dns_cache: unconditionally floor failure backoff at min_refresh_interval_

[Copilot]

Applies review feedback from @agrawroh on envoyproxy#44570: simplify the failure backoff capping logic in finishResolve by removing the conditional guard around the min_refresh_interval_ floor.

Changes

• dns_cache_impl.cc: Drop the uncapped_refresh_interval local variable and the if (refresh_interval < uncapped_refresh_interval) guard; replace with a single unconditional std::max floor:

// Before
const auto uncapped_refresh_interval = refresh_interval;
// ... TTL cap logic ...
if (refresh_interval < uncapped_refresh_interval) {
  const auto min_refresh_ms =
      std::chrono::duration_cast<std::chrono::milliseconds>(min_refresh_interval_);
  refresh_interval = std::max(refresh_interval, min_refresh_ms);
}

// After
// ... TTL cap logic ...
refresh_interval = std::max(refresh_interval,
    std::chrono::duration_cast<std::chrono::milliseconds>(min_refresh_interval_));

• Update the comment to drop the "if and only if" qualifier, reflecting that the floor now applies unconditionally.

Original prompt

Address review feedback from @agrawroh on PR envoyproxy#44570 ("shared_pool: fix leak when last reference drops on worker thread during dispatcher teardown"). The branch to work on is flake-dfp-no on this fork (phlax/envoy). Push commits directly to this branch so the upstream PR updates — do not open a new PR.

The files currently touched by the PR are:

• source/common/shared_pool/shared_pool.h
• source/extensions/common/dynamic_forward_proxy/dns_cache_impl.cc
• test/extensions/common/dynamic_forward_proxy/dns_cache_impl_test.cc
• changelogs/current.yaml

Please apply the following changes. Do not change unrelated code.

---

1. source/extensions/common/dynamic_forward_proxy/dns_cache_impl.cc — simplify and fix invariant

The current diff in finishResolve (around line 570) looks like:

const auto uncapped_refresh_interval = refresh_interval;
if (elapsed >= host_ttl_) {
  refresh_interval = std::chrono::milliseconds(0);
} else {
  const auto until_eviction =
      std::chrono::duration_cast<std::chrono::milliseconds>(host_ttl_ - elapsed);
  refresh_interval = std::min(refresh_interval, until_eviction);
}
// If (and only if) the host_ttl cap above reduced the failure backoff, floor the
// resulting interval at min_refresh_interval_ ...
if (refresh_interval < uncapped_refresh_interval) {
  const auto min_refresh_ms =
      std::chrono::duration_cast<std::chrono::milliseconds>(min_refresh_interval_);
  refresh_interval = std::max(refresh_interval, min_refresh_ms);
}
primary_host_info->refresh_timer_->enableTimer(refresh_interval);
ENVOY_LOG(debug, "DNS refresh rate reset for host '{}', (failure) refresh rate {} ms", host,
          refresh_interval.count());

Reviewer feedback (agrawroh):

• Rename the local uncapped_refresh_interval → raw_backoff_ms.
• Unconditionally floor at min_refresh_interval_. Replace the whole conditional floor block with a single line:

  refresh_interval = std::max(refresh_interval,
      std::chrono::duration_cast<std::chrono::milliseconds>(min_refresh_interval_));

  If min_refresh_interval_ is already std::chrono::milliseconds, drop the cast.
• Preserve the eviction invariant that the previous PR added (a failed resolve's re-arm fires no later than the host becomes eligible for eviction). Reviewer's preferred option (Newer release available build_bazel_rules_apple: 0.31.3 (current: 0.31.2) #2): change > to >= in onReResolveAlarm in the same file — find the branch that compares elapsed time against host_ttl_ for eviction and flip the strict > to >= so a re-arm exactly at the TTL boundary still evicts. Do NOT restore the +1ms magic offsets. Keep refresh_interval = std::chrono::milliseconds(0); in the past-TTL branch and keep until_eviction with no +1ms.
• Update the debug log to include both the raw (pre-cap) and the armed (post-cap+floor) interval:

  ENVOY_LOG(debug,
            "DNS refresh rate reset for host '{}', (failure) raw={} ms armed={} ms",
            host, raw_backoff_ms.count(), refresh_interval.count());

• Tidy the block comment — since the floor is now unconditional, drop the "if and only if the cap kicked in" language. Trim to: the cap bounds eviction delay; the floor prevents arming sub-dns_min_refresh_rate alarms (including the nextBackOffMs() == 0 case) which could kick rapid-fire resolves and race with dispatcher/resolver teardown.

2. test/extensions/common/dynamic_forward_proxy/dns_cache_impl_test.cc — add tests

a. A test with a seeded random generator that forces nextBackOffMs() == 0 and asserts the refresh alarm is armed at min_refresh_interval_ (the existing fixture around ResolveFailureBackoffCappedByHostTtl uses dns_min_refresh_rate = 1s). Wire in a NiceMock<Random::MockRandomGenerator> that feeds the failure backoff strategy if one isn't already there; set ON_CALL(random_, random()).WillByDefault(Return(0)); so the jittered exponential backoff returns 0. The test should configure the same refresh/ttl/min as the existing test, trigger a DNS failure, and assert enableTimer is called with 1000ms (the min_refresh_interval_), NOT 0. Name it ResolveFailureBackoffZeroFlooredByMinRefresh.

b. A test for the past-TTL branch (elapsed >= host_ttl_). Advance sim time past host_ttl before the failure; assert enableTimer is called with min_refresh_interval_ (not 1ms, not 0). Name it ResolveFailurePastHostTtlFlooredByMinRefresh. Keep style consistent with ResolveFailureBackoffCappedByHostTtl.

Also update the existing ResolveFailureBackoffCappedByHostTtl test's comment to match the new unconditional-floor semantics if it's still slightly stale.

3. source/common/shared_pool/shared_pool.h and a new test for the leak fix

Reviewer: "Could we please also add a regression test for the leak fix exercising the dispatcher teardown-drops-callback path?"

Find the existing shared_pool unit tests under test/common/shared_pool/ (likely shared_pool_test.cc). Add a new test exercisin...

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