Add and use parseUserConfig

- Throws a `ConfigurationError` if parsing the YAML fails
- Add a couple of tests for it

src/config/db-config.test.ts

@@ -2,7 +2,7 @@ import test, { ExecutionContext } from "ava";
 
 import { RepositoryProperties } from "../feature-flags/properties";
 import { KnownLanguage, Language } from "../languages";
-import { prettyPrintPack } from "../util";
+import { ConfigurationError, prettyPrintPack } from "../util";
 
 import * as dbConfig from "./db-config";
 
@@ -391,3 +391,42 @@ test(
   {},
   /"a-pack-without-a-scope" is not a valid pack/,
 );
+
+test("parseUserConfig - successfully parses valid YAML", (t) => {
+  const result = dbConfig.parseUserConfig(
+    "test",
+    `
+    paths-ignore:
+      - "some/path"
+    queries:
+      - uses: foo
+    `,
+  );
+  t.truthy(result);
+  if (t.truthy(result["paths-ignore"])) {
+    t.is(result["paths-ignore"].length, 1);
+    t.is(result["paths-ignore"][0], "some/path");
+  }
+  if (t.truthy(result["queries"])) {
+    t.is(result["queries"].length, 1);
+    t.deepEqual(result["queries"][0], { uses: "foo" });
+  }
+});
+
+test("parseUserConfig - throws a ConfigurationError if the file is not valid YAML", (t) => {
+  t.throws(
+    () =>
+      dbConfig.parseUserConfig(
+        "test",
+        `
+        paths-ignore:
+         - "some/path"
+         queries:
+         - foo
+        `,
+      ),
+    {
+      instanceOf: ConfigurationError,
+    },
+  );
+});

src/config/db-config.ts

@@ -1,5 +1,6 @@
 import * as path from "path";
 
+import * as yaml from "js-yaml";
 import * as semver from "semver";
 
 import * as errorMessages from "../error-messages";
@@ -474,3 +475,27 @@ export function generateCodeScanningConfig(
 
   return augmentedConfig;
 }
+
+/**
+ * Attempts to parse `contents` into a `UserConfig` value.
+ *
+ * @param pathInput The path to the file where `contents` was obtained from, for use in error messages.
+ * @param contents The string contents of a YAML file to try and parse as a `UserConfig`.
+ * @returns The `UserConfig` corresponding to `contents`, if parsing was successful.
+ * @throws A `ConfigurationError` if parsing failed.
+ */
+export function parseUserConfig(
+  pathInput: string,
+  contents: string,
+): UserConfig {
+  try {
+    return yaml.load(contents) as UserConfig;
+  } catch (error) {
+    if (error instanceof yaml.YAMLException) {
+      throw new ConfigurationError(
+        errorMessages.getConfigFileParseErrorMessage(pathInput, error.message),
+      );
+    }
+    throw error;
+  }
+}