Fix source input edge case

lib/src/main/java/graphql/nadel/engine/transform/hydration/batch/NadelNewBatchHydrator.kt

@@ -258,8 +258,12 @@ internal class NadelNewBatchHydrator(
                     extractNode(sourceId, instruction)
                 }
             } else {
-                val (sourceId, instruction) = sourceIdsPairedWithInstruction.single()
-                extractNode(sourceId, instruction)
+                if (sourceIdsPairedWithInstruction.isEmpty()) {
+                    JsonNode.Null
+                } else {
+                    val (sourceId, instruction) = sourceIdsPairedWithInstruction.single()
+                    extractNode(sourceId, instruction)
+                }
             }
         } else {
             if (sourceIdsPairedWithInstruction == null) {
@@ -272,8 +276,12 @@ internal class NadelNewBatchHydrator(
                         },
                 )
             } else {
-                val (sourceId, instruction) = sourceIdsPairedWithInstruction.single()
-                extractNode(sourceId, instruction)
+                if (sourceIdsPairedWithInstruction.isEmpty()) {
+                    JsonNode.Null
+                } else {
+                    val (sourceId, instruction) = sourceIdsPairedWithInstruction.single()
+                    extractNode(sourceId, instruction)
+                }
             }
         }
     }

test/src/test/kotlin/graphql/nadel/tests/hooks/new-batching-absent-source-input.kt

@@ -0,0 +1,57 @@
+package graphql.nadel.tests.hooks
+
+import graphql.nadel.Nadel
+import graphql.nadel.NadelExecutionHints
+import graphql.nadel.engine.blueprint.NadelGenericHydrationInstruction
+import graphql.nadel.engine.transform.artificial.NadelAliasHelper
+import graphql.nadel.engine.transform.result.json.JsonNode
+import graphql.nadel.hooks.NadelExecutionHooks
+import graphql.nadel.tests.EngineTestHook
+import graphql.nadel.tests.UseHook
+
+@UseHook
+class `new-batching-absent-source-input` : EngineTestHook {
+    override fun makeExecutionHints(builder: NadelExecutionHints.Builder): NadelExecutionHints.Builder {
+        return super.makeExecutionHints(builder)
+            .newBatchHydrationGrouping { true }
+    }
+
+    override fun makeNadel(builder: Nadel.Builder): Nadel.Builder {
+        return super.makeNadel(builder)
+            .executionHooks(
+                object : NadelExecutionHooks {
+                    override fun <T : NadelGenericHydrationInstruction> getHydrationInstruction(
+                        instructions: List<T>,
+                        sourceId: JsonNode,
+                        userContext: Any?,
+                    ): T {
+                        val type = (sourceId.value as String).substringBefore("/")
+
+                        return instructions
+                            .first {
+                                it.actorService.name.startsWith(type, ignoreCase = true)
+                            }
+                    }
+
+                    override fun <T : NadelGenericHydrationInstruction> getHydrationInstruction(
+                        instructions: List<T>,
+                        parentNode: JsonNode,
+                        aliasHelper: NadelAliasHelper,
+                        userContext: Any?,
+                    ): T? {
+                        val nodeString = parentNode.value.toString()
+
+                        return when  {
+                            nodeString.contains("comment/") -> instructions.single {
+                                it.actorFieldDef.name.contains("comment")
+                            }
+                            nodeString.contains("issue/") -> instructions.single {
+                                it.actorFieldDef.name.contains("issue")
+                            }
+                            else -> null
+                        }
+                    }
+                },
+            )
+    }
+}

test/src/test/kotlin/graphql/nadel/tests/hooks/new-batching-no-source-inputs.kt

@@ -0,0 +1,37 @@
+package graphql.nadel.tests.hooks
+
+import graphql.nadel.Nadel
+import graphql.nadel.NadelExecutionHints
+import graphql.nadel.engine.blueprint.NadelGenericHydrationInstruction
+import graphql.nadel.engine.transform.result.json.JsonNode
+import graphql.nadel.hooks.NadelExecutionHooks
+import graphql.nadel.tests.EngineTestHook
+import graphql.nadel.tests.UseHook
+
+@UseHook
+class `new-batching-no-source-inputs` : EngineTestHook {
+    override fun makeExecutionHints(builder: NadelExecutionHints.Builder): NadelExecutionHints.Builder {
+        return super.makeExecutionHints(builder)
+            .newBatchHydrationGrouping { true }
+    }
+
+    override fun makeNadel(builder: Nadel.Builder): Nadel.Builder {
+        return super.makeNadel(builder)
+            .executionHooks(
+                object : NadelExecutionHooks {
+                    override fun <T : NadelGenericHydrationInstruction> getHydrationInstruction(
+                        instructions: List<T>,
+                        sourceId: JsonNode,
+                        userContext: Any?,
+                    ): T {
+                        val type = (sourceId.value as String).substringBefore("/")
+
+                        return instructions
+                            .first {
+                                it.actorService.name.startsWith(type, ignoreCase = true)
+                            }
+                    }
+                },
+            )
+    }
+}

test/src/test/resources/fixtures/new hydration/new batching/new-batching-absent-source-input.yml

@@ -0,0 +1,215 @@
+name: "new batching absent source input"
+enabled: true
+# language=GraphQL
+overallSchema:
+  activity: |
+    type Query {
+      activity: [Activity]
+    }
+    union ActivityContent = Issue | Comment
+    type Activity {
+      id: ID!
+      contentId: ID
+      content: ActivityContent
+        @hydrated(
+          service: "comments"
+          field: "commentsByIds"
+          arguments: [
+            {name: "ids" value: "$source.contentId"}
+          ]
+        )
+        @hydrated(
+          service: "issues"
+          field: "issuesByIds"
+          arguments: [
+            {name: "ids" value: "$source.contentId"}
+          ]
+        )
+    }
+  # language=GraphQL
+  issues: |
+    type Query {
+      issuesByIds(ids: [ID!]!): [Issue!]
+    }
+    type Issue {
+      id: ID!
+      title: String
+    }
+  # language=GraphQL
+  comments: |
+    type Query {
+      commentsByIds(ids: [ID!]!): [Comment!]
+    }
+    type Comment {
+      id: ID!
+      content: String
+    }
+# language=GraphQL
+underlyingSchema:
+  # language=GraphQL
+  activity: |
+    type Query {
+      activity: [Activity]
+    }
+    type Activity {
+      id: ID!
+      contentId: ID
+    }
+  # language=GraphQL
+  issues: |
+    type Query {
+      issuesByIds(ids: [ID!]!): [Issue!]
+    }
+    type Issue {
+      id: ID!
+      title: String
+    }
+  # language=GraphQL
+  comments: |
+    type Query {
+      commentsByIds(ids: [ID!]!): [Comment!]
+    }
+    type Comment {
+      id: ID!
+      content: String
+    }
+# language=GraphQL
+query: |
+  {
+    activity {
+      content {
+        __typename
+        ... on Issue {
+          id
+          title
+        }
+        ... on Comment {
+          id
+          content
+        }
+      }
+    }
+  }
+variables: { }
+serviceCalls:
+  - serviceName: "activity"
+    request:
+      # language=GraphQL
+      query: |
+        {
+          activity {
+            __typename__batch_hydration__content: __typename
+            batch_hydration__content__contentId: contentId
+            batch_hydration__content__contentId: contentId
+          }
+        }
+      variables: { }
+    # language=JSON
+    response: |-
+      {
+        "data": {
+          "activity": [
+            {
+              "__typename__batch_hydration__content": "Activity"
+            },
+            {
+              "__typename__batch_hydration__content": "Activity",
+              "batch_hydration__content__contentId": ""
+            },
+            {
+              "__typename__batch_hydration__content": "Activity",
+              "batch_hydration__content__contentId": "comment/9001"
+            },
+            {
+              "__typename__batch_hydration__content": "Activity",
+              "batch_hydration__content__contentId": "issue/1234"
+            }
+          ]
+        },
+        "extensions": {}
+      }
+  - serviceName: "issues"
+    request:
+      # language=GraphQL
+      query: |
+        {
+          issuesByIds(ids: ["issue/1234"]) {
+            __typename
+            id
+            batch_hydration__content__id: id
+            title
+          }
+        }
+      variables: { }
+    # language=JSON
+    response: |-
+      {
+        "data": {
+          "issuesByIds": [
+            {
+              "__typename": "Issue",
+              "id": "issue/1234",
+              "batch_hydration__content__id": "issue/1234",
+              "title": "One Two Three Four"
+            }
+          ]
+        },
+        "extensions": {}
+      }
+  - serviceName: "comments"
+    request:
+      # language=GraphQL
+      query: |
+        {
+          commentsByIds(ids: ["comment/9001"]) {
+            __typename
+            content
+            id
+            batch_hydration__content__id: id
+          }
+        }
+      variables: { }
+    # language=JSON
+    response: |-
+      {
+        "data": {
+          "commentsByIds": [
+            {
+              "__typename": "Comment",
+              "id": "comment/9001",
+              "batch_hydration__content__id": "comment/9001",
+              "content": "It's over 9000"
+            }
+          ]
+        },
+        "extensions": {}
+      }
+# language=JSON
+response: |-
+  {
+    "data": {
+      "activity": [
+        {
+          "content": null
+        },
+        {
+          "content": null
+        },
+        {
+          "content": {
+            "__typename": "Comment",
+            "id": "comment/9001",
+            "content": "It's over 9000"
+          }
+        },
+        {
+          "content": {
+            "__typename": "Issue",
+            "id": "issue/1234",
+            "title": "One Two Three Four"
+          }
+        }
+      ]
+    },
+    "errors": []
+  }