Refactor getModulePath and fix decorator errors (#87)

* fix undefined "this" error for decorators

* refactor getModulePath so it doesn't require string parsing and gets file names for decorators correctly as well

* add tests

* update changelog

* use typecasting instead of ts-ignore

* return values immediately

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## [Unreleased]
+
+- Fixed an issue where decorators were changing `this` values for the methods they'd be wrapping, breaking them.
+- Improved how `getModulePath` utility works, passing stack trace as structured data, and making it more robust.
+
 ## [v0.5.3] - 2023-06-26
 
 ### Changed

packages/lib/src/utils.ts

@@ -20,13 +20,20 @@ export function getRuntime(): Runtime {
 // HACK: this entire function is a hacky way to acquire the module name for a
 // given function e.g.: dist/index.js
 export function getModulePath(): string | undefined {
-  Error.stackTraceLimit = 5;
-  const stack = new Error()?.stack?.split("\n");
+  const defaultPrepareStackTrace = Error.prepareStackTrace;
+  Error.prepareStackTrace = (_, stack) => stack;
+  const { stack: stackConstructor } = new Error() as Error & {
+    stack: NodeJS.CallSite[];
+  };
+  Error.prepareStackTrace = defaultPrepareStackTrace; // we have to make sure to reset this to normal
+
+  const stack = stackConstructor.map((callSite) => ({
+    name: callSite.getFunctionName(),
+    file: callSite.getFileName(),
+  }));
 
   let rootDir: string;
-
   const runtime = getRuntime();
-
   if (runtime === "browser") {
     rootDir = "";
   } else if (runtime === "deno") {
@@ -45,30 +52,27 @@ export function getModulePath(): string | undefined {
   }
 
   /**
+   * Finds the original wrapped function, first it checks if it's a decorator,
+   * and returns that filename or gets the 3th item of the stack trace:
    *
    * 0: Error
-   * 1: at getModulePath() ...
-   * 2: at autometrics() ...
-   * 3: at ... -> 4th line is always the original caller
+   * 1: at getModulePath ...
+   * 2: at autometrics ...
+   * 3: at ... -> 4th line is always the original wrapped function
    */
-  const originalCaller = 3 as const;
-
-  // The last element in this array will have the full path
-  const fullPath = stack[originalCaller].split(" ").pop();
+  const wrappedFunctionPath =
+    stack.find((call) => {
+      if (call.name === "__decorateClass") return true;
+    })?.file ?? stack[2]?.file;
 
-  const containsFileProtocol = fullPath.includes("file://");
+  const containsFileProtocol = wrappedFunctionPath.includes("file://");
 
   // We split away everything up to the root directory of the project,
   // if the path contains file:// we need to remove it
-  let modulePath = fullPath.replace(
+  return wrappedFunctionPath.replace(
     containsFileProtocol ? `file://${rootDir}` : rootDir,
     "",
   );
-
-  // We split away the line and column numbers index.js:14:6
-  modulePath = modulePath.substring(0, modulePath.indexOf(":"));
-
-  return modulePath;
 }
 
 type ALSContext = {

packages/lib/src/wrappers.ts

@@ -250,7 +250,7 @@ export function autometrics<F extends FunctionSig>(
 
     function instrumentedFunction() {
       try {
-        const result = fn(...params);
+        const result = fn.apply(this, params);
         if (isPromise<ReturnType<F>>(result)) {
           return result
             .then((res: Awaited<ReturnType<typeof result>>) => {

packages/lib/tests/integration.test.ts

@@ -1,4 +1,4 @@
-import { autometrics, init } from "../src";
+import { Autometrics, autometrics, init } from "../src";
 import { describe, test, expect, beforeAll, afterEach } from "vitest";
 import {
   AggregationTemporality,
@@ -57,4 +57,39 @@ describe("Autometrics integration test", () => {
 
     expect(serialized).toMatch(errorCountMetric);
   });
+
+  test("class method", async () => {
+    const callCountMetric =
+      /function_calls_count_total\{\S*function="helloWorld"\S*module="\/packages\/lib\/tests\/integration.test.ts"\S*\} 2/gm;
+    const durationMetric =
+      /function_calls_duration_bucket\{\S*function="helloWorld"\S*module="\/packages\/lib\/tests\/integration.test.ts"\S*\}/gm;
+
+    // @Autometrics decorator is likely to be used along-side other decorators
+    // this tests for any conflicts
+    function Bar() {
+      return function Bar(
+        target: Object,
+        propertyKey: string,
+        descriptor: PropertyDescriptor,
+      ) {
+        const originalMethod = descriptor.value;
+        descriptor.value = originalMethod;
+      };
+    }
+
+    @Autometrics()
+    class Foo {
+      @Bar()
+      helloWorld() {}
+    }
+
+    const foo = new Foo();
+    foo.helloWorld();
+    foo.helloWorld();
+
+    const serialized = await collectAndSerialize(exporter);
+
+    expect(serialized).toMatch(callCountMetric);
+    expect(serialized).toMatch(durationMetric);
+  });
 });