fix #2427 compileAsync a schema with discriminator and $ref

Make the discriminator code generation throw MissingRefError when the $ref does not synchronously resolve so that compileAsync can loadSchema and retry.

lib/vocabularies/discriminator/index.ts

@@ -3,6 +3,7 @@ import type {KeywordCxt} from "../../compile/validate"
 import {_, getProperty, Name} from "../../compile/codegen"
 import {DiscrError, DiscrErrorObj} from "../discriminator/types"
 import {resolveRef, SchemaEnv} from "../../compile"
+import MissingRefError from "../../compile/ref_error"
 import {schemaHasRulesButRef} from "../../compile/util"
 
 export type DiscriminatorError = DiscrErrorObj<DiscrError.Tag> | DiscrErrorObj<DiscrError.Mapping>
@@ -66,8 +67,10 @@ const def: CodeKeywordDefinition = {
       for (let i = 0; i < oneOf.length; i++) {
         let sch = oneOf[i]
         if (sch?.$ref && !schemaHasRulesButRef(sch, it.self.RULES)) {
-          sch = resolveRef.call(it.self, it.schemaEnv.root, it.baseId, sch?.$ref)
+          const ref = sch.$ref
+          sch = resolveRef.call(it.self, it.schemaEnv.root, it.baseId, ref)
           if (sch instanceof SchemaEnv) sch = sch.schema
+          if (sch === undefined) throw new MissingRefError(it.opts.uriResolver, it.baseId, ref)
         }
         const propSch = sch?.properties?.[tagName]
         if (typeof propSch != "object") {

spec/discriminator.spec.ts

@@ -159,6 +159,67 @@ describe("discriminator keyword", function () {
     })
   })
 
+  describe("schema with external $refs", () => {
+    const schemas = {
+      main: {
+        type: "object",
+        discriminator: {propertyName: "foo"},
+        required: ["foo"],
+        oneOf: [
+          {
+            $ref: "schema1",
+          },
+          {
+            $ref: "schema2",
+          },
+        ],
+      },
+      schema1: {
+        type: "object",
+        properties: {
+          foo: {const: "x"},
+        },
+      },
+      schema2: {
+        type: "object",
+        properties: {
+          foo: {enum: ["y", "z"]},
+        },
+      },
+    }
+
+    const data = {foo: "x"}
+    const badData = {foo: "w"}
+
+    it("compile should resolve each $ref to a schema that was added with addSchema", () => {
+      const options = {
+        discriminator: true,
+      }
+      const ajv = new _Ajv(options)
+      ajv.addSchema(schemas.main, "https://host/main")
+      ajv.addSchema(schemas.schema1, "https://host/schema1")
+      ajv.addSchema(schemas.schema2, "https://host/schema2")
+
+      const validate = ajv.compile({$ref: "https://host/main"})
+      assert.strictEqual(validate(data), true)
+      assert.strictEqual(validate(badData), false)
+    })
+    it("compileAsync should loadSchema each $ref", async () => {
+      const options = {
+        discriminator: true,
+        async loadSchema(url) {
+          if (!url.startsWith("https://host/")) return undefined
+          const name = url.substring("https://host/".length)
+          return schemas[name]
+        },
+      }
+      const ajv = new _Ajv(options)
+      const validate = await ajv.compileAsync({$ref: "https://host/main"})
+      assert.strictEqual(validate(data), true)
+      assert.strictEqual(validate(badData), false)
+    })
+  })
+
   describe("validation with deeply referenced schemas", () => {
     const schema = [
       {