServer/Hosting/BTCpayMiddleware.cs:line 95
   at Microsoft.AspNetCore.Diagnostics.StatusCodePagesMiddleware.Invoke(HttpContext context)
   at BTCPayServer.Hosting.HeadersOverrideMiddleware.Invoke(HttpContext httpContext) in /source/BTCPayServer/Hosting/HeadersOverrideMiddleware.cs:line 30
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)

Additional context

In the API docs the accountKeyPath example seems to have a fingerprint prefixed e.g. abcd82a1/84'/0'/0' from what I saw in searching some code it seems to be a fingerprint of the derivationScheme? Not sure how this get computed, any link to read about it + maybe docs need to be more explicit? Nevertheless using the normal account path should also work and if not it should throw a proper error I assume.

[NicolasDorier]

Updated the doc documenting an unused field, and also fixed the underlying issue of not properly reporting when we can’t deserialize a JsonException.

[NicolasDorier]

Other problem is that this call shouldn’t even have a accountKey parameter, we don’t use it.