DI: backfill CodeTracker registry with iseqs for pre-loaded files

[p-datadog]

What does this PR do?

When CodeTracker starts, backfills the iseq registry with instruction sequences for files that were loaded before tracking began. Also adds a DI.iseq_type C extension (Ruby 3.1+) for precise whole-file iseq detection.

Motivation:

CodeTracker only captures files loaded after DI starts via :script_compiled TracePoint events. Most application code and all gems are loaded at boot time before DI activates, making them invisible to line probes. This change walks the Ruby object space via the all_iseqs C extension (#5111) and recovers whole-file iseqs for already-loaded code.

Implementation:

• CodeTracker#backfill_registry walks the object space via DI.file_iseqs, filters to whole-file iseqs, and stores them in the registry without overwriting entries from :script_compiled.
• Whole-file iseq detection uses DI.iseq_type on Ruby 3.1+ (returns :top for require/load, :main for the entry script). On Ruby < 3.1, falls back to first_lineno == 0 — which produces the same result because Ruby's rb_iseq_new_top and rb_iseq_new_main pass INT2FIX(0) while method/class/block definitions get their source line (>= 1).
• DI.iseq_type wraps the internal rb_iseq_type() function added in Ruby 3.1 (commit 89a02d89 by Koichi Sasada). Guarded behind have_func('rb_iseq_type') in extconf.rb — only compiled when the symbol exists.
• Backfill runs inside start after the trace point is enabled, so files loaded concurrently are captured by the trace point (backfill won't overwrite them).

Change log entry

Yes. DI/LD: more third-party libraries are now instrumentable in DI/LD.

Additional Notes:

• Depends on DI: add a C extension #5111 (the all_iseqs and exception_message C extensions)
• Only whole-file iseqs are backfilled — per-method iseqs require instrumenter changes to select the correct iseq for a target line
• Some stdlib and gem iseqs are garbage collected and unavailable via object space walk. This will be addressed in a follow-up PR (DI: support per-method iseqs for line probes on pre-loaded files #5501)

How to test the change?

Unit tests for backfill_registry (filtering, overwrite protection, error boundary, fallback path). Integration test loading a file before tracking starts, then verifying probe installation and snapshot capture work via backfill.

Manually tested in gobo by inspecting the DI code tracer registry. Before:

After:

Manual testing via gobo targeting stdlib:

[github-actions]

Typing analysis

Note: Ignored files are excluded from the next sections.

steep:ignore comments

This PR introduces 1 steep:ignore comment.

steep:ignore comments (+1-0)

❌ Introduced:

lib/datadog/di/base.rb:109

Untyped methods

This PR introduces 1 partially typed method, and clears 1 partially typed method. It increases the percentage of typed methods from 61.66% to 61.7% (+0.04%).

Partially typed methods (+1-1)

❌ Introduced:

sig/datadog/di/code_tracker.rbs:15
└── def iseqs_for_path_suffix: (String suffix) -> untyped

✅ Cleared:

sig/datadog/di/code_tracker.rbs:14
└── def iseqs_for_path_suffix: (String suffix) -> untyped

If you believe a method or an attribute is rightfully untyped or partially typed, you can add # untyped:accept on the line before the definition to remove it from the stats.

[datadog-prod-us1-6]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 99.27%
• Overall Coverage: 95.36% (+0.01%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: a058463 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-03-28 03:54:55

Comparing candidate commit a058463 in PR branch di-iseq-backfill with baseline commit 609f934 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 46 metrics, 0 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 ----------------------------------'