feat: add support for non-standard pagination termination via a truncation flag (#858)

Author: ianbotsf

.changes/0af8edd3-bbff-4bb2-b1b6-43ac5b27033d.json

@@ -0,0 +1,5 @@
+{
+    "id": "0af8edd3-bbff-4bb2-b1b6-43ac5b27033d",
+    "type": "misc",
+    "description": "Support non-standard pagination termination"
+}

codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/model/traits/PaginationTruncationMember.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package software.amazon.smithy.kotlin.codegen.model.traits
+
+import software.amazon.smithy.model.node.Node
+import software.amazon.smithy.model.node.ObjectNode
+import software.amazon.smithy.model.shapes.ShapeId
+import software.amazon.smithy.model.traits.AnnotationTrait
+
+/**
+ * Indicates the annotated member is a truncation indicator which conveys a non-standard termination condition for
+ * pagination.
+ */
+class PaginationTruncationMember(node: ObjectNode) : AnnotationTrait(ID, node) {
+    companion object {
+        val ID: ShapeId = ShapeId.from("smithy.kotlin.traits#paginationTruncationMember")
+    }
+
+    constructor() : this(Node.objectNode())
+}

codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/rendering/PaginatorGenerator.kt

@@ -19,6 +19,7 @@ import software.amazon.smithy.kotlin.codegen.lang.KotlinTypes
 import software.amazon.smithy.kotlin.codegen.model.SymbolProperty
 import software.amazon.smithy.kotlin.codegen.model.expectShape
 import software.amazon.smithy.kotlin.codegen.model.hasTrait
+import software.amazon.smithy.kotlin.codegen.model.traits.PaginationTruncationMember
 import software.amazon.smithy.kotlin.codegen.utils.getOrNull
 import software.amazon.smithy.model.Model
 import software.amazon.smithy.model.knowledge.PaginatedIndex
@@ -28,6 +29,7 @@ import software.amazon.smithy.model.shapes.MapShape
 import software.amazon.smithy.model.shapes.OperationShape
 import software.amazon.smithy.model.shapes.ServiceShape
 import software.amazon.smithy.model.shapes.Shape
+import software.amazon.smithy.model.shapes.StructureShape
 import software.amazon.smithy.model.traits.PaginatedTrait
 
 /**
@@ -77,6 +79,7 @@ class PaginatorGenerator : KotlinIntegration {
             serviceSymbol,
             paginatedOperation,
             inputSymbol,
+            paginationInfo.output,
             outputSymbol,
             paginationInfo,
             cursorSymbol,
@@ -100,6 +103,7 @@ class PaginatorGenerator : KotlinIntegration {
         serviceSymbol: Symbol,
         operationShape: OperationShape,
         inputSymbol: Symbol,
+        outputShape: StructureShape,
         outputSymbol: Symbol,
         paginationInfo: PaginationInfo,
         cursorSymbol: Symbol,
@@ -141,18 +145,26 @@ class PaginatorGenerator : KotlinIntegration {
             ) {
                 withBlock("#T {", "}", ExternalTypes.KotlinxCoroutines.FlowGenerator) {
                     write("var cursor: #F = null", cursorSymbol)
-                    write("var isFirstPage: Boolean = true")
+                    write("var hasNextPage: Boolean = true")
                     write("")
-                    withBlock("while (isFirstPage || (cursor?.isNotEmpty() == true)) {", "}") {
+                    withBlock("while (hasNextPage) {", "}") {
                         withBlock("val req = initialRequest.copy {", "}") {
                             write("this.$markerLiteral = cursor")
                         }
                         write(
                             "val result = this@#1LPaginated.#1L(req)",
                             operationShape.defaultName(),
                         )
-                        write("isFirstPage = false")
                         write("cursor = result.$nextMarkerLiteral")
+
+                        val hasNextPageFlag = outputShape
+                            .members()
+                            .singleOrNull { it.hasTrait(PaginationTruncationMember.ID) }
+                            ?.defaultName()
+                            ?.let { "result.$it" }
+                            ?: "cursor?.isNotEmpty()"
+
+                        write("hasNextPage = #L == true", hasNextPageFlag)
                         write("emit(result)")
                     }
                 }