fix(tracers): bounded retry for DNSTracer gadget startup - #906
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Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe DNS tracer now retries failed gadget startup attempts with bounded exponential backoff. Each attempt uses a fresh gadget context. Shutdown cancels both the retry loop and active gadget context. Tests cover retry limits and cancellation. ChangesDNS startup retry flow
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The PR adds bounded, context-cancellable retries for DNS tracer startup failures without changing the tracer’s broader behavior. No actionable merge-blocking risk remains after normal checks and review. Sequence Diagram(s)sequenceDiagram
participant DNSTracer
participant GadgetContext
participant Runtime
DNSTracer->>GadgetContext: Create context for attempt
DNSTracer->>Runtime: RunGadget
Runtime-->>DNSTracer: Return transient failure
DNSTracer->>DNSTracer: Wait with bounded backoff
DNSTracer->>GadgetContext: Create fresh context
DNSTracer->>Runtime: RunGadget
Runtime-->>DNSTracer: Return success or final failure
DNSTracer->>GadgetContext: Cancel active context during Stop
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✅ Passed checks (4 passed)
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Inline comments:
In `@pkg/containerwatcher/v2/tracers/dns.go`:
- Around line 99-115: Create a tracer-owned cancellable child context in
DNSTracer.Start, use it for backoff.WithContext and every newGadgetContext call,
and store its cancel function for Stop to invoke. Ensure Stop cancels this
shared context so RetryNotify cannot start another attempt, including races
while replacing dt.gadgetCtx. Add a regression test covering Stop during a
failed attempt or backoff and assert RunGadget is not called again.
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pkg/containerwatcher/v2/tracers/dns.gopkg/containerwatcher/v2/tracers/dns_retry_test.go
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Good, well-scoped fix for the transient CO-RE/BTF startup race, and the write-up/tests are thorough. But there's a real correctness bug in the retry/cancellation interplay that needs fixing before merge — see the inline comment on Start/Stop. Short version: Stop() can race with the retry loop and end up letting the tracer start the gadget again after it was supposedly stopped, since the retry loop is scoped to Start's ctx rather than anything Stop() cancels. CodeRabbit's automated review flagged the same issue independently, which corroborates it.
Separately, the DCO check is currently failing (action_required) — that'll need a signed-off commit before this can merge regardless of the code review outcome.
Not approving yet (and note: GitHub won't let this identity formally request changes on its own PR anyway) — this is a blocker, please treat it as such. Happy to take another look once the cancellation path is fixed and DCO is green.
Stop only canceled the currently in-flight GadgetContext, not the retry loop's own backoff.WithContext(bo, ctx) - that ctx was the raw one passed to Start, which Stop never touched. So canceling the current attempt just made that one RunGadget call fail; RetryNotify saw its context still alive and started another attempt from a fresh GadgetContext Stop never touches either, i.e. Stop could leave the tracer starting the DNS gadget again after the caller believed it was stopped - worse than the pre-retry behavior, which had nothing to race with Stop at all. Give Start its own cancelable context (derived from the one it's called with) and store the CancelFunc. Use that context - not the raw Start ctx - for backoff.WithContext and every newGadgetContext call, and have Stop cancel it before canceling the current GadgetContext. After the in-flight attempt's RunGadget call returns, RetryNotify sees the canceled context and stops instead of scheduling another attempt. Also drops Start's initial newGadgetContext call: it existed to keep dt.gadgetCtx non-nil during the narrow window between Start returning and the goroutine's first attempt, but with the retry-loop context now canceled synchronously inside Stop, any GadgetContext built during that same window already inherits a canceled context regardless of whether this call ever ran, so keeping it was YAGNI. TestDNSTracerStopStopsRetryingMidBackoff reproduces the bug (fails on the pre-fix code: a second RunGadget call happens after Stop returns) and passes with the fix (Stop leaves the call count unchanged). Reported-by: matthyx in PR review #906 (comment) Docs-exempt: bug fix to unreleased internal retry logic added earlier in this same PR, no documented behavior or public API change.
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Re-reviewed after commit 81ad939. The fix is correct: Remaining item before merge: the DCO check is still |
trace_dns's Start() only logged a failed RunGadget call and never retried, so a single transient failure permanently stopped DNS event collection (utils.DnsEventType) until the node-agent pod restarted. That failure class is inherently transient: armosec/private-node-agent#511 root-caused an intermittent "apply CO-RE relocations: load BTF for kmod kvm_intel: rebase split spec: raw BTF differs" coming from a cache race in the pinned cilium/ebpf fork's CO-RE/BTF loading layer, unrelated to trace_dns itself. A fix for that race is proposed upstream (matthyx/ebpf#1), but any gadget startup failure in this class - CO-RE relocation racing other concurrently-loading gadgets - is timing-dependent by nature, so retrying is worthwhile independent of that fix. Wrap the existing background RunGadget call in a bounded retry (dnsStartMaxRetries = 5) using this repo's existing github.com/cenkalti/backoff convention (see containercallback.go's setSharedWatchedContainerData), with each attempt getting its own GadgetContext (the previous one is left terminated after a failed run) and retries stopping promptly once the tracer's context is canceled (Stop()). Scope note: only dns.go is touched. The sibling files #511 mentions (http.go, gotls.go, ssl.go, pkg/tracermanager/tailcalls.go) either don't exist in this OSS repo (gotls.go/ssl.go/tailcalls.go are armosec/private-node-agent's own custom TLS tracers, using a different attach mechanism not exposed to this CO-RE/BTF race) or weren't part of this issue's evidence, so they're left alone. Docs-exempt: internal resilience fix to one tracer's Start(), no documented behavior or public API change. Signed-off-by: Matthias Bertschy <matthias.bertschy@gmail.com>
Stop only canceled the currently in-flight GadgetContext, not the retry loop's own backoff.WithContext(bo, ctx) - that ctx was the raw one passed to Start, which Stop never touched. So canceling the current attempt just made that one RunGadget call fail; RetryNotify saw its context still alive and started another attempt from a fresh GadgetContext Stop never touches either, i.e. Stop could leave the tracer starting the DNS gadget again after the caller believed it was stopped - worse than the pre-retry behavior, which had nothing to race with Stop at all. Give Start its own cancelable context (derived from the one it's called with) and store the CancelFunc. Use that context - not the raw Start ctx - for backoff.WithContext and every newGadgetContext call, and have Stop cancel it before canceling the current GadgetContext. After the in-flight attempt's RunGadget call returns, RetryNotify sees the canceled context and stops instead of scheduling another attempt. Also drops Start's initial newGadgetContext call: it existed to keep dt.gadgetCtx non-nil during the narrow window between Start returning and the goroutine's first attempt, but with the retry-loop context now canceled synchronously inside Stop, any GadgetContext built during that same window already inherits a canceled context regardless of whether this call ever ran, so keeping it was YAGNI. TestDNSTracerStopStopsRetryingMidBackoff reproduces the bug (fails on the pre-fix code: a second RunGadget call happens after Stop returns) and passes with the fix (Stop leaves the call count unchanged). Reported-by: matthyx in PR review #906 (comment) Docs-exempt: bug fix to unreleased internal retry logic added earlier in this same PR, no documented behavior or public API change. Signed-off-by: Matthias Bertschy <matthias.bertschy@gmail.com>
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Performance Benchmark ResultsNode-Agent Resource Usage
Dedup EffectivenessNo data available. |
Performance Benchmark ResultsNode-Agent Resource Usage
Dedup EffectivenessNo data available. |
1 participant
Problem
DNSTracer.Start()only logs a failedRunGadgetcall and never retries:A single transient failure permanently stops DNS event collection
(
utils.DnsEventType) until the node-agent pod restarts.That failure class is inherently transient.
armosec/private-node-agent#511root-caused an intermittent
coming from a cache race in the pinned
cilium/ebpffork's CO-RE/BTF loadinglayer (unrelated to
trace_dnsitself —trace_dnsis an unmodifiedupstream OCI gadget image). A fix for that race is proposed upstream:
matthyx/ebpf#1. But any gadget startup failure in this class — CO-RE
relocation racing other concurrently-loading gadgets — is timing-dependent
by nature, so a bounded retry is worthwhile independent of that fix landing.
Fix
Wrap the existing background
RunGadgetcall in a bounded retry(
dnsStartMaxRetries = 5), using this repo's existinggithub.com/cenkalti/backoffconvention (seecontainercallback.go'ssetSharedWatchedContainerDatafor the establishedpattern). Each attempt gets its own
GadgetContext(the previous one isleft terminated after a failed run, and it's also what
Stop()willcancel), and retries stop promptly once the tracer's context is canceled.
Scope note
Only
dns.gois touched.armosec/private-node-agent#511's suggestedsecondary fix also mentions
http.go,gotls.go,ssl.go, andpkg/tracermanager/tailcalls.goas "sibling tracers" — checking this repo'sactual
pkg/containerwatcher/v2/tracers/directory,gotls.go/ssl.go/tailcalls.godon't exist here at all (they'rearmosec/private-node-agent'sown custom TLS tracers, using a manual uprobe-attach mechanism that never
goes through this CO-RE/BTF loading path, so they aren't exposed to this
particular race). This repo's own
http.goexists but wasn't part of#511's evidence, so it's left alone to keep this PR scoped to the actual
reported failure.
Testing
dns_retry_test.goadds a minimalruntime.Runtimefake and three tests:TestDNSTracerStartRetriesOnTransientFailure: fails every attempt but thelast, confirms the tracer retries through to success.
TestDNSTracerStartGivesUpAfterMaxRetries: always fails, confirms exactlydnsStartMaxRetriesattempts happen and no more.TestDNSTracerStartStopsRetryingWhenContextCanceled: confirms cancelingthe tracer's context stops further retries promptly.
go build ./...andgo vet ./pkg/containerwatcher/...are clean. The restof the
tracerspackage's pre-existing*Fieldstests fail in thischeckout for an unrelated reason (missing
tracers.tarfixture, not presentin a shallow clone — every
*Fieldstest across every tracer file failsidentically, confirming it's fixture-wide and not caused by this change).
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