[Backport 7.76.x] Rework portlist poller to open /proc/net/* fresh on each poll

[dd-octo-sts]

Backport f4d3c85 from #46927.

---

What does this PR do?

After a tentative fix, the present change implements the consensus established in a review comment.

It replaces the fd-caching + seek pattern with plain open-per-poll:

• drops procNetFiles []*os.File from linuxImpl,
• opens each /proc/net/{tcp,tcp4,udp,udp6} anew in AppendListeningPorts, closed immediately after parsing,
• eliminates lseek on /proc/net/* entirely, sidestepping the ESPIPE regression introduced in Linux 6.6.102/6.12.42.

With no fds to release, Close() became a no-op on all platforms; it is therefore removed from osImpl, Poller, and all (empty) implementations, along with TestClose and the sole external call site in port.go.

This allows to always enable portlist unit test by also reverting:

• [AGENTRUN-1108] Skip portlist unit test when kernel return ESPIPE #46909.

Motivation

Triggered by:

• #incident-50065
• #incident-50070
• #incident-50071

---

(for 7.76.x, that's #incident-50204 and #incident-50205)

[agent-platform-auto-pr]

Static quality checks

✅ Please find below the results from static quality gates
Comparison made with ancestor 58a4560
📊 Static Quality Gates Dashboard

Successful checks

Info

	Quality gate	Change	Size (prev → curr → max)
✅	agent_heroku_amd64	-4.0 KiB (0.00% reduction)	325.617 → 325.613 → 329.530
✅	iot_agent_deb_armhf	-4.0 KiB (0.01% reduction)	40.713 → 40.709 → 41.030

29 successful checks with minimal change (< 2 KiB)

	Quality gate	Current Size
✅	agent_deb_amd64	752.164 MiB
✅	agent_deb_amd64_fips	712.154 MiB
✅	agent_msi	662.484 MiB
✅	agent_rpm_amd64	752.148 MiB
✅	agent_rpm_amd64_fips	712.137 MiB
✅	agent_rpm_arm64	730.898 MiB
✅	agent_rpm_arm64_fips	693.349 MiB
✅	agent_suse_amd64	752.148 MiB
✅	agent_suse_amd64_fips	712.137 MiB
✅	agent_suse_arm64	730.898 MiB
✅	agent_suse_arm64_fips	693.349 MiB
✅	docker_agent_amd64	814.281 MiB
✅	docker_agent_arm64	817.614 MiB
✅	docker_agent_jmx_amd64	1005.192 MiB
✅	docker_agent_jmx_arm64	997.308 MiB
✅	docker_cluster_agent_amd64	181.193 MiB
✅	docker_cluster_agent_arm64	196.991 MiB
✅	docker_cws_instrumentation_amd64	7.135 MiB
✅	docker_cws_instrumentation_arm64	6.689 MiB
✅	docker_dogstatsd_amd64	38.859 MiB
✅	docker_dogstatsd_arm64	37.128 MiB
✅	dogstatsd_deb_amd64	30.074 MiB
✅	dogstatsd_deb_arm64	28.219 MiB
✅	dogstatsd_rpm_amd64	30.074 MiB
✅	dogstatsd_suse_amd64	30.074 MiB
✅	iot_agent_deb_amd64	43.041 MiB
✅	iot_agent_deb_arm64	40.143 MiB
✅	iot_agent_rpm_amd64	43.042 MiB
✅	iot_agent_suse_amd64	43.042 MiB

On-wire sizes (compressed)

	Quality gate	Change	Size (prev → curr → max)
✅	agent_deb_amd64	-14.65 KiB (0.01% reduction)	183.775 → 183.760 → 184.810
✅	agent_deb_amd64_fips	-13.62 KiB (0.01% reduction)	176.151 → 176.138 → 177.560
✅	agent_heroku_amd64	neutral	87.095 MiB → 88.450
✅	agent_msi	+28.0 KiB (0.02% increase)	143.156 → 143.184 → 143.300
✅	agent_rpm_amd64	-16.82 KiB (0.01% reduction)	186.854 → 186.838 → 188.160
✅	agent_rpm_amd64_fips	-16.39 KiB (0.01% reduction)	177.856 → 177.840 → 178.900
✅	agent_rpm_arm64	-50.37 KiB (0.03% reduction)	168.364 → 168.315 → 169.930
✅	agent_rpm_arm64_fips	-23.69 KiB (0.01% reduction)	161.504 → 161.481 → 163.120
✅	agent_suse_amd64	-16.82 KiB (0.01% reduction)	186.854 → 186.838 → 188.160
✅	agent_suse_amd64_fips	-16.39 KiB (0.01% reduction)	177.856 → 177.840 → 178.900
✅	agent_suse_arm64	-50.37 KiB (0.03% reduction)	168.364 → 168.315 → 169.930
✅	agent_suse_arm64_fips	-23.69 KiB (0.01% reduction)	161.504 → 161.481 → 163.120
✅	docker_agent_amd64	+15.36 KiB (0.01% increase)	276.383 → 276.398 → 277.400
✅	docker_agent_arm64	neutral	263.850 MiB → 266.040
✅	docker_agent_jmx_amd64	+3.6 KiB (0.00% increase)	345.047 → 345.051 → 346.020
✅	docker_agent_jmx_arm64	+7.37 KiB (0.00% increase)	328.470 → 328.477 → 330.660
✅	docker_cluster_agent_amd64	neutral	64.009 MiB → 64.510
✅	docker_cluster_agent_arm64	neutral	60.281 MiB → 61.170
✅	docker_cws_instrumentation_amd64	neutral	2.995 MiB → 3.330
✅	docker_cws_instrumentation_arm64	neutral	2.726 MiB → 3.090
✅	docker_dogstatsd_amd64	neutral	15.037 MiB → 15.820
✅	docker_dogstatsd_arm64	neutral	14.360 MiB → 14.830
✅	dogstatsd_deb_amd64	neutral	7.951 MiB → 8.790
✅	dogstatsd_deb_arm64	neutral	6.829 MiB → 7.710
✅	dogstatsd_rpm_amd64	neutral	7.960 MiB → 8.800
✅	dogstatsd_suse_amd64	neutral	7.960 MiB → 8.800
✅	iot_agent_deb_amd64	neutral	11.291 MiB → 12.040
✅	iot_agent_deb_arm64	neutral	9.652 MiB → 10.450
✅	iot_agent_deb_armhf	+2.51 KiB (0.02% increase)	9.848 → 9.851 → 10.620
✅	iot_agent_rpm_amd64	neutral	11.312 MiB → 12.060
✅	iot_agent_suse_amd64	neutral	11.312 MiB → 12.060

[cit-pr-commenter-54b7da]

Regression Detector

Regression Detector Results

Metrics dashboard
Target profiles
Run ID: e95294c3-e387-4891-9ca4-710e5773cb9b

Baseline: 42b6772
Comparison: 894d5bf
Diff

Optimization Goals: ✅ No significant changes detected

Experiments ignored for regressions

Regressions in experiments with settings containing erratic: true are ignored.

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	docker_containers_cpu	% cpu utilization	+0.87	[-2.21, +3.96]	1	Logs

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	quality_gate_metrics_logs	memory utilization	+1.98	[+1.76, +2.20]	1	Logs bounds checks dashboard
➖	otlp_ingest_metrics	memory utilization	+0.89	[+0.73, +1.04]	1	Logs
➖	docker_containers_cpu	% cpu utilization	+0.87	[-2.21, +3.96]	1	Logs
➖	ddot_logs	memory utilization	+0.44	[+0.37, +0.51]	1	Logs
➖	quality_gate_idle	memory utilization	+0.41	[+0.36, +0.45]	1	Logs bounds checks dashboard
➖	ddot_metrics_sum_cumulativetodelta_exporter	memory utilization	+0.40	[+0.17, +0.62]	1	Logs
➖	quality_gate_idle_all_features	memory utilization	+0.23	[+0.19, +0.26]	1	Logs bounds checks dashboard
➖	file_tree	memory utilization	+0.07	[+0.02, +0.12]	1	Logs
➖	otlp_ingest_logs	memory utilization	+0.04	[-0.05, +0.13]	1	Logs
➖	file_to_blackhole_1000ms_latency	egress throughput	+0.02	[-0.42, +0.45]	1	Logs
➖	uds_dogstatsd_to_api	ingress throughput	+0.01	[-0.11, +0.14]	1	Logs
➖	ddot_metrics_sum_delta	memory utilization	+0.01	[-0.20, +0.22]	1	Logs
➖	uds_dogstatsd_to_api_v3	ingress throughput	+0.01	[-0.12, +0.13]	1	Logs
➖	file_to_blackhole_0ms_latency	egress throughput	+0.01	[-0.50, +0.51]	1	Logs
➖	tcp_dd_logs_filter_exclude	ingress throughput	-0.01	[-0.11, +0.09]	1	Logs
➖	uds_dogstatsd_20mb_12k_contexts_20_senders	memory utilization	-0.02	[-0.07, +0.03]	1	Logs
➖	docker_containers_memory	memory utilization	-0.05	[-0.12, +0.03]	1	Logs
➖	file_to_blackhole_100ms_latency	egress throughput	-0.05	[-0.09, -0.01]	1	Logs
➖	file_to_blackhole_500ms_latency	egress throughput	-0.11	[-0.50, +0.28]	1	Logs
➖	ddot_metrics_sum_cumulative	memory utilization	-0.36	[-0.52, -0.19]	1	Logs
➖	tcp_syslog_to_blackhole	ingress throughput	-0.58	[-0.65, -0.51]	1	Logs
➖	ddot_metrics	memory utilization	-1.17	[-1.39, -0.96]	1	Logs
➖	quality_gate_logs	% cpu utilization	-1.91	[-3.41, -0.41]	1	Logs bounds checks dashboard

Bounds Checks: ✅ Passed

perf	experiment	bounds_check_name	replicates_passed	links
✅	docker_containers_cpu	simple_check_run	10/10
✅	docker_containers_memory	memory_usage	10/10
✅	docker_containers_memory	simple_check_run	10/10
✅	file_to_blackhole_0ms_latency	lost_bytes	10/10
✅	file_to_blackhole_0ms_latency	memory_usage	10/10
✅	file_to_blackhole_1000ms_latency	lost_bytes	10/10
✅	file_to_blackhole_1000ms_latency	memory_usage	10/10
✅	file_to_blackhole_100ms_latency	lost_bytes	10/10
✅	file_to_blackhole_100ms_latency	memory_usage	10/10
✅	file_to_blackhole_500ms_latency	lost_bytes	10/10
✅	file_to_blackhole_500ms_latency	memory_usage	10/10
✅	quality_gate_idle	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_idle	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_idle_all_features	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_idle_all_features	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_logs	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_logs	lost_bytes	10/10	bounds checks dashboard
✅	quality_gate_logs	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	cpu_usage	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	lost_bytes	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	memory_usage	10/10	bounds checks dashboard

Explanation

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".

CI Pass/Fail Decision

✅ Passed. All Quality Gates passed.

• quality_gate_idle_all_features, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle_all_features, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check lost_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check lost_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check cpu_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.