W3CBaggagePropagator drops valid empty baggage values

[Kielek]

Describe the bug

W3CBaggagePropagator does not extract baggage members with an empty value, for example baggage: key1=.

According to the W3C baggage grammar, value = *baggage-octet, so an empty value is valid. Java currently treats this as invalid and drops the entry.

Steps to reproduce

Extract baggage from a carrier containing:

baggage: key1=

For example, the current test behavior is:

Context result =
    W3CBaggagePropagator.getInstance()
        .extract(Context.root(), ImmutableMap.of("baggage", "key1="), getter);

assertThat(Baggage.fromContext(result)).isEqualTo(Baggage.empty());

What did you expect to see?

The extracted baggage should contain key1 with an empty string value:

Baggage.builder().put("key1", "").build()

What did you see instead?

The entry is dropped and extraction returns empty baggage.

What version and what artifacts are you using?

Artifacts: opentelemetry-api

Version: current main

How did you reference these artifacts? N/A

Environment

Compiler: N/A

OS: N/A

Runtime: N/A

OS, if different from OS compiled on: N/A

Additional context

This was found while reviewing open-telemetry/opentelemetry-dotnet#7376. with W3C baggage behavior across four OpenTelemetry implementations: .NET, Go, Java, and Rust.

The W3C baggage definition allows empty values:

list-member = key OWS "=" OWS value *( OWS ";" OWS property )
value       = *baggage-octet

Because value is zero-or-more baggage-octet, key1= is valid.

The other reviewed implementations accept empty baggage values:

• OpenTelemetry .NET accepts SomeKey=.
• OpenTelemetry Go accepts key1=.
• OpenTelemetry Rust accepts key1=.

Java appears to be the outlier by treating empty values as invalid.

[dahyvuun]

Hi I'd like to work on this. I'll look into the W3CBaggagePropagator parsing logic to allow empty baggage values per the W3C spec.

[jack-berg]

Thanks for the report @Kielek!

Go for it @dahyvuun!