fix: Don't replace malformed appmap.yml

- Use defaults only when there is no config file (ENOENT).
- Rethrow every other error thrown from readFileSync (ex: EACCESS).
- Rethrow YAML parsing error when appmap.yml is malformed, appending
  an informative message.

src/__tests__/config.test.ts

@@ -1,4 +1,4 @@
-import { mkdirSync, writeFileSync } from "node:fs";
+import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { basename } from "node:path";
 import { chdir, cwd } from "node:process";
 
@@ -101,6 +101,17 @@ describe(Config, () => {
     });
   });
 
+  it("if the appmap.yml is malformed throws with an informative message", () => {
+    const malformedAppMapFileContent = "{ appmap-dir: 'abc/xyz' ";
+    writeFileSync("appmap.yml", malformedAppMapFileContent);
+
+    expect(() => new Config()).toThrowError(
+      /You can remove the file to use the default configuration\.$/,
+    );
+
+    expect(readFileSync("appmap.yml").toString()).toEqual(malformedAppMapFileContent);
+  });
+
   let dir: string;
   beforeEach(() => {
     chdir((dir = tmp.dirSync().name));

src/config.ts

@@ -11,6 +11,7 @@ import PackageMatcher, { Package, parsePackages } from "./PackageMatcher";
 import locateFileUp from "./util/findFileUp";
 import lazyOpt from "./util/lazyOpt";
 import tryOr from "./util/tryOr";
+import { isNativeError } from "node:util/types";
 
 const responseBodyMaxLengthDefault = 10000;
 const kResponseBodyMaxLengthEnvar = "APPMAP_RESPONSE_BODY_MAX_LENGTH";
@@ -98,7 +99,24 @@ interface ConfigFile {
 
 function readConfigFile(path: string | undefined): ConfigFile | undefined {
   if (!path) return;
-  const config = tryOr(() => YAML.parse(readFileSync(path, "utf8")) as unknown);
+
+  let fileContent: string;
+  try {
+    fileContent = readFileSync(path, "utf-8");
+  } catch (exn) {
+    if (exn && typeof exn == "object" && "code" in exn && exn.code == "ENOENT") return;
+    throw exn;
+  }
+
+  let config;
+  try {
+    config = YAML.parse(fileContent) as unknown;
+  } catch (exn) {
+    assert(isNativeError(exn));
+    throw new Error(
+      `Error parsing config file at ${path}: ${exn.message}\nYou can remove the file to use the default configuration.`,
+    );
+  }
   if (!config) return;
 
   const result: ConfigFile = {};