Parallelize Flow in CI (#20794)

* Parallelize Flow in CI

We added more host configs recently, and we run all the checks in
sequence, so sometimes Flow ends up being the slowest CI job.

This splits the job across multiple processes.

* Fix environment variable typo

Co-authored-by: Ricky <rickhanlonii@gmail.com>

Co-authored-by: Ricky <rickhanlonii@gmail.com>

.circleci/config.yml

@@ -83,6 +83,7 @@ jobs:
   yarn_flow:
     docker: *docker
     environment: *environment
+    parallelism: 5
 
     steps:
       - checkout
@@ -384,9 +385,6 @@ workflows:
       - yarn_lint:
           requires:
             - setup
-      - yarn_flow:
-          requires:
-            - setup
       - RELEASE_CHANNEL_stable_yarn_build:
           requires:
             - setup
@@ -426,6 +424,9 @@ workflows:
     unless: << pipeline.parameters.prerelease_commit_sha >>
     jobs:
       - setup
+      - yarn_flow:
+          requires:
+            - setup
       - yarn_test:
           requires:
             - setup

scripts/tasks/flow-ci.js

@@ -14,12 +14,22 @@ process.on('unhandledRejection', err => {
 const runFlow = require('../flow/runFlow');
 const inlinedHostConfigs = require('../shared/inlinedHostConfigs');
 
+// Parallelize tests across multiple tasks.
+const nodeTotal = process.env.CIRCLE_NODE_TOTAL
+  ? parseInt(process.env.CIRCLE_NODE_TOTAL, 10)
+  : 1;
+const nodeIndex = process.env.CIRCLE_NODE_INDEX
+  ? parseInt(process.env.CIRCLE_NODE_INDEX, 10)
+  : 0;
+
 async function checkAll() {
-  // eslint-disable-next-line no-for-of-loops/no-for-of-loops
-  for (let rendererInfo of inlinedHostConfigs) {
-    if (rendererInfo.isFlowTyped) {
-      await runFlow(rendererInfo.shortName, ['check']);
-      console.log();
+  for (let i = 0; i < inlinedHostConfigs.length; i++) {
+    if (i % nodeTotal === nodeIndex) {
+      const rendererInfo = inlinedHostConfigs[i];
+      if (rendererInfo.isFlowTyped) {
+        await runFlow(rendererInfo.shortName, ['check']);
+        console.log();
+      }
     }
   }
 }