Fix intermittent NRE in ServerSettings.Create due to config injection race#3419
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Aaronontheweb merged 1 commit intoakkadotnet:devfrom Mar 18, 2026
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…llback race Pre-inject Akka.Http default config at builder time in WithAkkaManagement hosting extensions so it's part of the initial ActorSystem config, avoiding a race in Settings.RebuildConfig() where concurrent InjectTopLevelFallback calls can overwrite Config with a stale value. Also add a defensive null guard with re-injection retry in ServerSettings.Create().
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Summary
Fixes an intermittent
NullReferenceExceptioninServerSettings.Create()that occurs on Windows CI whenakka.http.serverconfig is not yet visible due to a race condition in Akka.NET core'sSettings.RebuildConfig().Root cause:
Settings.InjectTopLevelFallbackuses CAS for the_fallbackConfigatomic reference butRebuildConfig()writes to theConfigproperty (a plain auto-property) non-atomically. When two threads callInjectTopLevelFallbackconcurrently, one can overwriteConfigwith a stale value that's missing the other's injected config. This was partially fixed in akkadotnet/akka.net#7721 (1.5.45) but the stale-write race inRebuildConfigremains.Impact: When the
HttpExtextension injectsakka.httpconfig viaInjectTopLevelFallbackand another extension does the same concurrently,ServerSettings.Create()can read aConfigsnapshot that's missingakka.http.server, causingGetConfig()to returnnulland the next line to throw NRE.Observed on: Windows CI only (PR #3418), due to thread pool scheduling and timer resolution differences. The lease conflict fix in #3418 is unrelated — this is a pre-existing issue.
Changes
AkkaHostingExtensions.cs: Pre-injectAkka.Http.Http.DefaultConfig()at builder time in bothWithAkkaManagementoverloads, so the config is part of the initialActorSystemconfig before any concurrent threads can interfereServerSettings.cs: Add a defensive null guard — ifGetConfig("akka.http.server")returns null, re-inject the default config and retry before throwing a descriptive exceptionRelated
Test plan
Akka.Management.Tests.HostingSpecstests pass (these are the tests that were flaking)