feat(ddtrace/tracer): add tracer.StartSpanFromPropagatedContext to start a span from a propagated context by darccio · Pull Request #4492 · DataDog/dd-trace-go

darccio · 2026-03-03T11:49:43Z

What does this PR do?

Adds tracer.StartSpanFromPropagatedContext to start spans from propagated context.

Implements variant D from RFC: https://docs.google.com/document/d/12s7DyHu4AqoC2u_-1aFhod1BcStRAHFlDn1tFuERlMM/edit?tab=t.0. It's A+C because using generics and restricting the carriers to TextMapReader effectively implement - at compile time - functions that don't accept carriers of any type.

It also avoids coupling ddtrace/tracer more with net/http and allows custom TextMapReader implementers. It's also more flexible if we want to support more carriers.

Motivation

Resolves #3598.

Reviewer's Checklist

Changed code has unit tests for its functionality at or near 100% coverage.
There is a benchmark for any new code, or changes to existing code.
New code is free of linting errors. You can check this by running make lint locally.
New code doesn't break existing tests. You can check this by running make test locally.
Add an appropriate team label so this PR gets put in the right place for the release notes.

Unsure? Have a question? Request a review!

codecov · 2026-03-03T11:57:33Z

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.03%. Comparing base (bd8a42c) to head (05a3625).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
ddtrace/tracer/tracer.go	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
ddtrace/tracer/tracer.go	`87.54% <66.66%> (ø)`

... and 433 files with indirect coverage changes

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❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
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pr-commenter · 2026-03-03T12:11:59Z

Benchmarks

Benchmark execution time: 2026-04-14 11:51:12

Comparing candidate commit 6a5ffed in PR branch dario.castane/poc/continue-span with baseline commit dd184a0 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 215 metrics, 9 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

🟩 = significantly better candidate vs. baseline
🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'

mtoffl01

I think this will be a nice win for our API users 🥳

But, I don't like the name ContinueSpan 😬. I don't think it's accurate -- we're not extending or "continuing" an existing span, we're creating a new one, either as a child of the parent span, or as a root span of a new trace. What about StartSpanFromCarrier? It aligns with the StartSpanFromContext format, and these functions pretty much do the same thing, except one starts a span from a carrier, and the other from a context 😉

kakkoyun

Nice work on this, Dario. The generic TextMapReader constraint is a really elegant choice — it gives us type safety at compile time while keeping things flexible for all carrier types. Much better than having to maintain separate functions per carrier.

I have a few suggestions below, mostly around error semantics and the SpanLinks pattern. Nothing blocking the direction, just things worth discussing.

…ogging it instead

datadog-official · 2026-03-11T11:25:12Z

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 63.64%
• Overall Coverage: 60.11%

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: 6a5ffed | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

darccio · 2026-03-11T11:34:50Z

Feedback applied in ad23248 and 13ef564.

genesor

Great work!

…StartSpanFromPropagatedContext

rarguelloF

nice!! LGTM 🚀

kakkoyun · 2026-03-12T14:44:42Z

@mtoffl01 it is waiting for you :)

mtoffl01

This should be a nice improvement to our user experience, thanks for doing it!!!

RomainMuller

That API surface looks fine to me at this stage.

It does not prohibit adding specific helpers (in the future) to cut some boilerplate out if we determine this is useful/necessary (i.e, StartSpanFromHTTPHeadersPropagatedContext or something like that).

…-span

feat: add tracer.ContinueSpan to start a span from a propagated context

fc2c279

darccio requested review from a team as code owners March 3, 2026 11:49

feat(ddtrace/tracer): add benchmark for tracer.ContinueSpan

486350f

darccio added the team:apm-go label Mar 3, 2026

feat(ddtrace/tracer): update API stability check file

1ef8bec

darccio added 2 commits March 3, 2026 16:44

Merge branch 'main' into dario.castane/poc/continue-span

d9b1ff4

Merge branch 'main' into dario.castane/poc/continue-span

34b5c68

mtoffl01 requested changes Mar 4, 2026

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Comment thread ddtrace/tracer/tracer.go Outdated

darccio requested review from genesor and mtoffl01 March 9, 2026 18:21

kakkoyun reviewed Mar 10, 2026

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Comment thread ddtrace/tracer/tracer.go

kakkoyun reviewed Mar 10, 2026

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Comment thread ddtrace/tracer/tracer_test.go

darccio added 3 commits March 11, 2026 09:46

fix: rename ContinueSpan as StartSpanFromPropagatedContext

aabb2c8

feat(ddtrace/tracer): change signature to avoid returning error and l…

ad23248

…ogging it instead

feat(ddtrace/tracer): process span links

13ef564

chore: make lint happy

1a48dc2

darccio changed the title ~~feat(ddtrace/tracer): add tracer.ContinueSpan to start a span from a propagated context~~ feat(ddtrace/tracer): add tracer.StartSpanFromPropagatedContext to start a span from a propagated context Mar 11, 2026

darccio mentioned this pull request Mar 11, 2026

Lack of convenient API to start span with a parent context in v2 #3598

Closed

kakkoyun approved these changes Mar 12, 2026

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kakkoyun reviewed Mar 12, 2026

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Comment thread ddtrace/tracer/api.txt Outdated

genesor approved these changes Mar 12, 2026

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darccio requested a review from a team March 12, 2026 11:29

rarguelloF reviewed Mar 12, 2026

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Comment thread ddtrace/tracer/tracer.go

feedback(ddtrace/tracer): clarify carrier and TextMapReader roles in …

05a3625

…StartSpanFromPropagatedContext

darccio requested review from a team and rarguelloF March 12, 2026 14:20

rarguelloF approved these changes Mar 12, 2026

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mtoffl01 approved these changes Mar 12, 2026

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RomainMuller approved these changes Mar 12, 2026

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darccio requested a review from felixge March 17, 2026 16:13

Merge remote-tracking branch 'origin' into dario.castane/poc/continue…

6a5ffed

…-span

darccio removed the request for review from felixge April 14, 2026 11:22

gh-worker-dd-devflow-36fce6 Bot added mergequeue-status: waiting mergequeue-status: queued mergequeue-status: in_progress and removed mergequeue-status: waiting mergequeue-status: queued labels Apr 14, 2026

gh-worker-dd-mergequeue-cf854d Bot merged commit 42ba2e8 into main Apr 14, 2026
217 checks passed

gh-worker-dd-mergequeue-cf854d Bot deleted the dario.castane/poc/continue-span branch April 14, 2026 12:08

gh-worker-dd-devflow-36fce6 Bot added mergequeue-status: done and removed mergequeue-status: in_progress labels Apr 14, 2026

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What does this PR do?

Motivation

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