Fix BaggageBuilder.put() silently accepting empty string keys

[itsmehotpants]

Problem

The W3C Baggage spec requires that a baggage-name be a non-empty token (§3 definition). An empty string "" is therefore an invalid key and should be silently ignored, the same way null keys already are.

Currently, ImmutableBaggage.Builder.put() accepts empty string keys and stores them. When the W3CBaggagePropagator later serialises the Baggage into a header, it includes the empty-key entry, which can produce malformed baggage headers like:

baggage: =somevalue, valid-key=other

This corrupts propagation for downstream services.

Inconsistency with the propagator

The parsing path (W3CBaggagePropagator.isValidBaggageKey) already correctly rejects empty/blank keys when reading incoming headers:

// W3CBaggagePropagator.java
private static boolean isValidBaggageKey(String name) {
  return name != null && !name.trim().isEmpty() && StringUtils.isPrintableString(name);
}

This PR closes the same gap on the programmatic builder path.

Change

• ImmutableBaggage.Builder.put(): add key.isEmpty() to the existing early-return null-guard
• BaggageBuilder.java: document the empty-key contract in Javadoc (links W3C spec)
• ImmutableBaggageTest: correct the existing put_keyEmpty test (it was asserting the buggy behaviour); add put_keyEmpty_withMetadata variant

Fixes #8657

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: itsmehotpants / name: itsmehotpants (a669383)

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Pull request dashboard status

Waiting on the author · refreshed 2026-08-07 07:35 UTC

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.48%. Comparing base (6678f8e) to head (0b9a556).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8660      +/-   ##
============================================
+ Coverage     91.46%   91.48%   +0.01%     
+ Complexity    10457    10456       -1     
============================================
  Files          1021     1021              
  Lines         27647    27647              
  Branches       3242     3242              
============================================
+ Hits          25288    25293       +5     
+ Misses         1616     1611       -5     
  Partials        743      743              

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[ivamly]

isEmpty() only checks for an empty string, but doesn't account for whitespace. According to the spec, a key cannot consist solely of whitespace, so maybe isBlank() should be used instead?

[opentelemetry-pr-dashboard]

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