Break the mutual GetHashCode recursion between StreamConfiguration<T> and SynchronizationStream<T> #2306

What is failing

MeshWeaver.Blazor.Infrastructure.UserIdentityCache.As<User>() is handed a value whose runtime type is MeshWeaver.Graph.Configuration.NodeTypeDefinition, and its type guard rejects it. The captured stack shows the read path then enters an unbounded hash cycle:

SynchronizationStream<T>.GetHashCode()
  -> ObjectEqualityComparer<T>.GetHashCode(T)
     -> StreamConfiguration<T>.GetHashCode()
        -> SynchronizationStream<T>.GetHashCode()
           -> ... (repeats to truncation)

StreamConfiguration<T>.GetHashCode() and SynchronizationStream<T>.GetHashCode() each hash the other, with no depth guard, cycle check or memoized hash.

Probable cause

High confidence on the recursion — it is not inferred, it is the literal repeating frame sequence in the sample, and it is structural rather than data-dependent: any code path that hashes either type (or puts one in a dictionary/set, which is what ObjectEqualityComparer in the trace means) can walk it forever. Sibling incident 786f05aa5de0c166 captured the same class of cycle one layer down (InstanceCollection.GetHashCode <-> EntityStore.GetHashCode) and there it terminated in an explicit Stack overflow. — i.e. a process kill, which .NET cannot catch.

Low confidence on the trigger — why a NodeTypeDefinition occupies a slot the cache expects to hold a User is not established by this evidence. Do not invent a cache-key collision from this ticket; that thread is already being pulled on separately (see below).

Likely duplicate — read before starting work

This is almost certainly the same defect as #2163 (incident 6e643370636ed34a), which describes this exact two-type cycle. The only difference is which frame of the cycle happened to be on top when the burst was sampled — SynchronizationStream<T> here, StreamConfiguration<T> there. Because the fingerprint incorporates topFrame, one infinite recursion keeps minting new incidents, one per frame that lands on top. Related: #2171 (786f05aa5de0c166, the EntityStore variant) and a95282b4d12cfd2b (already Filed, very high volume, no stack captured — that one tracks the cache type-confusion trigger).

A human can likely close this as a duplicate of #2163 in seconds. It is worth keeping open only if you want the SynchronizationStream<T> entry point covered by a regression test explicitly.

Impact

This fingerprint on its own is a single event on a single pod — negligible in isolation. The reason to act is the family, not this row: the untriggered form is a StackOverflowException, which is an uncatchable process termination and a pod restart, and the trigger condition it rides on is running at very high volume on the same deployment. Severity is high per event, low in frequency so far, and will rise if the trigger worsens.

Where to look

Framework source is not mirrored into the mesh, so these are type/method references rather than links:

1. MeshWeaver.Data.Serialization.StreamConfiguration<T>.GetHashCode() — make it hash only its own fields, or cache the hash.
2. MeshWeaver.Data.Serialization.SynchronizationStream<T>.GetHashCode() — make the delegation one-way; two types must not hash through each other.
3. Consider whether either type should be hashable at all — a mutable stream used as a dictionary key is a smell independent of the recursion.
4. MeshWeaver.Blazor.Infrastructure.UserIdentityCache — validate type at write time so a non-User is refused on insert rather than detected on read.

Evidence

2026-08-26 02:35:48Z memex-portal-deployment-86779d495b-ksxqq fail: MeshWeaver.Blazor.Infrastructure.UserIdentityCache[0]
      As<User> for User: value is MeshWeaver.Graph.Configuration.NodeTypeDefinition (MeshWeaver.Graph), not convertible
   at MeshWeaver.Data.Serialization.SynchronizationStream`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode()
   at System.Collections.Generic.ObjectEqualityComparer`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode(System.__Canon)
   at MeshWeaver.Data.Serialization.StreamConfiguration`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode()
   at MeshWeaver.Data.Serialization.SynchronizationStream`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode()
   at System.Collections.Generic.ObjectEqualityComparer`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode(System.__Canon)
   at MeshWeaver.Data.Serialization.StreamConfiguration`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode()
   at MeshWeaver.Data.Serialization.SynchronizationStream`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode()
   at System.Collections.Generic.ObjectEqualityComparer`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode(System.__Canon)
   at MeshWeaver.Data.Serialization.StreamConfiguration`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetHashCode()
   at MeshWeaver.Data.Serialization.SynchronizationStream`1[[System.__Canon, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral…[truncated]

_{Opened automatically from Admin/_LogIncident/557ddfbfbfbc70e8. Recurrences are folded into this issue rather than opening new ones.}