K8SPG-1053 make PMM sidecar work with readOnlyRootFilesystem

[hors]

The pmm-agent config (pmm-agent.yaml) was written to the container root filesystem, so the sidecar failed on pods enforcing readOnlyRootFilesystem: true (e.g. OpenShift):
Config file /usr/local/percona/pmm/config/pmm-agent.yaml is not writable: read-only file system.

The config is stateless — pmm-agent regenerates it on every start (PMM_AGENT_SETUP_FORCE=1) — so from crVersion 3.1.0 relocate it to the writable /tmp/pmm path (inside the /tmp emptyDir already mounted into the sidecar) for both PMM2 and PMM3, and align the PMM2 tempdir to /tmp/pmm. Older crVersions keep the previous paths to avoid changing running pods.

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Are all needed new/changed options added to the Helm Chart?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported PG version?
• Does the change support oldest and newest supported Kubernetes version?

[Copilot]

Pull request overview

This PR updates the PMM sidecar container configuration so that the pmm-agent.yaml file is written to a writable location (/tmp/pmm) when readOnlyRootFilesystem: true is enforced, while preserving legacy paths for older CR versions.

Changes:

• Added version-gated helpers to select a writable PMM agent config path and (for PMM2) an aligned temp directory starting from CR version 3.1.0.
• Updated PMM2 and PMM3 sidecar environment variables to use the new config path logic (and PMM2 tempdir logic).
• Extended/unit-updated tests to validate the new config path behavior and updated expected env var values.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
percona/pmm/pmm.go	Introduces version-based selection of PMM_AGENT_CONFIG_FILE (and PMM2 tempdir) to support read-only root filesystems.
percona/pmm/pmm_test.go	Adds coverage for config path selection and updates expected env var values for PMM sidecars.

[egegunes]

LGTM but please check monitoring tests

[JNKPercona]

Test Name	Result	Time
backup-enable-disable	passed	00:12:28
builtin-extensions	passed	00:06:00
cert-manager-tls	passed	00:09:21
custom-envs	passed	00:18:45
custom-tls	passed	00:05:49
database-init-sql	passed	00:02:28
demand-backup	passed	00:23:16
demand-backup-offline-snapshot	passed	00:12:49
dynamic-configuration	passed	00:03:37
finalizers	passed	00:06:00
init-deploy	passed	00:03:16
huge-pages	passed	00:02:55
major-upgrade-14-to-15	passed	00:10:21
major-upgrade-15-to-16	passed	00:10:31
major-upgrade-16-to-17	passed	00:10:38
major-upgrade-17-to-18	passed	00:10:45
ldap	passed	00:03:52
ldap-tls	passed	00:05:40
monitoring	passed	00:08:12
monitoring-pmm3	passed	00:09:14
one-pod	passed	00:05:56
operator-self-healing	passed	00:10:27
pitr	passed	00:11:30
scaling	passed	00:05:23
scheduled-backup	passed	00:27:02
self-healing	passed	00:09:24
sidecars	passed	00:02:43
standby-pgbackrest	passed	00:19:44
standby-streaming	passed	00:12:48
start-from-backup	passed	00:11:39
tablespaces	passed	00:06:48
telemetry-transfer	passed	00:04:35
upgrade-consistency	passed	00:06:34
upgrade-minor	passed	00:06:55
users	passed	00:04:54

Summary	Value
Tests Run	35/35
Job Duration	01:41:30
Total Test Time	05:22:36

commit: 808852d
image: perconalab/percona-postgresql-operator:PR-1639-808852d24