Add span based feature flags

[adinauer]

📜 Description

Add addFeatureFlag method to span and transaction.

It'll allow setting a max of 10 feature flag evaluations per span.
Spans do not inherit or share flags, so each span is separate.
When the limit has been reached new values are rejected but existing values may be updated.

💡 Motivation and Context

Another part of #4763

💚 How did you test it?

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against f07c6d1

[cursor]

Bug: Feature Flag Data Handling Causes Unintended Mutations

The feature flag logic in the SentrySpan constructor directly modifies the data map parameter. This results in feature flags being silently dropped if data is null, and it mutates the original Span's data map if data is not null, which can cause unintended side effects.

 

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	296.57 ms	361.98 ms	65.41 ms
Size	1.58 MiB	2.13 MiB	555.39 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
806307f	357.85 ms	424.64 ms	66.79 ms
ee747ae	554.98 ms	611.50 ms	56.52 ms
17a0955	372.53 ms	446.70 ms	74.17 ms
2124a46	319.19 ms	415.04 ms	95.85 ms
9fbb112	404.51 ms	475.65 ms	71.14 ms
b6702b0	395.86 ms	409.98 ms	14.12 ms
ee747ae	357.79 ms	421.84 ms	64.05 ms
27d7cf8	314.17 ms	347.00 ms	32.83 ms
9fbb112	361.43 ms	427.57 ms	66.14 ms
96449e8	361.30 ms	423.39 ms	62.09 ms

App size

Revision	Plain	With Sentry	Diff
806307f	1.58 MiB	2.10 MiB	533.42 KiB
ee747ae	1.58 MiB	2.10 MiB	530.95 KiB
17a0955	1.58 MiB	2.10 MiB	533.20 KiB
2124a46	1.58 MiB	2.12 MiB	551.51 KiB
9fbb112	1.58 MiB	2.11 MiB	539.18 KiB
b6702b0	1.58 MiB	2.12 MiB	551.79 KiB
ee747ae	1.58 MiB	2.10 MiB	530.95 KiB
27d7cf8	1.58 MiB	2.12 MiB	549.42 KiB
9fbb112	1.58 MiB	2.11 MiB	539.18 KiB
96449e8	1.58 MiB	2.11 MiB	539.35 KiB

Previous results on branch: feat/feature-flags-on-spans

Startup times

Revision	Plain	With Sentry	Diff
ef06876	312.94 ms	370.33 ms	57.39 ms
cb9cf5e	313.21 ms	359.20 ms	46.00 ms
8f87720	367.76 ms	460.02 ms	92.26 ms
8a5f7d1	322.31 ms	431.93 ms	109.63 ms

App size

Revision	Plain	With Sentry	Diff
ef06876	1.58 MiB	2.12 MiB	553.23 KiB
cb9cf5e	1.58 MiB	2.12 MiB	549.76 KiB
8f87720	1.58 MiB	2.12 MiB	549.76 KiB
8a5f7d1	1.58 MiB	2.12 MiB	549.68 KiB

[lbloder]

Nice work! Added some minor comments/questions

[cursor]

Bug: Feature Flags Vanish During Context Duplication

The SpanContext copy constructor doesn't copy the newly added featureFlags field. When contexts are copied through Contexts or MonitorContexts constructors, feature flags added to the span will be lost. This causes the copied SpanContext to have an empty feature flag buffer instead of preserving the flags from the source.

sentry/src/main/java/io/sentry/SpanContext.java#L120-L148

sentry-java/sentry/src/main/java/io/sentry/SpanContext.java

Lines 120 to 148 in 36be3b2

	*/
	public SpanContext(final @NotNull SpanContext spanContext) {
	this.traceId = spanContext.traceId;
	this.spanId = spanContext.spanId;
	this.parentSpanId = spanContext.parentSpanId;
	setSamplingDecision(spanContext.samplingDecision);
	this.op = spanContext.op;
	this.description = spanContext.description;
	this.status = spanContext.status;
	final Map<String, String> copiedTags = CollectionUtils.newConcurrentHashMap(spanContext.tags);
	if (copiedTags != null) {
	this.tags = copiedTags;
	}
	final Map<String, Object> copiedUnknown =
	CollectionUtils.newConcurrentHashMap(spanContext.unknown);
	if (copiedUnknown != null) {
	this.unknown = copiedUnknown;
	}
	this.baggage = spanContext.baggage;
	final Map<String, Object> copiedData = CollectionUtils.newConcurrentHashMap(spanContext.data);
	if (copiedData != null) {
	this.data = copiedData;
	}
	}
	public void setOperation(final @NotNull String operation) {
	this.op = Objects.requireNonNull(operation, "operation is required");
	}

 

---

[cursor]

Bug: Feature Flags Attach to Wrong Active Span.

The test expects transaction-feature-flag on the transaction and my-feature-flag on the child span, but both flags are added using Sentry.addFeatureFlag() which writes to both scope and the current active span. When transaction-feature-flag is added before starting the child span, the current span is the transaction (root span). When my-feature-flag is added inside the child span, the current span is still the parent (since startChild() doesn't automatically make the child current unless ScopeBindingMode.ON is set). Both flags should end up on the transaction, not split between transaction and child span.

 

[lbloder]

LGTM 👍

[cursor]

Bug: Context Cloning: Feature Flags Vanish

The SpanContext copy constructor doesn't copy the featureFlags field, causing feature flags to be lost when contexts are cloned (e.g., when Scope is cloned via new Contexts(scope.contexts)). This results in feature flag data being discarded unexpectedly since featureFlags will be initialized to a fresh empty buffer instead of preserving the original flags.

sentry/src/main/java/io/sentry/SpanContext.java#L120-L141

sentry-java/sentry/src/main/java/io/sentry/SpanContext.java

Lines 120 to 141 in e10c410

	*/
	public SpanContext(final @NotNull SpanContext spanContext) {
	this.traceId = spanContext.traceId;
	this.spanId = spanContext.spanId;
	this.parentSpanId = spanContext.parentSpanId;
	setSamplingDecision(spanContext.samplingDecision);
	this.op = spanContext.op;
	this.description = spanContext.description;
	this.status = spanContext.status;
	final Map<String, String> copiedTags = CollectionUtils.newConcurrentHashMap(spanContext.tags);
	if (copiedTags != null) {
	this.tags = copiedTags;
	}
	final Map<String, Object> copiedUnknown =
	CollectionUtils.newConcurrentHashMap(spanContext.unknown);
	if (copiedUnknown != null) {
	this.unknown = copiedUnknown;
	}
	this.baggage = spanContext.baggage;
	final Map<String, Object> copiedData = CollectionUtils.newConcurrentHashMap(spanContext.data);
	if (copiedData != null) {
	this.data = copiedData;

 

---

[cursor]

Bug: Flags Lost in Context Duplication

The SpanContext copy constructor doesn't copy the featureFlags field, causing feature flags to be lost when contexts are cloned (e.g., via Contexts or MonitorContexts copy constructors). The field should be cloned similar to how tags and data are copied.

sentry/src/main/java/io/sentry/SpanContext.java#L120-L144

sentry-java/sentry/src/main/java/io/sentry/SpanContext.java

Lines 120 to 144 in f07c6d1

	*/
	public SpanContext(final @NotNull SpanContext spanContext) {
	this.traceId = spanContext.traceId;
	this.spanId = spanContext.spanId;
	this.parentSpanId = spanContext.parentSpanId;
	setSamplingDecision(spanContext.samplingDecision);
	this.op = spanContext.op;
	this.description = spanContext.description;
	this.status = spanContext.status;
	final Map<String, String> copiedTags = CollectionUtils.newConcurrentHashMap(spanContext.tags);
	if (copiedTags != null) {
	this.tags = copiedTags;
	}
	final Map<String, Object> copiedUnknown =
	CollectionUtils.newConcurrentHashMap(spanContext.unknown);
	if (copiedUnknown != null) {
	this.unknown = copiedUnknown;
	}
	this.baggage = spanContext.baggage;
	final Map<String, Object> copiedData = CollectionUtils.newConcurrentHashMap(spanContext.data);
	if (copiedData != null) {
	this.data = copiedData;
	}
	}

 

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