libbeat/testing: use ephemeral ports to avoid TOCTOU collisions

[orestisfl]

Proposed commit message

libbeat/testing: use ephemeral ports to avoid TOCTOU collisions

Allows us to stress-test OTel tests.

Tests that needed a free TCP port called MustAvailableTCP4Port, which
bound :0, read the assigned port, closed the listener, and returned the
number. That available port can become unavailable before the caller
tries to use it. Running the same test many times in parallel (e.g. via
script/stresstest.sh -p 30) makes those cross-process collisions likely,
producing flaky bind failures.

Details:

Remove the MustAvailableTCP4Port(s) and AvailableTCP4Port(s) helpers and
convert every caller to let the OS assign an ephemeral port at bind time
and read the bound port back:

- OTel integration tests (filebeat/metricbeat/packetbeat) set
  http.port: 0 and discover the resolved monitoring port from the
  "Metrics endpoint listening on:" log line via the new shared
  libbeat/testing.ParseMonitoringPort, exposed through
  oteltestcol.Collector.MonitoringPort(s) (collector logs) and
  integration.BeatProc.MonitoringPort (beat log file).
- packetbeat's test HTTP servers bind ephemeral ports and the sniffer
  is configured from the resolved port.
- the filebeat ES-outage test and the heartbeat connection-refused
  tests bind :0 in-process, releasing the port for the
  connection-refused / restart cases.

The test OTel collector is also made parallel-safe: oteltestcol.New
merges service.telemetry.metrics.level: none into every collector
config so collectors no longer bind the fixed :8888 Prometheus port,
and Collector.Shutdown blocks until the collector has fully exited so
TestNoDuplicates can restart a collector without polling :8888 as an
exit signal. The now-redundant per-config telemetry metrics-off blocks
are removed.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• [ ] I have added an entry in ./changelog/fragments — test-only change, skip-changelog applied.

Disruptive User Impact

None. The change is limited to test code and helpers.

How to test this PR locally

The flakiness this fixes only shows up under parallel stress, so verify both the unit and integration tests stay green under load.

Unit tests

script/stresstest.sh ./heartbeat/monitors/active/tcp '^TestConnectionRefusedEndpointJob$' -p 30
script/stresstest.sh ./heartbeat/monitors/active/http '^TestConnRefusedJob$' -p 30

Both ran ~5 minutes with 0 failures (~150k and ~139k runs respectively).

Integration tests (OTel collectors)

The collectors no longer bind any fixed port, so many can run concurrently against a single Elasticsearch. Bring Elasticsearch up once, then stress each beat's OTel tests in parallel:

cd x-pack/filebeat && mage docker:composeUp   # one-time; the ES on :9200 is shared by all three beats
cd ../..

# Each OTel E2E run creates a per-run data stream. Raise the shard limit so a
# long parallel run does not hit the default 1000-shards-per-node cap
curl -s -u admin:testing -XPUT localhost:9200/_cluster/settings \
  -H 'Content-Type: application/json' \
  -d '{"persistent":{"cluster.max_shards_per_node":100000}}'

script/stresstest.sh --tags integration ./x-pack/filebeat/tests/integration   '^TestFilebeatOTelE2E$' -p 12
script/stresstest.sh --tags integration ./x-pack/filebeat/tests/integration   '^TestNoDuplicates$' -p 6
script/stresstest.sh --tags integration ./x-pack/metricbeat/tests/integration '^TestMetricbeatOTelE2E$|^TestMetricbeatOTelMultipleReceiversE2E$' -p 10
script/stresstest.sh --tags integration ./x-pack/packetbeat/tests/integration '^TestPacketbeatOTelE2E$|^TestPacketbeatOTelMultipleReceiversE2E$|^TestPacketbeatOTelBeatE2E$' -p 10

[github-actions]

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[infra-vault-gh-plugin-prod]

Pinging @elastic/obs-ds-hosted-services (Team:obs-ds-hosted-services)

[infra-vault-gh-plugin-prod]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[infra-vault-gh-plugin-prod]

Pinging @elastic/sec-linux-platform (Team:Security-Linux Platform)

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR removes the libbeat/testing package's AvailableTCP4Port* helper functions and migrates all call sites to OS-assigned ephemeral ports via net.Listen(":0"). It adds ParseMonitoringPort and MonitoringEndpointSnippet to libbeat/tests/integration for extracting the bound monitoring port from Beat startup logs, plus a BeatProc.MonitoringPort(timeout) polling helper. The oteltestcol.Collector harness gains a done channel and sync.Once for deterministic shutdown, and now writes a metrics-off.yaml config override merged via a multi-URI newCollectorSettings. All OTel integration tests across filebeat, metricbeat, and packetbeat are updated to set http.port: 0 and retrieve actual ports via collector.MonitoringPort(t) / MonitoringPorts(t, n).

Possibly related PRs

• elastic/beats#51091: Modifies the same celOTelConfig collector configuration in otel_cel_test.go that this PR updates.
• elastic/beats#51428: Introduces the packetbeat OTel integration test suite in otel_test.go whose monitoring-port handling this PR refactors.

Suggested labels

bugfix

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

• 🛠️ Update Documentation

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@filebeat/input/net/tcp/input_test.go`:
- Line 106: The benchmark and related connection-refused tests still use
ephemeralTCPAddr, which closes the listener before inp.Run binds, leaving a
TOCTOU race on the chosen port. Replace this bind-then-close pattern in
TestInputRunTCP and the other affected tests with a setup that keeps the
listener reserved until the TCP input is ready to accept, or otherwise
coordinate the bind so serverAddr is not exposed prematurely. Use the existing
inp.Run and serverAddr call sites to update the test flow consistently across
this cohort.

In `@x-pack/filebeat/tests/integration/otel_test.go`:
- Around line 1487-1496: The ephemeral port selection in the mock Elasticsearch
setup reintroduces a TOCTOU race by closing the listener before the mock server
owns the port. Update the setup around the port reservation logic in the test
helpers that use createMockServer so the server keeps ownership of the bound
endpoint throughout its lifecycle. Prefer an always-bound mock endpoint and
toggle outage behavior at the handler/transport layer instead of releasing
serverPort early. Apply the same fix anywhere the same port reservation pattern
is duplicated in this test.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Enterprise

Run ID: f585fc90-bb72-4ec9-af43-5b6638b3a806

📥 Commits

Reviewing files that changed from the base of the PR and between 7764586 and 0e6f94d.

📒 Files selected for processing (14)

• filebeat/input/net/tcp/input_test.go
• heartbeat/monitors/active/http/http_test.go
• heartbeat/monitors/active/tcp/tcp_test.go
• libbeat/testing/available_port.go
• libbeat/testing/available_port_test.go
• libbeat/tests/integration/framework.go
• libbeat/tests/integration/monitoring.go
• x-pack/filebeat/tests/integration/otel_cel_test.go
• x-pack/filebeat/tests/integration/otel_kafka_test.go
• x-pack/filebeat/tests/integration/otel_lsexporter_test.go
• x-pack/filebeat/tests/integration/otel_test.go
• x-pack/metricbeat/tests/integration/otel_test.go
• x-pack/otel/oteltestcol/collector.go
• x-pack/packetbeat/tests/integration/otel_test.go

💤 Files with no reviewable changes (5)

• libbeat/testing/available_port_test.go
• libbeat/testing/available_port.go
• x-pack/filebeat/tests/integration/otel_cel_test.go
• x-pack/filebeat/tests/integration/otel_kafka_test.go
• x-pack/filebeat/tests/integration/otel_lsexporter_test.go

[orestisfl]

@elastic/sec-linux-platform @elastic/obs-ds-hosted-services can i get a review please?