From e85e8cf0b2ccd2bdcef6e813efd1f95e64b34567 Mon Sep 17 00:00:00 2001
From: Benj-AI-e <benjie+ai@graphile.com>
Date: Thu, 5 Feb 2026 09:42:58 +0000
Subject: [PATCH] Fix flagUnion propagation for incremental buckets

---
 .../__tests__/flagUnionIncremental-test.ts    | 178 ++++++++++++++++++
 grafast/grafast/src/prepare.ts                |  13 +-
 2 files changed, 189 insertions(+), 2 deletions(-)
 create mode 100644 grafast/grafast/__tests__/flagUnionIncremental-test.ts

diff --git a/grafast/grafast/__tests__/flagUnionIncremental-test.ts b/grafast/grafast/__tests__/flagUnionIncremental-test.ts
new file mode 100644
index 0000000000..ac81913376
--- /dev/null
+++ b/grafast/grafast/__tests__/flagUnionIncremental-test.ts
@@ -0,0 +1,178 @@
+/* eslint-disable graphile-export/exhaustive-deps, graphile-export/export-methods, graphile-export/export-plans, graphile-export/export-instances, graphile-export/export-subclasses, graphile-export/no-nested */
+import { expect } from "chai";
+import { resolvePreset } from "graphile-config";
+import { it } from "mocha";
+
+import type { AsyncExecutionResult } from "graphql";
+import type { UnbatchedExecutionExtra } from "../dist/index.js";
+import {
+  DEFAULT_ACCEPT_FLAGS,
+  TRAP_ERROR,
+  UnbatchedStep,
+  constant,
+  flagError,
+  grafast,
+  lambda,
+  makeGrafastSchema,
+  Step,
+} from "../dist/index.js";
+
+import { resolveStreamDefer, streamToArray } from "./incrementalUtils.ts";
+
+const resolvedPreset = resolvePreset({});
+const requestContext = {};
+
+class FlagUnionCheckStep extends UnbatchedStep<number> {
+  isSyncAndSafe = false;
+  constructor($dep: Step<unknown>) {
+    super();
+    this.addDependency({
+      step: $dep,
+      acceptFlags: DEFAULT_ACCEPT_FLAGS | TRAP_ERROR,
+    });
+  }
+  unbatchedExecute(
+    _extra: UnbatchedExecutionExtra,
+    _value: unknown,
+  ): number {
+    if ((_extra._bucket.flagUnion & TRAP_ERROR) === 0) {
+      throw new Error("Missing flagUnion error flag");
+    }
+    return 1;
+  }
+}
+
+it("carries error flags into stream buckets", async () => {
+  let $bad: Step<unknown> | null = null;
+  const schema = makeGrafastSchema({
+    typeDefs: /* GraphQL */ `
+      type Thing {
+        id: Int
+        check: Int
+      }
+      type Query {
+        list: [Thing!]!
+      }
+    `,
+    objects: {
+      Query: {
+        plans: {
+          list() {
+            $bad = lambda(null, () =>
+              flagError(new Error("Root error flag")),
+            );
+            return constant([1, 2]);
+          },
+        },
+      },
+      Thing: {
+        plans: {
+          id($i: Step<number>) {
+            return $i;
+          },
+          check() {
+            if ($bad == null) {
+              throw new Error("Expected $bad to be initialised");
+            }
+            return new FlagUnionCheckStep($bad);
+          },
+        },
+      },
+    },
+    enableDeferStream: true,
+  });
+
+  const source = /* GraphQL */ `
+    {
+      list @stream(initialCount: 0) {
+        id
+        check
+      }
+    }
+  `;
+
+  const result = await grafast({
+    schema,
+    source,
+    resolvedPreset,
+    requestContext,
+  });
+  const payloads = (await streamToArray(result)) as AsyncExecutionResult[];
+  payloads.forEach((payload) => {
+    expect(payload.errors).to.equal(undefined);
+  });
+  const merged = resolveStreamDefer(payloads);
+  expect(merged.data).to.deep.equal({
+    list: [
+      { id: 1, check: 1 },
+      { id: 2, check: 1 },
+    ],
+  });
+});
+
+it("carries error flags into deferred buckets", async () => {
+  let $bad: Step<unknown> | null = null;
+  const schema = makeGrafastSchema({
+    typeDefs: /* GraphQL */ `
+      type Thing {
+        id: Int
+        check: Int
+      }
+      type Query {
+        thing: Thing
+      }
+    `,
+    objects: {
+      Query: {
+        plans: {
+          thing() {
+            $bad = lambda(null, () =>
+              flagError(new Error("Root error flag")),
+            );
+            return constant(1);
+          },
+        },
+      },
+      Thing: {
+        plans: {
+          id($i: Step<number>) {
+            return $i;
+          },
+          check() {
+            if ($bad == null) {
+              throw new Error("Expected $bad to be initialised");
+            }
+            return new FlagUnionCheckStep($bad);
+          },
+        },
+      },
+    },
+    enableDeferStream: true,
+  });
+
+  const source = /* GraphQL */ `
+    {
+      thing {
+        id
+        ... @defer {
+          check
+        }
+      }
+    }
+  `;
+
+  const result = await grafast({
+    schema,
+    source,
+    resolvedPreset,
+    requestContext,
+  });
+  const payloads = (await streamToArray(result)) as AsyncExecutionResult[];
+  payloads.forEach((payload) => {
+    expect(payload.errors).to.equal(undefined);
+  });
+  const merged = resolveStreamDefer(payloads);
+  expect(merged.data).to.deep.equal({
+    thing: { id: 1, check: 1 },
+  });
+});
diff --git a/grafast/grafast/src/prepare.ts b/grafast/grafast/src/prepare.ts
index 63a2029186..5664a39a79 100644
--- a/grafast/grafast/src/prepare.ts
+++ b/grafast/grafast/src/prepare.ts
@@ -42,6 +42,7 @@ import { establishOperationPlan } from "./establishOperationPlan.ts";
 import type {
   ErrorBehavior,
   EstablishOperationPlanEvent,
+  ExecutionEntryFlags,
   GrafastExecutionArgs,
   GrafastTimeouts,
   JSONValue,
@@ -104,6 +105,14 @@ const bypassGraphQLObj = Object.assign(Object.create(null), {
 
 function noop() {}
 
+function getFlagUnionFromStore(store: Bucket["store"]): ExecutionEntryFlags {
+  let flagUnion = NO_FLAGS;
+  for (const ev of store.values()) {
+    flagUnion |= ev._getStateUnion();
+  }
+  return flagUnion;
+}
+
 function processRoot(
   // errors should already have been handled, and this ctx isn't suitable to be reused.
   ctx: Omit<OutputPlanContext, "errors">,
@@ -885,7 +894,7 @@ async function processStream(
       layerPlan: directLayerPlanChild,
       size,
       store,
-      flagUnion: NO_FLAGS,
+      flagUnion: getFlagUnionFromStore(store),
       polymorphicPathList,
       polymorphicType: null,
       iterators,
@@ -1054,7 +1063,7 @@ function processSingleDeferred(
     layerPlan: outputPlan.layerPlan,
     size,
     store,
-    flagUnion: NO_FLAGS,
+    flagUnion: getFlagUnionFromStore(store),
     polymorphicPathList,
     polymorphicType: null,
     iterators,