`AddHeaderPropagation()` without providing `Action<HeaderPropagationOptions> configureOptions` prevents middleware from working correctly

[jjanuszkiewicz]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When trying to configure the header propagation feature using the HeaderPropagationServiceCollectionExtensions.AddHeaderPropagation(this IServiceCollection services) (
link) method, there is no HeaderPropagationOptions instance registered, so the middleware doesn't capture any headers and header propagation won't work, even when header names are specified for a specific HttpClient, e.g.:

services.AddHeaderPropagation();
services.AddHttpClient<FooClient>().AddHeaderPropagation(o => o.Headers.Add("Authorization"));
services.AddHttpClient<BarClient>().AddHeaderPropagation(o => o.Headers.Add("X-Another-Header"));

This results in no headers being forwarded.

Instead, this needs to be done:

services.AddHeaderPropagation(o =>
{
    o.Headers.Add("Authorization");
    o.Headers.Add("X-Another-Header");
});
services.AddHttpClient<FooClient>().AddHeaderPropagation(o => o.Headers.Add("Authorization"));
services.AddHttpClient<BarClient>().AddHeaderPropagation(o => o.Headers.Add("X-Another-Header"));

This is especially problematic if different HttpClients require forwarding different headers - the services.AddHeaderPropagation call needs to list all those headers, which is code duplication and leads to bugs.

I propose to either:

1. Make AddHeaderPropagation(this IServiceCollection services) internal, as it's useless for users of this library. This way all headers still need to be listed explicitly in the services.AddHeaderPropagation() call, but at least the not working method is hidden.
2. Change AddHeaderPropagation(Action<HeaderPropagationMessageHandlerOptions>) so that it adds the configured headers to the registered HeaderPropagationOptions (and/or registers it if it's not yet registered).

I can create a PR for either of these (I prefer option 2), but looking for some feedback first.

Expected Behavior

No response

Steps To Reproduce

No response

Exceptions (if any)

No response

.NET Version

No response

Anything else?

No response

[kavatari]

@jjanuszkiewicz Did you register the middleware?
app.UseHeaderPropagation()

see: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-6.0#header-propagation-middleware

also an example in this repo: https://github.com/dotnet/aspnetcore/blob/main/src/Middleware/HeaderPropagation/samples/HeaderPropagationSample/Startup.cs

[jjanuszkiewicz]

Yes, I register the middleware.

The problem is that:

• HeaderPropagationMiddleware takes an instance of IOptions<HeaderPropagationOptions> and captures only the headers included in those options
• but the parameterless IServiceCollection.AddHeaderPropagation() method doesn't register the options,
• so even if options are later provided for a specific HttpClient via IHttpClientBuilder.AddHeaderPropagation(Action<HeaderPropagationMessageHandlerOptions> configure) it doesn't make the middleware capture any headers, because this method doesn't register or modify the registered instance of HeaderPropagationOptions.

My suggestion 2 would address that:

Change AddHeaderPropagation(Action) so that it adds the configured headers to the registered HeaderPropagationOptions (and/or registers it if it's not yet registered).

[jjanuszkiewicz]

Like I said, I can create a PR with proposed changes and hopefully also with some tests demonstrating the problem, but I'd like to get some feedback first (e.g. if this is for some reason by design and the PR won't be considered for merging?)

[davidfowl]

API changes require an API proposal and review and number 1 is a non-starter because it's a binary breaking change.

[rmja]

I am seeing this as well. I was expecting services.AddHttpClient<FooClient>().AddHeaderPropagation(o => o.Headers.Add("Authorization")); to automatically add Authorization to the set of headers being captured by the middleware.

[jjanuszkiewicz]

If I have some spare time during holiday, I'll create a MR with tests showing the problem and maybe a possible solution (#2 from my original post). If time permits.