Accepts Extension method for null Content-Type in Minimal API

[rachna-lad]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I have used Accepts extension method to allow only specific Content-Type. Refer below code snippet for reference.
webApplication.MapPost("v1/todo", async (long id, [FromBody] TODO request) => { // Perform operation }) .Accepts<TODO>("application/json") .Produces(StatusCodes.Status415UnsupportedMediaType);
But this Accepts doesn't handle null Content-Type in header.

Expected Behavior

Should throw 415 if define Content-type is not passed in header.

Steps To Reproduce

1. Use extension method Accepts which supports only specific Content-Type otherwise throw 415. Here i have taken application/json.
2. Now make API request without request body(body is null) and Content-Type is also null
3. Verify response

Exceptions (if any)

No response

.NET Version

6.0.101

Anything else?

No response

[captainsafia]

This appears to be a bug in the way the AcceptsMatcherPolicy is applied on endpoints. In particular, it doesn't look like it is registered as a policy on the endpoint when the DfaMatcher is constructed. AFAIK, the logic below looks incomplete because it doesn't actually go in and evaluate if the endpoint has the correct metadata if it is not dynamic. It's assumed that if there are no dynamic endpoints then the AcceptsMatcher policy is not registered at all.

aspnetcore/src/Http/Routing/src/Matching/AcceptsMatcherPolicy.cs

Lines 37 to 46 in 2c55745

	bool IEndpointSelectorPolicy.AppliesToEndpoints(IReadOnlyList<Endpoint> endpoints)
	{
	if (endpoints == null)
	{
	throw new ArgumentNullException(nameof(endpoints));
	}
	// When the node contains dynamic endpoints we can't make any assumptions.
	return ContainsDynamicEndpoints(endpoints);
	}

To validate this assumption, I updated the implementation of AppliesToEndpoints to as follows:

  bool IEndpointSelectorPolicy.AppliesToEndpoints(IReadOnlyList<Endpoint> endpoints)
  {
      if (endpoints == null)
      {
          throw new ArgumentNullException(nameof(endpoints));
      }

      // When the node contains dynamic endpoints we can't make any assumptions.
      if (ContainsDynamicEndpoints(endpoints))
      {
          return false;
      }

      return AppliesToEndpointsCore(endpoints);
  }

This seems to populate some nodes in the matcher with the correct policies, however the matcher doesn't pick up those candidate nodes during routing here.

aspnetcore/src/Http/Routing/src/Matching/DfaMatcher.cs

Line 51 in 2c55745

var (candidates, policies) = FindCandidateSet(httpContext, path, segments);

A sidenote to all this analysis is that we do have content-type checks for minimal APIs generated in the RequestDelegateFactory. However, the rules are not nearly as robust as what you would get in the AcceptsMatcherPolicy and the rules behave rather esoterically depending on whether or not you have (a) provided a payload in your request and (b) provided a malformed payload.

More notes incoming...

[captainsafia]

OK, we spent some time discussing this in the team chat and @brunolins16 was able to provide some illuminating insights in this regard.

TL;DR: this is expected behavior given the way the AcceptsMatcherPolicy is expected to behave. For real-world scenarios, the user will still get a 415 in dev.

Request content types in minimal API applications are validated at two layers:

• During routing by the AcceptsMatcherPolicy
• At runtime when validated by the JSON deserialization logic codegened in the RequestDelegateFactory

What logic comes into play depends on some rules that are enforced in the AcceptsMatcherPolicy. In particular, it creates the following edges in the routing jump table:

1. Edges for each of the content types defined in Accepts metadata (just application/json for most minimal apps)
2. Edges for */*: the wildcard content-type
3. Edges for string.Empty

#1 and #3 will evaluate to the underlying endpoint. The AcceptsMatcherPolicy makes an explicit call-out that it will no-op in the event that the request contains an empty content type. That means that content-type validation falls back to the logic in the RDF. #2 will invoke the validation logic in the AcceptsMatcherPolicy which will verify whether arbitrary content types are allowed.

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