Drop redundant rootPath from fleet installer config

[rdesgroppes]

What does this PR do?

1. remove the rootPath parameter from FileOperation.apply(), replacing filepath.Join(rootPath, ...) calls with filepath.Join(root.Name(), ...).
2. pass *os.Root into setFileOwnershipAndPermissions and use root.Chmod() in place of os.Chmod(), but:
   • root.Chown() is not used as it requires numeric uid/gid,
   • file.Chown() still handles the name lookup internally and requires a full path.

Motivation

1. address a trivial TODO, follow-up of Use Go 1.25 os.Root API in fleet installer config #46592: once root.RemoveAll() replaced the last os-level filepath.Join(rootPath, ...) call, rootPath's sole remaining purpose was feeding setFileOwnershipAndPermissions: root.Name() returns the equivalent value,
2. address review comment: use os.Root APIs where possible instead of reconstructing full paths at call sites.

Describe how you validated your changes

Existing unit tests pass unchanged.

[agent-platform-auto-pr]

Static quality checks

✅ Please find below the results from static quality gates
Comparison made with ancestor cded3a1
📊 Static Quality Gates Dashboard
🔗 SQG Job

Successful checks

Info

	Quality gate	Change	Size (prev → curr → max)
✅	agent_heroku_amd64	+3.97 KiB (0.00% increase)	323.754 → 323.758 → 329.530
✅	agent_rpm_arm64_fips	+4.31 KiB (0.00% increase)	696.201 → 696.205 → 703.390
✅	agent_suse_arm64_fips	+4.31 KiB (0.00% increase)	696.201 → 696.205 → 703.390
✅	docker_cluster_agent_amd64	+24.0 KiB (0.01% increase)	192.318 → 192.342 → 193.610
✅	docker_cluster_agent_arm64	+63.99 KiB (0.03% increase)	207.582 → 207.644 → 208.190

26 successful checks with minimal change (< 2 KiB)

	Quality gate	Current Size
✅	agent_deb_amd64	755.988 MiB
✅	agent_deb_amd64_fips	715.077 MiB
✅	agent_msi	622.073 MiB
✅	agent_rpm_amd64	755.972 MiB
✅	agent_rpm_amd64_fips	715.061 MiB
✅	agent_rpm_arm64	734.126 MiB
✅	agent_suse_amd64	755.972 MiB
✅	agent_suse_amd64_fips	715.061 MiB
✅	agent_suse_arm64	734.126 MiB
✅	docker_agent_amd64	817.022 MiB
✅	docker_agent_arm64	819.911 MiB
✅	docker_agent_jmx_amd64	1007.933 MiB
✅	docker_agent_jmx_arm64	999.605 MiB
✅	docker_cws_instrumentation_amd64	7.135 MiB
✅	docker_cws_instrumentation_arm64	6.689 MiB
✅	docker_dogstatsd_amd64	38.500 MiB
✅	docker_dogstatsd_arm64	36.812 MiB
✅	dogstatsd_deb_amd64	29.720 MiB
✅	dogstatsd_deb_arm64	27.881 MiB
✅	dogstatsd_rpm_amd64	29.720 MiB
✅	dogstatsd_suse_amd64	29.720 MiB
✅	iot_agent_deb_amd64	42.617 MiB
✅	iot_agent_deb_arm64	39.723 MiB
✅	iot_agent_deb_armhf	40.447 MiB
✅	iot_agent_rpm_amd64	42.618 MiB
✅	iot_agent_suse_amd64	42.618 MiB

On-wire sizes (compressed)

	Quality gate	Change	Size (prev → curr → max)
✅	agent_deb_amd64	+19.91 KiB (0.01% increase)	185.426 → 185.446 → 186.090
✅	agent_deb_amd64_fips	-39.45 KiB (0.02% reduction)	176.326 → 176.288 → 180.330
✅	agent_heroku_amd64	+2.89 KiB (0.00% increase)	87.103 → 87.106 → 88.440
✅	agent_msi	+4.0 KiB (0.00% increase)	149.215 → 149.219 → 154.470
✅	agent_rpm_amd64	-16.7 KiB (0.01% reduction)	187.310 → 187.294 → 189.170
✅	agent_rpm_amd64_fips	+27.43 KiB (0.02% increase)	178.398 → 178.424 → 181.060
✅	agent_rpm_arm64	+11.34 KiB (0.01% increase)	169.718 → 169.729 → 170.020
✅	agent_rpm_arm64_fips	+45.52 KiB (0.03% increase)	162.456 → 162.500 → 164.130
✅	agent_suse_amd64	-16.7 KiB (0.01% reduction)	187.310 → 187.294 → 189.170
✅	agent_suse_amd64_fips	+27.43 KiB (0.02% increase)	178.398 → 178.424 → 181.060
✅	agent_suse_arm64	+11.34 KiB (0.01% increase)	169.718 → 169.729 → 170.020
✅	agent_suse_arm64_fips	+45.52 KiB (0.03% increase)	162.456 → 162.500 → 164.130
✅	docker_agent_amd64	+6.25 KiB (0.00% increase)	277.759 → 277.765 → 279.410
✅	docker_agent_arm64	neutral	265.053 MiB → 267.960
✅	docker_agent_jmx_amd64	neutral	346.405 MiB → 348.040
✅	docker_agent_jmx_arm64	neutral	329.696 MiB → 332.560
✅	docker_cluster_agent_amd64	+4.09 KiB (0.01% increase)	67.199 → 67.203 → 68.000
✅	docker_cluster_agent_arm64	+9.98 KiB (0.02% increase)	63.179 → 63.189 → 63.640
✅	docker_cws_instrumentation_amd64	neutral	2.995 MiB → 3.330
✅	docker_cws_instrumentation_arm64	neutral	2.726 MiB → 3.090
✅	docker_dogstatsd_amd64	neutral	14.900 MiB → 15.820
✅	docker_dogstatsd_arm64	neutral	14.239 MiB → 14.830
✅	dogstatsd_deb_amd64	neutral	7.854 MiB → 8.790
✅	dogstatsd_deb_arm64	neutral	6.741 MiB → 7.710
✅	dogstatsd_rpm_amd64	neutral	7.865 MiB → 8.800
✅	dogstatsd_suse_amd64	neutral	7.865 MiB → 8.800
✅	iot_agent_deb_amd64	neutral	11.239 MiB → 12.040
✅	iot_agent_deb_arm64	neutral	9.601 MiB → 10.450
✅	iot_agent_deb_armhf	neutral	9.805 MiB → 10.620
✅	iot_agent_rpm_amd64	neutral	11.257 MiB → 12.060
✅	iot_agent_suse_amd64	neutral	11.257 MiB → 12.060

[cit-pr-commenter-54b7da]

Regression Detector

Regression Detector Results

Metrics dashboard
Target profiles
Run ID: 7eb252d9-3697-467a-88e1-42d4eda07438

Baseline: a88881b
Comparison: b774baf
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	-2.80	[-5.79, +0.18]	1	Logs

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	ddot_metrics_sum_cumulative	memory utilization	+0.52	[+0.37, +0.67]	1	Logs
➖	ddot_metrics	memory utilization	+0.34	[+0.11, +0.57]	1	Logs
➖	quality_gate_metrics_logs	memory utilization	+0.20	[+0.00, +0.40]	1	Logs bounds checks dashboard
➖	quality_gate_idle_all_features	memory utilization	+0.18	[+0.15, +0.22]	1	Logs bounds checks dashboard
➖	otlp_ingest_metrics	memory utilization	+0.15	[-0.01, +0.30]	1	Logs
➖	quality_gate_logs	% cpu utilization	+0.01	[-1.50, +1.51]	1	Logs bounds checks dashboard
➖	uds_dogstatsd_to_api_v3	ingress throughput	+0.00	[-0.12, +0.13]	1	Logs
➖	file_to_blackhole_1000ms_latency	egress throughput	-0.00	[-0.42, +0.42]	1	Logs
➖	uds_dogstatsd_to_api	ingress throughput	-0.01	[-0.13, +0.12]	1	Logs
➖	tcp_dd_logs_filter_exclude	ingress throughput	-0.01	[-0.10, +0.09]	1	Logs
➖	file_to_blackhole_0ms_latency	egress throughput	-0.02	[-0.48, +0.44]	1	Logs
➖	file_to_blackhole_500ms_latency	egress throughput	-0.05	[-0.42, +0.32]	1	Logs
➖	ddot_metrics_sum_cumulativetodelta_exporter	memory utilization	-0.06	[-0.29, +0.17]	1	Logs
➖	file_to_blackhole_100ms_latency	egress throughput	-0.07	[-0.11, -0.02]	1	Logs
➖	file_tree	memory utilization	-0.16	[-0.21, -0.11]	1	Logs
➖	quality_gate_idle	memory utilization	-0.17	[-0.22, -0.13]	1	Logs bounds checks dashboard
➖	uds_dogstatsd_20mb_12k_contexts_20_senders	memory utilization	-0.23	[-0.28, -0.18]	1	Logs
➖	ddot_metrics_sum_delta	memory utilization	-0.31	[-0.50, -0.12]	1	Logs
➖	docker_containers_memory	memory utilization	-0.33	[-0.41, -0.26]	1	Logs
➖	otlp_ingest_logs	memory utilization	-0.34	[-0.43, -0.24]	1	Logs
➖	ddot_logs	memory utilization	-0.53	[-0.59, -0.47]	1	Logs
➖	tcp_syslog_to_blackhole	ingress throughput	-0.81	[-0.89, -0.73]	1	Logs
➖	docker_containers_cpu	% cpu utilization	-2.80	[-5.79, +0.18]	1	Logs

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

[clarkb7]

❓ question
should setFileOwnershipAndPermissions use os.Root instead of filepath.Join ?

[rdesgroppes]

@clarkb7 I've added a 2nd commit that I hope addresses your comment, esp.:

use os.Root APIs where possible instead of reconstructing full paths at call sites.
root.Chown() is not used as it requires numeric uid/gid: file.Chown() still handles the name lookup internally and requires a full path.

[agent-platform-auto-pr]

Files inventory check summary

File checks results against ancestor cded3a15:

Results for datadog-agent_7.77.0~devel.git.778.b774baf.pipeline.98033369-1_amd64.deb:

Detected file changes:

1 Removed files:

• opt/datadog-agent/embedded/lib/libdatadog-agent-rtloader.so.2 (34.0 B)

1 Changed files:

• opt/datadog-agent/embedded/lib/libdatadog-agent-rtloader.so:
  • Size changed: +13.33% (30.0 B -> 34.0 B)