[Danger] Use the PR's mergebase for a branch in the dangerfile (#12049)

* [Danger] Use the PR's mergebase for a branch in the dangerfile instead of
the root commit's parent.

* [Danger] Get the full history to find the merge base

dangerfile.js

@@ -7,11 +7,30 @@
 
 'use strict';
 
+// Hi, if this is your first time editing/reading a Dangerfile, here's a summary:
+// It's a JS runtime which helps you provide continuous feedback inside GitHub.
+//
+// You can see the docs here: http://danger.systems/js/
+//
+// If you want to test changes Danger, I'd recommend checking out an existing PR
+// and then running the `danger pr` command.
+//
+// You'll need a GitHub token, you can re-use this one:
+//
+//  e622517d9f1136ea8900 07c6373666312cdfaa69
+//
+// (Just remove the space)
+//
+// So, for example:
+//
+// `DANGER_GITHUB_API_TOKEN=[ENV_ABOVE] yarn danger pr https://github.com/facebook/react/pull/11865
+
 const {markdown, danger} = require('danger');
 const fetch = require('node-fetch');
 
 const {generateResultsArray} = require('./scripts/rollup/stats');
 const {readFileSync} = require('fs');
+const {exec} = require('child_process');
 
 const currentBuildResults = JSON.parse(
   readFileSync('./scripts/rollup/results.json')
@@ -54,13 +73,40 @@ function setBoldness(row, isBold) {
   }
 }
 
-// Grab the results.json before we ran CI via the GH API
-// const baseMerge = danger.github.pr.base.sha
-const parentOfOldestCommit = danger.git.commits[0].parents[0];
-const commitURL = sha =>
-  `http://react.zpao.com/builds/master/_commits/${sha}/results.json`;
+/**
+ * Gets the commit that represents the merge between the current branch
+ * and master.
+ */
+function getMergeBase() {
+  return git('merge-base HEAD origin/master');
+}
 
-fetch(commitURL(parentOfOldestCommit)).then(async response => {
+/**
+ * Gets the commit that represents the merge between the current branch
+ * and master.
+ */
+function git(args) {
+  return new Promise(res => {
+    exec('git ' + args, (err, stdout, stderr) => {
+      if (err) {
+        throw err;
+      } else {
+        res(stdout.trim());
+      }
+    });
+  });
+}
+
+(async function() {
+  // Use git locally to grab the commit which represents the place
+  // where the branches differ
+  const mergeBaseCommit = await getMergeBase();
+  const commitURL = sha =>
+    `http://react.zpao.com/builds/master/_commits/${sha}/results.json`;
+  const response = await fetch(commitURL(mergeBaseCommit));
+
+  // Take the JSON of the build response and
+  // make an array comparing the results for printing
   const previousBuildResults = await response.json();
   const results = generateResultsArray(
     currentBuildResults,
@@ -74,6 +120,7 @@ fetch(commitURL(parentOfOldestCommit)).then(async response => {
         Math.abs(r.prevFileSizeChange) > percentToWarrentShowing ||
         Math.abs(r.prevGzipSizeChange) > percentToWarrentShowing
     )
+
     .map(r => r.packageName);
 
   if (packagesToShow.length) {
@@ -152,16 +199,16 @@ fetch(commitURL(parentOfOldestCommit)).then(async response => {
     }
 
     const summary = `
-<details>
-<summary>Details of bundled changes.</summary>
+  <details>
+  <summary>Details of bundled changes.</summary>
 
-<p>Comparing: ${parentOfOldestCommit}...${danger.github.pr.head.sha}</p>
+  <p>Comparing: ${mergeBaseCommit}...${danger.github.pr.head.sha}</p>
 
 
-${allTables.join('\n')}
+  ${allTables.join('\n')}
 
-</details>
-`;
+  </details>
+  `;
     markdown(summary);
   }
-});
+})();

scripts/circleci/test_entry_point.sh

@@ -25,7 +25,7 @@ if [ $((2 % CIRCLE_NODE_TOTAL)) -eq "$CIRCLE_NODE_INDEX" ]; then
   COMMANDS_TO_RUN+=('./scripts/circleci/build.sh')
   COMMANDS_TO_RUN+=('yarn test-build --maxWorkers=2')
   COMMANDS_TO_RUN+=('yarn test-build-prod --maxWorkers=2')
-#  COMMANDS_TO_RUN+=('node ./scripts/tasks/danger')
+  COMMANDS_TO_RUN+=('node ./scripts/tasks/danger')
   COMMANDS_TO_RUN+=('./scripts/circleci/upload_build.sh')
 fi
 

scripts/rollup/stats.js

@@ -21,7 +21,13 @@ function saveResults() {
 }
 
 function percentChange(prev, current) {
-  return Math.floor((current - prev) / prev * 100);
+  const change = Math.floor((current - prev) / prev * 100);
+  // When a new package is created
+  if (isFinite(change)) {
+    return change;
+  } else {
+    return 100;
+  }
 }
 
 function percentChangeString(change) {