fix(lib-injection): don't close stdout/stderr

[betterengineering]

Overview

This commit resolves an issue where telemetry forwarding never completes when DD_APM_INSTRUMENTATION_DEBUG is enabled. We are closing the pipe from the Python side, which means the telemetry forwarder will fill the buffer and be unable to complete.

Test

I updated the telemetry-forwarder.sh to look like the following:

#!/usr/bin/env bash

# Generate 64KB of output on stderr (writes to fd 2)
# This creates 1024 lines of 64 bytes each = 64KB
for i in {1..1024}; do
  echo "This is a line of telemetry data to fill the pipe buffer ---- $i" >&2
done

# Write stdin + $1 to a file as before
echo "$1	$(</dev/stdin)" >>mock-telemetry.out

I then ran this on main and this branch to confirm mock-telemetry.out is written on this branch but never gets written on main:

DD_APM_INSTRUMENTATION_DEBUG="true" DD_INJECTION_ENABLED=true DD_TRACE_DEBUG=true PYTHONPATH=/Users/mark.spicer/go/src/github.com/DataDog/dd-trace-py/lib-injection/sources:$PYTHONPATH DD_TELEMETRY_FORWARDER_PATH=/Users/mark.spicer/go/src/github.com/DataDog/dd-trace-py/lib-injection/sources/telemetry-forwarder.sh python3 /Users/mark.spicer/go/src/github.com/DataDog/injector-dev/apps/python/app.py

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/resolve-telemetry-forwarder-bug-e71f154d954eda4c.yaml  @DataDog/apm-python
lib-injection/sources/sitecustomize.py                                  @DataDog/apm-core-python

[brettlangdon]

is there a system test we are able to pair with this as well to show it is working in this PR?

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 282 ± 4 ms.

The average import time from base is: 283 ± 4 ms.

The import time difference between this PR and base is: -0.8 ± 0.2 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 1.901 ms (0.67%)

ddtrace.bootstrap.sitecustomize 1.225 ms (0.43%)

ddtrace.bootstrap.preload 1.225 ms (0.43%)

ddtrace.internal.remoteconfig.client 0.637 ms (0.23%)

ddtrace 0.676 ms (0.24%)

ddtrace.internal._unpatched 0.030 ms (0.01%)

json 0.030 ms (0.01%)

json.decoder 0.030 ms (0.01%)

re 0.030 ms (0.01%)

enum 0.030 ms (0.01%)

types 0.030 ms (0.01%)

[betterengineering]

@brettlangdon associated system-tests: DataDog/system-tests#4910

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-07-14 21:28:20

Comparing candidate commit 089ecaa in PR branch mark.spicer/INPLAT-448-resolve-telemetry-forwarder-bug with baseline commit ab1f13d in branch main.

Found 0 performance improvements and 3 performance regressions! Performance is the same for 545 metrics, 2 unstable metrics.

scenario:iastaspects-format_map_aspect

• 🟥 execution_time [+442.538ns; +532.894ns] or [+13.671%; +16.463%]

scenario:iastaspects-replace_aspect

• 🟥 execution_time [+661.858ns; +763.145ns] or [+13.967%; +16.104%]

scenario:iastaspectsospath-ospathjoin_aspect

• 🟥 execution_time [+821.735ns; +996.715ns] or [+13.447%; +16.310%]

[brettlangdon]

if we can clean up the unnecessary changes to system-tests.yml, then lgtm

[brettlangdon]

system tests changes are coming, but we need the fix merged first.

I have verified with @betterengineering that locally the system tests are passing with this change.