in_podman_metrics: do not abort startup when the initial scrape fails

[stondo]

Summary

With scrape_on_start enabled, a failing initial scrape makes in_metrics_init return -1. Fluent Bit treats a failed input initialization as fatal, so the whole process exits rather than that one input being degraded.

The usual trigger is a podman config file that does not exist yet: podman has not run on the host, or its storage is not mounted. That is a transient condition, not a misconfiguration.

Why this is inconsistent today

collect_container_data already reports the missing file with flb_plg_warn, not an error:

flb_utils_read_file(ctx->config, &buffer, &read_bytes);
if (!read_bytes) {
    flb_plg_warn(ctx->ins, "Failed to open %s", ctx->config);
    return -1;
}

but in_metrics_init turns that same -1 into a fatal init failure:

if (ctx->scrape_interval >= 2 && ctx->scrape_on_start) {
    if (scrape_metrics(config, ctx) == -1) {
        ...
        return -1;
    }
}

The interval collector cb_metrics_collect_runtime hits the identical failure with no consequence beyond a logged warning. So whether the process survives depends only on whether the first scrape happened during init or one interval later. scrape_on_start is a timing convenience; it should not change liveness semantics.

Note an empty container list is already handled correctly ([] parses as a JSMN array with zero entries). Only an absent file trips this.

Change

Log a warning and let the interval collector retry. A partially collected container list is discarded by destroy_container_list at the top of the next scrape, so no stale state survives.

How it shows up

On an embedded image where the container storage lives on a data partition that is empty on first boot:

fluent-bit.service: Main process exited, code=exited, status=255/EXCEPTION
fluent-bit.service: Scheduled restart job, restart counter is at 5.
fluent-bit.service: Start request repeated too quickly.
Failed to start Fluent Bit.

Fluent Bit never starts, so no inputs run at all, not just podman metrics.

Related

• in_podman_metrics: fix multiple cgroup v2 issues #11719 (in_podman_metrics cgroup v2 fixes) and in_podman_metrics: add exclude_name_regex option #11970 (exclude_name_regex), both still open.

Summary by CodeRabbit

• Bug Fixes
  • The metrics service now continues starting when the initial container metrics scrape fails.
  • A warning is shown, and the scrape is retried automatically after the configured interval.
  • When container configuration is missing or invalid, the service starts successfully without exposing container usage metrics.

[coderabbitai]

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---

📝 Walkthrough

Walkthrough

The Podman metrics plugin continues engine startup when its initial scrape fails. It logs a retry warning and retries after scrape_interval seconds. Runtime tests cover invalid and missing configuration.

Changes

Podman metrics startup

Layer / File(s)	Summary
Initial scrape failure handling plugins/in_podman_metrics/podman_metrics.c, tests/runtime/in_podman_metrics.c	The plugin continues initialization after an initial scrape failure. Runtime tests use metric_exists() to verify that invalid or missing configuration does not expose usage_bytes.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to c7e35

Podman metrics initialization now tolerates an unavailable or transiently malformed configuration and retries on its normal interval, preventing that input from aborting Fluent Bit startup. No current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: failed initial scrapes no longer abort in_podman_metrics startup.
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.

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[chatgpt-codex-connector]

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[coderabbitai]

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Inline comments:
In `@plugins/in_podman_metrics/podman_metrics.c`:
- Around line 496-497: Update flb_test_ipm_no_config to expect flb_start(ctx) to
succeed after an initial scrape failure, and add assertions that verify the
configured retry behavior occurs before the test completes.

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[stondo]

Testing

Built from this branch against master (f78472443) and compared with an otherwise identical build that only reverts this hunk. Test config:

[INPUT]
    Name            podman_metrics
    scrape_interval 2
    scrape_on_start true
    path.config     /tmp/does-not-exist/containers.json

Before (unpatched master) — the process refuses to start:

[ warn] [input:podman_metrics:podman_metrics.0] Failed to open /tmp/does-not-exist/containers.json
[error] [input:podman_metrics:podman_metrics.0] Could not collect container ids
[error] [input:podman_metrics:podman_metrics.0] Could not start collector for podman metrics plugin
[error] failed initialize input podman_metrics.0
[error] [engine] input initialization failed
$ echo $?
255

After — it starts, warns, and retries on the interval:

[ info] [input:podman_metrics:podman_metrics.0] Generating podman metrics (initial scrape)
[ warn] [input:podman_metrics:podman_metrics.0] Failed to open /tmp/does-not-exist/containers.json
[error] [input:podman_metrics:podman_metrics.0] Could not collect container ids
[ warn] [input:podman_metrics:podman_metrics.0] Initial scrape failed, retrying in 2 seconds
[ info] [sp] stream processor started
[ info] [output:stdout:stdout.0] worker #0 started

Recovery — creating the file mid-run at 17:19:46, the next interval picks it up with no restart:

17:19:45.233 [ warn]  Failed to open /tmp/does-not-exist/containers.json
17:19:45.233 [error]  Could not collect container ids
        >>> containers.json created at 17:19:46
17:19:47.235 [ info]  Scraping metrics took 1911477ns
17:19:49.238 [ info]  Scraping metrics took 4769140ns
17:19:51.237 [ info]  Scraping metrics took 4101318ns

The residual [error] Could not collect container ids line comes from scrape_metrics and is unchanged by this PR; only the fatality is removed. Happy to soften that one to a warning too if you would prefer the log to match the severity.

[stondo]

Thanks both, this was a real blocker and I have pushed a fix.

@coderabbitai flagged flb_test_ipm_no_config; Codex additionally caught flb_test_ipm_garbage_config, which hits the same collect_container_data return through jsmn_parse failing. Both asserted the old contract that startup must fail, so flb-rt-in_podman_metrics would indeed have gone red.

Codex also asked the right design question rather than assuming the flip: should malformed input stay fatal? I decided it should not, and I would rather argue it explicitly than have it change by accident.

Podman rewrites containers.json in place. A scrape can race with that write and read a truncated file, which jsmn_parse rejects exactly like the garbage fixture does. So the malformed case is just as transient as the missing-file case, and keeping it fatal means the process can die at startup purely on timing. Neither condition is something an operator can act on, and collect_container_data already reports both with flb_plg_warn. Treating them the same seems right; happy to split them by return code if you disagree, but that needs collect_container_data to distinguish "absent" from "unparsable", which it currently does not.

Both tests now expect a successful start plus check_metric(...) != 0, so they assert the engine survived init and that no container metrics are exposed, rather than only checking a return code.

Verification

$ ./bin/flb-rt-in_podman_metrics
SUCCESS: All unit tests have passed.          # all 8

$ ./bin/flb-rt-in_podman_metrics no_config garbage_config
Test no_config...        [ OK ]
Test garbage_config...   [ OK ]

And with only the plugin hunk reverted to master, to confirm the tests actually exercise the change:

Test no_config...        [ FAILED ]
Test garbage_config...   [ FAILED ]
FAILED: 2 of 2 unit tests have failed.

On the request to cover the retry itself: a genuine retry test needs the fixture to become valid part-way through the run, and the suite has no helper for mutating fixtures mid-test. I left that out rather than add a sleep that asserts nothing. The retry path is the ordinary interval collector already covered by regular, reversed and cgroup_v2. I verified recovery by hand instead, creating the file under a running instance and watching the next interval pick it up with no restart; that log is in the comment above.

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Actionable comments posted: 1

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Inline comments:
In `@tests/runtime/in_podman_metrics.c`:
- Line 136: Replace the `check_metric`-based assertions in both tests with an
explicit metric-presence check that distinguishes an absent `usage_bytes` from
an input containing zero counters, and assert that `usage_bytes` is absent.

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@stondo Thanks for the detailed validation.

The non-fatal behavior is correct for both missing and transiently malformed containers.json input. Podman can expose truncated JSON during an in-place rewrite, so an initial jsmn_parse failure must not stop Fluent Bit startup.

The updated tests verify the intended contract and fail when the plugin change is reverted. The manual recovery check also validates interval-based retry behavior without adding a timing-dependent test.

🐇 ✅

---

✏️ Learnings added

Learnt from: stondo
URL: https://github.com/fluent/fluent-bit/pull/12391

Timestamp: 2026-09-08T16:42:33.878Z
Learning: In `plugins/in_podman_metrics`, both a missing Podman `containers.json` file and a transient malformed file must be non-fatal during the initial scrape. Podman rewrites `containers.json` in place, so `collect_container_data` can observe truncated JSON and `jsmn_parse` can fail temporarily. The interval collector must retry later instead of preventing Fluent Bit startup.

Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure review instructions. You can manage existing learnings in the app.

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[stondo]

Good catch, fixed in the latest push. check_metric() returns 0 both when the counter is found and when the input has no counters at all, so != 0 did not actually assert absence. Added a metric_exists() helper that returns a real boolean and changed both assertions to metric_exists(ctx, "usage_bytes") == FLB_FALSE.

The helper uses the plain mk_list_foreach/cfl_list_foreach macros rather than the _safe variants, since it does not mutate the list.

Verified: all eight tests in the suite pass, and with the plugin change reverted both no_config and garbage_config fail, so they now exercise the new contract rather than passing on the presence of unrelated counters.

[cosmo0920]

It's reasonable to me. 👍