Validate value types returned by API against expectations

src/feature-flags/properties.test.ts

@@ -38,7 +38,7 @@ test.serial(
 );
 
 test.serial(
-  "loadPropertiesFromApi throws if response data contains unexpected objects",
+  "loadPropertiesFromApi throws if response data contains objects without `property_name`",
   async (t) => {
     sinon.stub(api, "getRepositoryProperties").resolves({
       headers: {},
@@ -197,7 +197,7 @@ test.serial(
 );
 
 test.serial(
-  "loadPropertiesFromApi throws if property value is not a string",
+  "loadPropertiesFromApi throws if known property value is not a string",
   async (t) => {
     sinon.stub(api, "getRepositoryProperties").resolves({
       headers: {},
@@ -217,7 +217,7 @@ test.serial(
       ),
       {
         message:
-          /Expected repository property 'github-codeql-extra-queries' to have a string value/,
+          /Unexpected value for repository property 'github-codeql-extra-queries', got: 123/,
       },
     );
   },

src/feature-flags/properties.ts

@@ -20,16 +20,56 @@ type AllRepositoryProperties = {
 /** Parsed repository properties. */
 export type RepositoryProperties = Partial<AllRepositoryProperties>;
 
+/** Maps known repository properties to the type we expect to get from the API. */
+type RepositoryPropertyApiType = {
+  [RepositoryPropertyName.DISABLE_OVERLAY]: string;
+  [RepositoryPropertyName.EXTRA_QUERIES]: string;
+};
+
+/** The type of functions which take the `value` from the API and try to convert it to the type we want. */
+export type PropertyParser<K extends RepositoryPropertyName> = (
+  name: K,
+  value: RepositoryPropertyApiType[K],
+  logger: Logger,
+) => AllRepositoryProperties[K];
+
+/** Possible types of `value`s we get from the API. */
+export type RepositoryPropertyValue = string | string[];
+
+/** The type of repository property configurations. */
+export type PropertyInfo<K extends RepositoryPropertyName> = {
+  /** A validator which checks that the value received from the API is what we expect. */
+  validate: (
+    value: RepositoryPropertyValue,
+  ) => value is RepositoryPropertyApiType[K];
+  /** A `PropertyParser` for the property. */
+  parse: PropertyParser<K>;
+};
+
+/** Determines whether a value from the API is a string or not. */
+function isString(value: RepositoryPropertyValue): value is string {
+  return typeof value === "string";
+}
+
+/** A repository property that we expect to contain a string value. */
+const stringProperty = {
+  validate: isString,
+  parse: parseStringRepositoryProperty,
+};
+
+/** A repository property that we expect to contain a boolean value. */
+const booleanProperty = {
+  // The value from the API should come as a string, which we then parse into a boolean.
+  validate: isString,
+  parse: parseBooleanRepositoryProperty,
+};
+
 /** Parsers that transform repository properties from the API response into typed values. */
 const repositoryPropertyParsers: {
-  [K in RepositoryPropertyName]: (
-    name: K,
-    value: string,
-    logger: Logger,
-  ) => AllRepositoryProperties[K];
+  [K in RepositoryPropertyName]: PropertyInfo<K>;
 } = {
-  [RepositoryPropertyName.DISABLE_OVERLAY]: parseBooleanRepositoryProperty,
-  [RepositoryPropertyName.EXTRA_QUERIES]: parseStringRepositoryProperty,
+  [RepositoryPropertyName.DISABLE_OVERLAY]: booleanProperty,
+  [RepositoryPropertyName.EXTRA_QUERIES]: stringProperty,
 };
 
 /**
@@ -37,7 +77,7 @@ const repositoryPropertyParsers: {
  */
 export interface GitHubRepositoryProperty {
   property_name: string;
-  value: string | string[];
+  value: RepositoryPropertyValue;
 }
 
 /**
@@ -86,14 +126,6 @@ export async function loadPropertiesFromApi(
       }
 
       if (isKnownPropertyName(property.property_name)) {
-        // Only validate the type of `value` if this is a property we care about, to avoid throwing
-        // on unrelated properties that may use representations we do not support.
-        if (typeof property.value !== "string") {
-          throw new Error(
-            `Expected repository property '${property.property_name}' to have a string value, but got: ${JSON.stringify(property)}`,
-          );
-        }
-
         setProperty(properties, property.property_name, property.value, logger);
       }
     }
@@ -119,14 +151,30 @@ export async function loadPropertiesFromApi(
   }
 }
 
-/** Update the partial set of repository properties with the parsed value of the specified property. */
+/**
+ * Validate that `value` has the correct type for `K` and, if so, update the partial set of repository
+ * properties with the parsed value of the specified property.
+ */
 function setProperty<K extends RepositoryPropertyName>(
   properties: RepositoryProperties,
   name: K,
-  value: string,
+  value: RepositoryPropertyValue,
   logger: Logger,
 ): void {
-  properties[name] = repositoryPropertyParsers[name](name, value, logger);
+  const propertyOptions = repositoryPropertyParsers[name];
+
+  // We perform the validation here for two reasons:
+  // 1. This function is only called if `name` is a property we care about, to avoid throwing
+  //    on unrelated properties that may use representations we do not support.
+  // 2. The `propertyOptions.validate` function checks that the type of `value` we received from
+  //    the API is what expect and narrows the type accordingly, allowing us to call `parse`.
+  if (propertyOptions.validate(value)) {
+    properties[name] = propertyOptions.parse(name, value, logger);
+  } else {
+    throw new Error(
+      `Unexpected value for repository property '${name}', got: ${JSON.stringify(value)}`,
+    );
+  }
 }
 
 /** Parse a boolean repository property. */