Title

Fix dataloader dispatch in @defer when multiple deferred fragments exist (#4269)

Description

Problem

When a query contains multiple @defer fragments at the same level, dataloaders invoked inside those deferred fragments are never dispatched. The deferred payloads hang until the request times out.

This was introduced in #3980 ("make dataloader work inside defer blocks").

Root Cause

In DeferredExecutionSupportImpl.createDeferredFragmentCall(), the AlternativeCallContext is constructed with deferredFields.size() — the total field count across all @defer fragments — instead of the field count for the specific fragment:

// BUG: deferredFields.size() is the total across ALL @defer fragments
AlternativeCallContext alternativeCallContext = new AlternativeCallContext(level, deferredFields.size());

// But mergedFields is only the fields for THIS specific fragment
List<MergedField> mergedFields = deferredExecutionToFields.get(deferredExecution);

Each DeferredFragmentCall only invokes its own fields, so the fetch counter will only reach the count of fields in that fragment, never the inflated total. Both PerLevelDataLoaderDispatchStrategy.fieldFetched() and ExhaustedDataLoaderDispatchStrategy.deferFieldFetched() compare against this inflated count to decide when to dispatch, so dispatch never triggers.

Example: 2 @defer fragments with 1 field each → expectedFirstLevelFetchCount = 2, but each fragment only fetches 1 field → dispatch condition 1 == 2 is never satisfied → dataloaders hang.

Fix

Pass mergedFields.size() (the per-fragment field count) instead of deferredFields.size():

 private DeferredFragmentCall createDeferredFragmentCall(DeferredExecution deferredExecution) {
     int level = parameters.getPath().getLevel() + 1;
-    AlternativeCallContext alternativeCallContext = new AlternativeCallContext(level, deferredFields.size());
-
-    List<MergedField> mergedFields = deferredExecutionToFields.get(deferredExecution);
+    List<MergedField> mergedFields = deferredExecutionToFields.get(deferredExecution);
+    AlternativeCallContext alternativeCallContext = new AlternativeCallContext(level, mergedFields.size());

This is safe for mixed deferred/non-deferred selection sets because the constructor already partitions fields — non-deferred fields are routed into nonDeferredFieldNames and never added to deferredExecutionToFields.

Tests Added

Two new test cases in DeferWithDataLoaderTest:

• multiple separate defer fragments with dataloader fields dispatch correctly — Two separate @defer blocks (departments and expensiveDepartments) on shops, each backed by dataloaders. Without the fix, all 3 dispatch strategy variants time out at ~10s each. With the fix, they complete in ~1s total.
• mixed deferred and non-deferred fields with dataloaders dispatch correctly — Non-deferred departments alongside a single deferred expensiveDepartments, confirming the fix correctly handles mixed selection sets.

Both tests cover all three dispatch strategy modes: default (PerLevel), ENABLE_DATA_LOADER_CHAINING, and ENABLE_DATA_LOADER_EXHAUSTED_DISPATCHING.