DI: capture real positional parameter names in non-virtual method probes

[p-datadog]

What does this PR do?

Captures real positional parameter names in probes on explicitly defined methods (i.e. NOT method_missing / C code).

Motivation:

The arg1 etc. placeholders are not discoverable by customers.

Change log entry

Yes. Dynamic Instrumentation: report positional method
argument names for methods explicitly defined in Ruby

Additional Notes:

Virtual methods (method_missing) and C code still doesn't reveal the argument names. arg1 etc. placeholders remain for those.

How to test the change?

Unit and integration tests added

[dd-octo-sts]

Typing analysis

Note: Ignored files are excluded from the next sections.

steep:ignore comments

This PR introduces 12 steep:ignore comments, and clears 12 steep:ignore comments.

steep:ignore comments (+12-12)

❌ Introduced:

lib/datadog/di/instrumenter.rb:197
lib/datadog/di/instrumenter.rb:202
lib/datadog/di/instrumenter.rb:206
lib/datadog/di/instrumenter.rb:211
lib/datadog/di/instrumenter.rb:215
lib/datadog/di/instrumenter.rb:234
lib/datadog/di/instrumenter.rb:295
lib/datadog/di/instrumenter.rb:297
lib/datadog/di/instrumenter.rb:940
lib/datadog/di/serializer.rb:284
lib/datadog/di/serializer.rb:416
lib/datadog/di/serializer.rb:531

✅ Cleared:

lib/datadog/di/instrumenter.rb:196
lib/datadog/di/instrumenter.rb:201
lib/datadog/di/instrumenter.rb:205
lib/datadog/di/instrumenter.rb:210
lib/datadog/di/instrumenter.rb:214
lib/datadog/di/instrumenter.rb:233
lib/datadog/di/instrumenter.rb:294
lib/datadog/di/instrumenter.rb:296
lib/datadog/di/instrumenter.rb:903
lib/datadog/di/serializer.rb:257
lib/datadog/di/serializer.rb:389
lib/datadog/di/serializer.rb:504

Untyped methods

This PR introduces 5 partially typed methods, and clears 5 partially typed methods. It increases the percentage of typed methods from 69.55% to 69.56% (+0.01%).

Partially typed methods (+5-5)

❌ Introduced:

sig/datadog/di/instrumenter.rbs:63
└── def run_method_probe: (::Array[untyped] args, ::Hash[::Symbol, untyped] kwargs, ::Proc? target_block, Object target_self, Probe probe, untyped responder, [::String, ::Integer]? loc, ::String method_name, ?::Array[::Symbol?]? positional_param_names) { () -> untyped } -> untyped
sig/datadog/di/instrumenter.rbs:75
└── def line_trace_point_callback: (Probe probe, RubyVM::InstructionSequence? iseq, untyped responder, TracePoint tp) -> void
sig/datadog/di/instrumenter.rbs:79
└── def check_and_disable_if_exceeded: (Probe probe, untyped responder, Float di_start_time, ?Float accumulated_duration) -> void
sig/datadog/di/serializer.rbs:24
└── def combine_args: (Array[untyped] args, Hash[Symbol, untyped] kwargs, Object target_self, ?Array[Symbol?]? param_names) -> Hash[Symbol, untyped]
sig/datadog/di/serializer.rbs:25
└── def serialize_args: (Array[untyped] args, Hash[Symbol, untyped] kwargs, untyped instance_vars, ?Array[Symbol?]? param_names, ?depth: Integer, ?attribute_count: Integer?, ?length: Integer?, ?collection_size: Integer?) -> Hash[Symbol, untyped]

✅ Cleared:

sig/datadog/di/instrumenter.rbs:63
└── def run_method_probe: (::Array[untyped] args, ::Hash[::Symbol, untyped] kwargs, ::Proc? target_block, Object target_self, Probe probe, untyped responder, [::String, ::Integer]? loc, ::String method_name) { () -> untyped } -> untyped
sig/datadog/di/instrumenter.rbs:73
└── def line_trace_point_callback: (Probe probe, RubyVM::InstructionSequence? iseq, untyped responder, TracePoint tp) -> void
sig/datadog/di/instrumenter.rbs:77
└── def check_and_disable_if_exceeded: (Probe probe, untyped responder, Float di_start_time, ?Float accumulated_duration) -> void
sig/datadog/di/serializer.rbs:24
└── def combine_args: (Array[untyped] args, Hash[Symbol, untyped] kwargs, Object target_self) -> Hash[Symbol, untyped]
sig/datadog/di/serializer.rbs:25
└── def serialize_args: (Array[untyped] args, Hash[Symbol, untyped] kwargs, untyped instance_vars, ?depth: Integer, ?attribute_count: Integer?, ?length: Integer?, ?collection_size: Integer?) -> Hash[Symbol, 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-official]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 13.64%
• Overall Coverage: 90.00% (-0.03%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 2aaa948 | Docs | Datadog PR Page | Give us feedback!}

[pr-commenter]

Benchmarks

Benchmark execution time: 2026-07-28 18:10:44

Comparing candidate commit 2aaa948 in PR branch di-method-probe-real-arg-names with baseline commit e1e1961 in branch master.

📊 Benchmarking dashboard

Found 0 performance improvements and 0 performance regressions! Performance is the same for 48 metrics, 1 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 ----------------------------------'

Unstable benchmarks

These benchmarks have a confidence interval too wide to call a change; treat them as noise rather than signal.

scenario:tracing - trace.to_digest - Continue

• unstable throughput [-1458.943op/s; +1549.838op/s] or [-4.986%; +5.296%]