[BUG] Request body is not captured if CaptureBody is "errors"

[mstrukov]

APM Agent version

1.20.0

Environment

Linux, Kubernetes, .NET 6, Web API

Describe the bug

I'd like to capture request body for failed transactions only so I set "CaptureBody": "errors" in the application config file but the requests body is not captured and http.request.body.original is N/A

On the Errors tab http.request.body.original is [REDACTED]

However, the request body is captured both for failed and successful transactions if I set "CaptureBody": "all" .

To Reproduce

Steps to reproduce the behavior:

1. Create a basic api with a post action which accepts a model
2. Make sure the action fails with an exception (unhandled) and 500 error code is returned
3. Set "CaptureBody": "errors"
4. Call the action and pass a json to fill the model
5. Verify that the response code is 500

Expected behavior

http.request.body.original contains the json passed when calling the action

Actual behavior

http.request.body.original is "N/A".

Again, it works as expected if CaptureBody is "all" and I can see http.request.body.original populated with actual request body both on Transactions and Errors tabs but I want request body for failed transactions or errors only.

[mstrukov]

Update:
It is important to have a model for an action which is automatically parsed from a request body by the built-in model binding mechanism to reproduce the issue. By default, it is not possible to read a request body twice, so if it is already done by model binding then Elastic APM middleware cannot do it any more if CaptureBody is "errors". For this to work, EnableBuffering() should be called before request body is read for the first time. If CaptureBody is "errors" then EnableBuffering() is called after the action execution and doesn't make any change already because the body has already been read by the model binding mechanism. It works fine if CaptureBody is "all" bacause in this case EnableBuffering() is called before action execution.

Workaround:
I had to add the following resource filter to call EnableBuffering() if CaptureBody is "errors" and set the Body stream position to 0 after the action execution.

public class ElasticCaptureBodyFilter : IResourceFilter
  {
      private readonly IConfiguration _configuration;

      public ElasticCaptureBodyFilter(IConfiguration configuration)
      {
          _configuration = configuration;
      }

      public void OnResourceExecuted(ResourceExecutedContext context)
      {
          if (IsCaptureBodyForErrorsEnabled())
          {
              context.HttpContext.Request.Body.Position = 0;
          }
      }

      public void OnResourceExecuting(ResourceExecutingContext context)
      {
          if (IsCaptureBodyForErrorsEnabled())
          { 
              context.HttpContext.Request.EnableBuffering();
          }
      }

      private bool IsCaptureBodyForErrorsEnabled()
      {
          return _configuration["ElasticApm:CaptureBody"] == "errors";
      }
  }

But I would prefer if it is fixed in the package and I don't have to make any workarounds if I use default model binding mechanism and want to record request body for failed requests only.

[danielcedric]

+1
I vote for fixing this bug, we also had to implement the workaround proposed by @mstrukov.