feat(data-point): add reducer stack trace functionality (ViacomInc#409)

Property `error.reducerStackTrace` now contains a stack trace of all reducers executed before and including the reducer with the error.

packages/data-point/src/Accumulator.js

@@ -30,9 +30,9 @@ class Accumulator {
     this.tracer = options.tracer;
 
     /**
-     * @type {Reducer} reference to the current reducer
+     * @type {string[]} stores the list of reducers executed per branch.
      */
-    this.reducer = options.reducer;
+    this.__reducerStackTrace = [];
 
     /**
      * @type {Number} current reducer process id, this number changes on the
@@ -59,6 +59,23 @@ class Accumulator {
     return copy;
   }
 
+  set reducer(reducer) {
+    this.__reducer = reducer;
+
+    // we create a new array to create a new reducer history branch.
+    this.__reducerStackTrace = this.__reducerStackTrace.concat(
+      this.__reducer.id
+    );
+  }
+
+  get reducer() {
+    return this.__reducer;
+  }
+
+  get reducerStackTrace() {
+    return this.__reducerStackTrace.join(" > ");
+  }
+
   resolve(reducers) {
     return this.__resolve(this, reducers);
   }

packages/data-point/src/Accumulator.test.js

@@ -5,15 +5,16 @@ describe("Accumulator", () => {
     it("should accept empty options object", () => {
       expect(new Accumulator()).toMatchInlineSnapshot(`
         Accumulator {
+          "__reducerStackTrace": Array [],
           "cache": Object {},
           "locals": undefined,
           "pid": undefined,
-          "reducer": undefined,
           "tracer": undefined,
           "value": undefined,
         }
       `);
     });
+
     it("should assign options object", () => {
       const resolve = () => true;
       const acc = new Accumulator({
@@ -23,12 +24,13 @@ describe("Accumulator", () => {
         tracer: "tracer",
         resolve
       });
+
       expect(acc).toMatchInlineSnapshot(`
         Accumulator {
+          "__reducerStackTrace": Array [],
           "cache": "cache",
           "locals": "locals",
           "pid": undefined,
-          "reducer": undefined,
           "tracer": "tracer",
           "value": "value",
         }
@@ -60,6 +62,43 @@ describe("Accumulator", () => {
       expect(Object.getPrototypeOf(acc2)).toEqual(acc1);
     });
   });
+
+  describe("reducer", () => {
+    const reducer = {
+      id: "myReducerId"
+    };
+
+    describe("set reducer", () => {
+      it("should append a new reducer to the stack of reducers", () => {
+        const acc = new Accumulator({
+          value: "a"
+        });
+
+        // eslint-disable-next-line no-underscore-dangle
+        const originalReducerStackTrace = acc.__reducerStackTrace;
+
+        acc.reducer = reducer;
+
+        // should not mutate the original stackTrace
+        expect(originalReducerStackTrace).toEqual([]);
+        // eslint-disable-next-line no-underscore-dangle
+        expect(acc.__reducerStackTrace).toEqual(["myReducerId"]);
+      });
+    });
+
+    describe("get reducer", () => {
+      it("should return stored internal reducer", () => {
+        const acc = new Accumulator({
+          value: "a"
+        });
+
+        acc.reducer = reducer;
+        // eslint-disable-next-line no-underscore-dangle
+        expect(acc.__reducer).toEqual(reducer);
+      });
+    });
+  });
+
   describe("resolve", () => {
     it("should call internal resolve method", () => {
       const mockResolve = jest.fn(() => "resolved");

packages/data-point/src/resolve.js

@@ -22,7 +22,15 @@ async function resolve(accumulator, reducer) {
     // NOTE: recursive call by passing resolve method
     result = await reducer.resolveReducer(acc, resolve);
   } catch (error) {
+    // because the error bubbles up, we only want to set `error.reducerStackTrace`
+    // once, otherwise we end up with only the first item in the stack.
+    if (!error.reducerStackTrace) {
+      error.reducerStackTrace = acc.reducerStackTrace;
+    }
+
     traceSpan.logError(span, error);
+
+    // rethrow the error up
     throw error;
   } finally {
     traceSpan.finish(span);

packages/data-point/src/resolve.test.js

@@ -35,6 +35,41 @@ describe("resolve", () => {
     });
   });
 
+  describe("reducerStackTrace", () => {
+    it("should augment error with reducer stacktrace", async () => {
+      const acc = new Accumulator();
+
+      const testError = new Error("resolveReducer failed");
+
+      const reducerRejected = {
+        id: "badReducer",
+        resolveReducer: jest.fn().mockRejectedValue(testError)
+      };
+
+      const result = await resolve(acc, reducerRejected).catch(error => error);
+
+      expect(result).toBeInstanceOf(Error);
+      expect(result.reducerStackTrace).toEqual("badReducer");
+    });
+
+    it("should not augment error with reducer stacktrace if already set", async () => {
+      const acc = new Accumulator();
+
+      const testError = new Error("resolveReducer failed");
+      testError.reducerStackTrace = "initialReducerStackTrace";
+
+      const reducerRejected = {
+        id: "badReducer",
+        resolveReducer: jest.fn().mockRejectedValue(testError)
+      };
+
+      const result = await resolve(acc, reducerRejected).catch(error => error);
+
+      expect(result).toBeInstanceOf(Error);
+      expect(result.reducerStackTrace).toEqual("initialReducerStackTrace");
+    });
+  });
+
   describe("tracing", () => {
     it("should call traceSpan.create with accumulator", async () => {
       const acc = new Accumulator();
@@ -49,25 +84,16 @@ describe("resolve", () => {
     it("should call traceSpan.logError when resolveReducer throws error", async () => {
       const acc = new Accumulator();
 
+      const testError = new Error("resolveReducer failed");
       const reducerRejected = {
-        resolveReducer: jest
-          .fn()
-          .mockRejectedValue(new Error("resolveReducer failed"))
+        resolveReducer: jest.fn().mockRejectedValue(testError)
       };
 
       await resolve(acc, reducerRejected).catch(error => error);
 
       expect(traceSpan.logError).toBeCalled();
 
-      // first argument would is an undefined span
-      expect(traceSpan.logError.mock.calls).toMatchInlineSnapshot(`
-                Array [
-                  Array [
-                    undefined,
-                    [Error: resolveReducer failed],
-                  ],
-                ]
-            `);
+      expect(traceSpan.logError.mock.calls[0]).toEqual([undefined, testError]);
     });
 
     it("should finally call traceSpan.finish()", async () => {