Mint.HTTP1.Request private fn validate_header_value throws Erlang error: {:bad_generator, nil} when value is nil

[marinac-dev]

If the value of header is nil it will just throw error.

** (ErlangError) Erlang error: {:bad_generator, nil}
    (mint 1.5.1) lib/mint/http1/request.ex:85: Mint.HTTP1.Request."-validate_header_value!/2-lc$^0/1-0-"/3
    (mint 1.5.1) lib/mint/http1/request.ex:85: Mint.HTTP1.Request.validate_header_value!/2
    (mint 1.5.1) lib/mint/http1/request.ex:27: anonymous fn/2 in Mint.HTT
    # . . . Stack strace

I think I could make a bug reproducing repo in free time, but what caused this in my case is a dependency.
In dependency, there is macro @api_key Application.compile_env(:my_app, :api_key) (yes I know I shouldn't use that, I will change it) which requires setting it in somewhere in config.exs but if I don't do that header value becomes nil and bum bug. :)

In my config, I use System.get_env to fetch header value
I am using Tesla with Finch adapter

[ericmj]

We do not accept nil in the headers as seen in the typespec: https://hexdocs.pm/mint/Mint.Types.html#t:headers/0. Handling nil would mean we have to make the decision if the header should be removed or the value should be "". This decision will be different for different applications so mint cannot make it.

If you are using System.get_env it takes a second argument where you can give a default value that isn't nil.

[goncalotomas]

Just ran into this issue but I'm seeing {:bad_generator, :error} instead. I'm not too proud to say this took me a while to track down, and I was wondering if it would be acceptable to return an error (e.g. {:invalid_header_value, some_invalid_value}) instead of letting it crash this deep into the call stack?

[wojtekmach]

@goncalotomas I created #468 to improve the error message.

While header strings with characters that are not allowed will result in an error tuple,

iex> {:ok, conn} = Mint.HTTP1.connect(:http, "httpbin.org", 80)
iex> Mint.HTTP.request(conn, "GET", "/json", [{"accept", "\x00"}], nil)
{:error,
 ...,
 %Mint.HTTPError{
   reason: {:invalid_header_value, "accept", <<0>>},
   module: Mint.HTTP1
 }}

I agree with above mentioned rationale that nil, or any other term, is not an accepted value so I think a FunctionClauseError/ArgumentError is more appropriate.