factor loadWeights into Common (#1377)

As suggested by @Beanow in [a review comment][1], this commit factors
loading weights from disk into a cli/common utility method.

The actual method is really generic, and we have a number of similar
constructions across the codebase (grep for `JSON.parse` to find them).
I considered factoring out a generic utility for loading and
deserializing JSON data from disk in general, but it didn't seem
valuable enough at this time.

Test plan: Unit tests added, existing tests pass.

[1]: https://github.com/sourcecred/sourcecred/pull/1374#discussion_r323149740

src/cli/common.js

@@ -4,6 +4,8 @@
 import os from "os";
 import path from "path";
 import deepFreeze from "deep-freeze";
+import fs from "fs-extra";
+import {type Weights, fromJSON as weightsFromJSON} from "../analysis/weights";
 
 import * as NullUtil from "../util/null";
 
@@ -27,3 +29,16 @@ export function githubToken(): string | null {
 export function discourseKey(): string | null {
   return NullUtil.orElse(process.env.SOURCECRED_DISCOURSE_KEY, null);
 }
+
+export async function loadWeights(path: string): Promise<Weights> {
+  if (!(await fs.exists(path))) {
+    throw new Error("Could not find the weights file");
+  }
+  const raw = await fs.readFile(path, "utf-8");
+  const weightsJSON = JSON.parse(raw);
+  try {
+    return weightsFromJSON(weightsJSON);
+  } catch (e) {
+    throw new Error(`provided weights file is invalid:\n${e}`);
+  }
+}

src/cli/common.test.js

@@ -1,6 +1,10 @@
 // @flow
 
 import path from "path";
+import tmp from "tmp";
+import fs from "fs-extra";
+import {defaultWeights, toJSON as weightsToJSON} from "../analysis/weights";
+import {NodeAddress} from "../core/graph";
 
 import {
   defaultPlugins,
@@ -8,6 +12,7 @@ import {
   sourcecredDirectory,
   githubToken,
   discourseKey,
+  loadWeights,
 } from "./common";
 
 describe("cli/common", () => {
@@ -66,4 +71,36 @@ describe("cli/common", () => {
       expect(discourseKey()).toBe(null);
     });
   });
+
+  describe("loadWeights", () => {
+    function tmpWithContents(contents: mixed) {
+      const name = tmp.tmpNameSync();
+      fs.writeFileSync(name, JSON.stringify(contents));
+      return name;
+    }
+    it("works in a simple success case", async () => {
+      const weights = defaultWeights();
+      // Make a modification, just to be sure we aren't always loading the
+      // default weights.
+      weights.nodeManualWeights.set(NodeAddress.empty, 3);
+      const weightsJSON = weightsToJSON(weights);
+      const file = tmpWithContents(weightsJSON);
+      const weights_ = await loadWeights(file);
+      expect(weights).toEqual(weights_);
+    });
+    it("rejects if the file is not a valid weights file", () => {
+      const file = tmpWithContents(1234);
+      expect.assertions(1);
+      return loadWeights(file).catch((e) =>
+        expect(e.message).toMatch("provided weights file is invalid:")
+      );
+    });
+    it("rejects if the file does not exist", () => {
+      const file = tmp.tmpNameSync();
+      expect.assertions(1);
+      return loadWeights(file).catch((e) =>
+        expect(e.message).toMatch("Could not find the weights file")
+      );
+    });
+  });
 });

src/cli/load.js

@@ -5,10 +5,9 @@ import dedent from "../util/dedent";
 import {LoggingTaskReporter} from "../util/taskReporter";
 import type {Command} from "./command";
 import * as Common from "./common";
-import {defaultWeights, fromJSON as weightsFromJSON} from "../analysis/weights";
+import {defaultWeights} from "../analysis/weights";
 import {load} from "../api/load";
 import {specToProject} from "../plugins/github/specToProject";
-import fs from "fs-extra";
 import {partialParams} from "../analysis/timeline/params";
 import {DEFAULT_PLUGINS} from "./defaultPlugins";
 
@@ -93,7 +92,7 @@ const loadCommand: Command = async (args, std) => {
 
   let weights = defaultWeights();
   if (weightsPath) {
-    weights = await loadWeightOverrides(weightsPath);
+    weights = await Common.loadWeights(weightsPath);
   }
 
   const githubToken = Common.githubToken();
@@ -124,20 +123,6 @@ const loadCommand: Command = async (args, std) => {
   return 0;
 };
 
-const loadWeightOverrides = async (path: string) => {
-  if (!(await fs.exists(path))) {
-    throw new Error("Could not find the weights file");
-  }
-
-  const raw = await fs.readFile(path, "utf-8");
-  const weightsJSON = JSON.parse(raw);
-  try {
-    return weightsFromJSON(weightsJSON);
-  } catch (e) {
-    throw new Error(`provided weights file is invalid:\n${e}`);
-  }
-};
-
 export const help: Command = async (args, std) => {
   if (args.length === 0) {
     usage(std.out);