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[BUGFIX] Agent: fix crash from invalid type in pool#17802

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bboreham merged 2 commits intoprometheus:release-3.9from
bboreham:fix-agent-appender
Jan 7, 2026
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[BUGFIX] Agent: fix crash from invalid type in pool#17802
bboreham merged 2 commits intoprometheus:release-3.9from
bboreham:fix-agent-appender

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@bboreham bboreham commented Jan 7, 2026

We have separate pools for Appender and AppenderV2 objects, and must not put another kind of object into them.

Add lines to tests which detect the problem. (When using V2 the symptom would be that no objects are reused from the pool, so harder to spot).

Which issue(s) does the PR fix:

Fixes: #17800

Does this PR introduce a user-facing change?

[BUGFIX] Agent: fix crash shortly after startup from invalid type of object.

We have separate pools for Appender and AppenderV2 objects, and must not
put another kind of object into them.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
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// Commit submits the collected samples and purges the batch.
func (a *appenderBase) Commit() error {
func (a *appender) Commit() error {
defer a.appenderPool.Put(a)
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The original code (before appender v2) leaked the appender on error.

It also explicitly tested Commit after Rollback which is forbidden in the interface documentation. Also it will call the commit on an object already returned to the pool.

Reading the scrape code I think there's some cases when we call Rollback twice possibly. E.g. when config is wrong and we stop the scrape loop with forced error, we may error on stale markers, do a Rollback() and then do another Rollback() in the deferred function.

So in general this automatic pool handling seems a bit dangerous, but we're not making it too much worse I guess and fix a bug.

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I think in the scrape code it gets a new appender after a forced error, so it's safe to Commit that.

I agree the lines in TestRollback that you highlighted are bad.

As for not pooling after an error - if clearData can leave something in the object which propagates the error, then that would be better. But I can't see that it does.

@bboreham bboreham merged commit f1719fa into prometheus:release-3.9 Jan 7, 2026
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@bboreham bboreham deleted the fix-agent-appender branch January 7, 2026 14:01
renovate bot added a commit to sdwilsh/ansible-playbooks that referenced this pull request Jan 10, 2026
##### [\`v3.9.1\`](https://github.com/prometheus/prometheus/releases/tag/v3.9.1)

- \[BUGFIX] Agent: fix crash shortly after startup from invalid type of object. [#17802](prometheus/prometheus#17802)
- \[BUGFIX] Scraping: fix relabel keep/drop not working. [#17807](prometheus/prometheus#17807)

---
##### [\`v3.9.0\`](https://github.com/prometheus/prometheus/releases/tag/v3.9.0)

#### Note for users of Native Histograms

In version 3.9, Native Histograms is no longer experimental, and the feature flag `native-histogram` has no effect.  You must now turn on
the config setting `scrape_native_histograms` to collect Native Histogram samples from exporters.

#### Changelog

- \[CHANGE] Native Histograms are no longer experimental! Make the `native-histogram` feature flag a no-op. Use `scrape_native_histograms` config option instead. [#17528](prometheus/prometheus#17528)
- \[CHANGE] API: Add maximum limit of 10,000 sets of statistics to TSDB status endpoint. [#17647](prometheus/prometheus#17647)
- \[FEATURE] API: Add /api/v1/features for clients to understand which features are supported. [#17427](prometheus/prometheus#17427)
- \[FEATURE] Promtool: Add `start_timestamp` field for unit tests. [#17636](prometheus/prometheus#17636)
- \[FEATURE] Promtool: Add `--format seriesjson` option to `tsdb dump` to output just series labels in JSON format. [#13409](prometheus/prometheus#13409)
- \[FEATURE] Add `--storage.tsdb.delay-compact-file.path` flag for better interoperability with Thanos. [#17435](prometheus/prometheus#17435)
- \[FEATURE] UI: Add an option on the query drop-down menu to duplicate that query panel. [#17714](prometheus/prometheus#17714)
- \[ENHANCEMENT]: TSDB: add flag `--storage.tsdb.block-reload-interval` to configure TSDB Block Reload Interval. [#16728](prometheus/prometheus#16728)
- \[ENHANCEMENT] UI: Add graph option to start the chart's Y axis at zero. [#17565](prometheus/prometheus#17565)
- \[ENHANCEMENT] Scraping: Classic protobuf format no longer requires the unit in the metric name. [#16834](prometheus/prometheus#16834)
- \[ENHANCEMENT] PromQL, Rules, SD, Scraping: Add native histograms to complement existing summaries. [#17374](prometheus/prometheus#17374)
- \[ENHANCEMENT] Notifications: Add a histogram `prometheus_notifications_latency_histogram_seconds` to complement the existing summary. [#16637](prometheus/prometheus#16637)
- \[ENHANCEMENT] Remote-write: Add custom scope support for AzureAD authentication. [#17483](prometheus/prometheus#17483)
- \[ENHANCEMENT] SD: add a `config` label with job name for most `prometheus_sd_refresh` metrics. [#17138](prometheus/prometheus#17138)
- \[ENHANCEMENT] TSDB: New histogram `prometheus_tsdb_sample_ooo_delta`, the distribution of out-of-order samples in seconds. Collected for all samples, accepted or not. [#17477](prometheus/prometheus#17477)
- \[ENHANCEMENT] Remote-read: Validate histograms received via remote-read. [#17561](prometheus/prometheus#17561)
- \[PERF] TSDB: Small optimizations to postings index. [#17439](prometheus/prometheus#17439)
- \[PERF] Scraping: Speed up relabelling of series. [#17530](prometheus/prometheus#17530)
- \[PERF] PromQL: Small optimisations in binary operators. [#17524](prometheus/prometheus#17524), [#17519](prometheus/prometheus#17519).
- \[BUGFIX] UI: PromQL autocomplete now shows the correct type and HELP text for OpenMetrics counters whose samples end in `_total`. [#17682](prometheus/prometheus#17682)
- \[BUGFIX] UI: Fixed codemirror-promql incorrectly showing label completion suggestions after the closing curly brace of a vector selector. [#17602](prometheus/prometheus#17602)
- \[BUGFIX] UI: Query editor no longer suggests a duration unit if one is already present after a number. [#17605](prometheus/prometheus#17605)
- \[BUGFIX] PromQL: Fix some "vector cannot contain metrics with the same labelset" errors when experimental delayed name removal is enabled. [#17678](prometheus/prometheus#17678)
- \[BUGFIX] PromQL: Fix possible corruption of PromQL text if the query had an empty `ignoring()` and non-empty grouping. [#17643](prometheus/prometheus#17643)
- \[BUGFIX] PromQL: Fix resets/changes to return empty results for anchored selectors when all samples are outside the range. [#17479](prometheus/prometheus#17479)
- \[BUGFIX] PromQL: Check more consistently for many-to-one matching in filter binary operators. [#17668](prometheus/prometheus#17668)
- \[BUGFIX] PromQL: Fix collision in unary negation with non-overlapping series. [#17708](prometheus/prometheus#17708)
- \[BUGFIX] PromQL: Fix collision in label\_join and label\_replace with non-overlapping series. [#17703](prometheus/prometheus#17703)
- \[BUGFIX] PromQL: Fix bug with inconsistent results for queries with OR expression when experimental delayed name removal is enabled. [#17161](prometheus/prometheus#17161)
- \[BUGFIX] PromQL: Ensure that `rate`/`increase`/`delta` of histograms results in a gauge histogram. [#17608](prometheus/prometheus#17608)
- \[BUGFIX] PromQL: Do not panic while iterating over invalid histograms. [#17559](prometheus/prometheus#17559)
- \[BUGFIX] TSDB: Reject chunk files whose encoded chunk length overflows int. [#17533](prometheus/prometheus#17533)
- \[BUGFIX] TSDB: Do not panic during resolution reduction of invalid histograms. [#17561](prometheus/prometheus#17561)
- \[BUGFIX] Remote-write Receive: Avoid duplicate labels when experimental type-and-unit-label feature is enabled. [#17546](prometheus/prometheus#17546)
- \[BUGFIX] OTLP Receiver: Only write metadata to disk when experimental metadata-wal-records feature is enabled. [#17472](prometheus/prometheus#17472)
renovate bot added a commit to sdwilsh/ansible-playbooks that referenced this pull request Jan 10, 2026
##### [\`v3.9.1\`](https://github.com/prometheus/prometheus/releases/tag/v3.9.1)

- \[BUGFIX] Agent: fix crash shortly after startup from invalid type of object. [#17802](prometheus/prometheus#17802)
- \[BUGFIX] Scraping: fix relabel keep/drop not working. [#17807](prometheus/prometheus#17807)

---
##### [\`v3.9.0\`](https://github.com/prometheus/prometheus/releases/tag/v3.9.0)

#### Note for users of Native Histograms

In version 3.9, Native Histograms is no longer experimental, and the feature flag `native-histogram` has no effect.  You must now turn on
the config setting `scrape_native_histograms` to collect Native Histogram samples from exporters.

#### Changelog

- \[CHANGE] Native Histograms are no longer experimental! Make the `native-histogram` feature flag a no-op. Use `scrape_native_histograms` config option instead. [#17528](prometheus/prometheus#17528)
- \[CHANGE] API: Add maximum limit of 10,000 sets of statistics to TSDB status endpoint. [#17647](prometheus/prometheus#17647)
- \[FEATURE] API: Add /api/v1/features for clients to understand which features are supported. [#17427](prometheus/prometheus#17427)
- \[FEATURE] Promtool: Add `start_timestamp` field for unit tests. [#17636](prometheus/prometheus#17636)
- \[FEATURE] Promtool: Add `--format seriesjson` option to `tsdb dump` to output just series labels in JSON format. [#13409](prometheus/prometheus#13409)
- \[FEATURE] Add `--storage.tsdb.delay-compact-file.path` flag for better interoperability with Thanos. [#17435](prometheus/prometheus#17435)
- \[FEATURE] UI: Add an option on the query drop-down menu to duplicate that query panel. [#17714](prometheus/prometheus#17714)
- \[ENHANCEMENT]: TSDB: add flag `--storage.tsdb.block-reload-interval` to configure TSDB Block Reload Interval. [#16728](prometheus/prometheus#16728)
- \[ENHANCEMENT] UI: Add graph option to start the chart's Y axis at zero. [#17565](prometheus/prometheus#17565)
- \[ENHANCEMENT] Scraping: Classic protobuf format no longer requires the unit in the metric name. [#16834](prometheus/prometheus#16834)
- \[ENHANCEMENT] PromQL, Rules, SD, Scraping: Add native histograms to complement existing summaries. [#17374](prometheus/prometheus#17374)
- \[ENHANCEMENT] Notifications: Add a histogram `prometheus_notifications_latency_histogram_seconds` to complement the existing summary. [#16637](prometheus/prometheus#16637)
- \[ENHANCEMENT] Remote-write: Add custom scope support for AzureAD authentication. [#17483](prometheus/prometheus#17483)
- \[ENHANCEMENT] SD: add a `config` label with job name for most `prometheus_sd_refresh` metrics. [#17138](prometheus/prometheus#17138)
- \[ENHANCEMENT] TSDB: New histogram `prometheus_tsdb_sample_ooo_delta`, the distribution of out-of-order samples in seconds. Collected for all samples, accepted or not. [#17477](prometheus/prometheus#17477)
- \[ENHANCEMENT] Remote-read: Validate histograms received via remote-read. [#17561](prometheus/prometheus#17561)
- \[PERF] TSDB: Small optimizations to postings index. [#17439](prometheus/prometheus#17439)
- \[PERF] Scraping: Speed up relabelling of series. [#17530](prometheus/prometheus#17530)
- \[PERF] PromQL: Small optimisations in binary operators. [#17524](prometheus/prometheus#17524), [#17519](prometheus/prometheus#17519).
- \[BUGFIX] UI: PromQL autocomplete now shows the correct type and HELP text for OpenMetrics counters whose samples end in `_total`. [#17682](prometheus/prometheus#17682)
- \[BUGFIX] UI: Fixed codemirror-promql incorrectly showing label completion suggestions after the closing curly brace of a vector selector. [#17602](prometheus/prometheus#17602)
- \[BUGFIX] UI: Query editor no longer suggests a duration unit if one is already present after a number. [#17605](prometheus/prometheus#17605)
- \[BUGFIX] PromQL: Fix some "vector cannot contain metrics with the same labelset" errors when experimental delayed name removal is enabled. [#17678](prometheus/prometheus#17678)
- \[BUGFIX] PromQL: Fix possible corruption of PromQL text if the query had an empty `ignoring()` and non-empty grouping. [#17643](prometheus/prometheus#17643)
- \[BUGFIX] PromQL: Fix resets/changes to return empty results for anchored selectors when all samples are outside the range. [#17479](prometheus/prometheus#17479)
- \[BUGFIX] PromQL: Check more consistently for many-to-one matching in filter binary operators. [#17668](prometheus/prometheus#17668)
- \[BUGFIX] PromQL: Fix collision in unary negation with non-overlapping series. [#17708](prometheus/prometheus#17708)
- \[BUGFIX] PromQL: Fix collision in label\_join and label\_replace with non-overlapping series. [#17703](prometheus/prometheus#17703)
- \[BUGFIX] PromQL: Fix bug with inconsistent results for queries with OR expression when experimental delayed name removal is enabled. [#17161](prometheus/prometheus#17161)
- \[BUGFIX] PromQL: Ensure that `rate`/`increase`/`delta` of histograms results in a gauge histogram. [#17608](prometheus/prometheus#17608)
- \[BUGFIX] PromQL: Do not panic while iterating over invalid histograms. [#17559](prometheus/prometheus#17559)
- \[BUGFIX] TSDB: Reject chunk files whose encoded chunk length overflows int. [#17533](prometheus/prometheus#17533)
- \[BUGFIX] TSDB: Do not panic during resolution reduction of invalid histograms. [#17561](prometheus/prometheus#17561)
- \[BUGFIX] Remote-write Receive: Avoid duplicate labels when experimental type-and-unit-label feature is enabled. [#17546](prometheus/prometheus#17546)
- \[BUGFIX] OTLP Receiver: Only write metadata to disk when experimental metadata-wal-records feature is enabled. [#17472](prometheus/prometheus#17472)
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