Summary

Fixes a JVM-level <clinit> deadlock that occurs when multiple threads concurrently trigger Cosmos SDK class loading for the first time. This is a permanent, unrecoverable deadlock that hangs all affected threads indefinitely.

Also fixes a latent CosmosItemSerializer.DEFAULT_SERIALIZER null bug caused by circular <clinit> dependencies between CosmosItemSerializer and DefaultCosmosItemSerializer.

Fixes: #48622, #48585

Root Cause

Deadlock

Consuming classes cached accessors in private static final fields:

private static final FeedResponseAccessor feedResponseAccessor =
    ImplementationBridgeHelpers.FeedResponseHelper.getFeedResponseAccessor();

During <clinit>, the getter finds the accessor null and calls initializeAllAccessors(), which eagerly loads 40+ classes. When two threads enter <clinit> of different classes simultaneously, the JVM's per-class initialization locks create a circular wait — permanent deadlock (JLS §12.4.2).

DEFAULT_SERIALIZER null

CosmosItemSerializer.DEFAULT_SERIALIZER was assigned from DefaultCosmosItemSerializer.DEFAULT_SERIALIZER. When DefaultCosmosItemSerializer.<clinit> ran first (e.g., via INTERNAL_DEFAULT_SERIALIZER access), the recursive <clinit> of CosmosItemSerializer read DefaultCosmosItemSerializer.DEFAULT_SERIALIZER before it was set (JLS §12.4.2 same-thread recursive init = no-op), resulting in null. This caused NullPointerException: serializer is null in Utils.parse() and GatewayAddressCache.

Fix

1. Uniform static getter pattern

Every accessor is now accessed via a short private static getter method — no fields, no <clinit> involvement:

// Before — triggers initializeAllAccessors() during <clinit>
private static final FeedResponseAccessor feedResponseAccessor =
    ImplementationBridgeHelpers.FeedResponseHelper.getFeedResponseAccessor();

// After — no <clinit> involvement, accessor resolved lazily on first use
private static FeedResponseAccessor feedResponseAccessor() {
    return ImplementationBridgeHelpers.FeedResponseHelper.getFeedResponseAccessor();
}

The accessor is already cached inside ImplementationBridgeHelpers via AtomicReference — the getter adds one volatile read (~1ns) per call, negligible vs actual Cosmos operations.

2. Break circular <clinit> for DEFAULT_SERIALIZER

// Before — circular <clinit> dependency
public final static CosmosItemSerializer DEFAULT_SERIALIZER =
    DefaultCosmosItemSerializer.DEFAULT_SERIALIZER;

// After — creates instance directly, no cross-class <clinit> dependency
public final static CosmosItemSerializer DEFAULT_SERIALIZER =
    new DefaultCosmosItemSerializer(
        Utils.getDocumentObjectMapper(Configs.getItemSerializationInclusionMode()));

Scope of changes

Documented exceptions (not converted to static getters)

• DefaultCosmosItemSerializer — instance field intentionally preserved. This class is instantiated during CosmosItemSerializer.<clinit>; the instance field initialization triggers the initializeAllAccessors() fallback at exactly the right time (after super(), before constructor body).
• HttpClient.java — Java interface; Java 8 doesn't support private static interface methods. Uses method-local variable.
• Utils.java — ensureItemSerializerAccessor() uses a CAS/caching pattern with AtomicReference. Preserved as-is.
• BridgeInternal/ModelBridgeInternal/UtilBridgeInternal — accessor registration sources, not consumers.

Why this approach

Tests

1. concurrentAccessorInitializationShouldNotDeadlock (invocationCount=5) — forks fresh child JVMs, 12 threads via CyclicBarrier concurrently triggering <clinit> of 6 high-risk classes. 30s timeout catches deadlock.

2. allAccessorClassesMustHaveStaticInitializerBlock — forked JVM iterates all *Helper inner classes, calls each getter, verifies accessor is non-null. Catches missing static { initialize(); } blocks.

3. noStaticOrInstanceAccessorFieldsInConsumingClasses — reflection-based enforcement. Collects all Accessor interface types from ImplementationBridgeHelpers, scans every class for static or final fields of those types. Catches reintroduction of dangerous patterns regardless of source formatting.

4. accessorInitialization — existing test, validates explicit initializeAllAccessors() bootstrap path.