chore: improve unit test CI performance via shared deps cache and Jest cache (#28419)

---

## **Description**

If we start to see OOM failures (exit code 137, "JavaScript heap out of
memory") after this change, the first thing to try would be lowering
--max_old_space_size from 20480 to something more realistic like 8192,
which forces Node's GC to be more aggressive rather than hoarding memory
until the OS kills the process.

<!--
Write a short description of the changes included in this pull request,
also include relevant motivation and context. Have in mind the following
questions:
1. What is the reason for the change?
2. What is the improvement/solution?
-->

Unit tests in CI were taking **~18+ minutes** due to several avoidable
bottlenecks. This PR addresses three root causes without requiring paid
infrastructure:

**Problems fixed:**
1. **Redundant dependency installation** — Each of 10 unit test shards,
2 component-view test shards, and the coverage merge job independently
ran `yarn install --immutable` + `yarn setup:github-ci --node` (~3–5 min
each), totalling 13 redundant installs per CI run.
2. **Jest transform cache disabled** — `cache: false` in
`jest.config.js` forced every shard to re-parse and re-transform all
~3,060 test files through `babel-jest` from scratch on every run.
3. **Low parallelism within shards** — `maxWorkers: '20%'` on a 4-vCPU
`ubuntu-latest` runner meant Jest used ~1 worker per shard, leaving 3
cores idle.

**Changes:**
- New `prepare-unit-test-deps` CI job runs `yarn install` +
`setup:github-ci --node` once and caches `node_modules` via
`actions/cache`. All downstream jobs restore from this cache (~30s)
instead of installing independently.
- Jest transform cache persisted across CI runs via `actions/cache`,
keyed on `yarn.lock` + `babel.config.tests.js`. Transforms are only
redone when dependencies or transform config change.
- `cache: false` → `cache: true` in `jest.config.js`, with a
configurable `cacheDirectory` via `JEST_CACHE_DIRECTORY` env var so CI
controls the cache path.
- `maxWorkers` changed from `'20%'` (always) to `'50%'` in CI / `'20%'`
locally, utilizing ~2 workers on the 4-core runner instead of ~1.


**Expected impact (warm cache):**

| Metric | Before | After |
|---|---|---|
| Setup time per shard | ~3–5 min | ~30s |
| Transform time per shard | Full rebuild every run | Cached |
| Jest workers per shard | ~1 | ~2 |
| **Estimated wall-clock time** | **~18+ min** | **~8–12 min** |

## **Changelog**

CHANGELOG entry: null

## **Related issues**

Fixes:

## **Manual testing steps**

N/A

## **Screenshots/Recordings**

### **Before**
<img width="1079" height="381" alt="image"
src="https://github.com/user-attachments/assets/4d2a32f5-6ab0-410d-838e-b902172567a2"
/>

N/A

### **After**
<img width="976" height="318" alt="image"
src="https://github.com/user-attachments/assets/25655008-6956-49b7-8a4f-db1bf72ea12c"
/>

N/A

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I've included tests if applicable
- [x] I've documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I've applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [x] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [x] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

<!-- Generated with the help of the pr-description AI skill -->

Made with [Cursor](https://cursor.com)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes CI test execution by introducing cross-job workspace caching
and enabling Jest caching, which could cause flaky runs or cache-miss
failures if the cache keys/paths are incorrect. No production runtime
code is affected.
> 
> **Overview**
> Improves CI test performance by introducing a new
`prepare-unit-test-deps` job that installs once and caches a prepared
workspace (`node_modules`, `.yarn`, and generated terms-of-use content)
for all unit-test shards, component-view shards, and the coverage merge
job.
> 
> Enables and persists Jest’s transform cache across CI runs (shared
`JEST_CACHE_DIRECTORY` at `/tmp/jest_rs` with an `actions/cache` key
based on `yarn.lock` and `babel.config.tests.js`) and increases Jest
parallelism in CI by setting `maxWorkers` to `50%` when `process.env.CI`
is set.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
cafd2e6. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: tommasini

.github/workflows/ci.yml

@@ -259,15 +259,14 @@ jobs:
         run: ${{ steps.download-actionlint.outputs.executable }} -color -config-file .github/actionlint.yaml
         shell: bash
 
-  unit-tests:
-    name: Unit tests (${{ matrix.shard }})
+  # === Unit Test Performance ===
+  # See docs/ci-unit-test-performance.md for full rationale, applied optimizations, and future proposals.
+  prepare-unit-test-deps:
+    name: Prepare unit test dependencies
     runs-on: ubuntu-latest
     if: github.event_name != 'merge_group' || needs.check-skip-merge-queue.outputs.skip-merge-queue != 'true'
     needs:
       - check-skip-merge-queue
-    strategy:
-      matrix:
-        shard: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
@@ -283,13 +282,54 @@ jobs:
           command: yarn install --immutable
       - name: Clean state and following up dependencies installation
         run: yarn setup:github-ci --node
+      - name: Cache prepared workspace
+        uses: actions/cache/save@v4
+        with:
+          path: |
+            node_modules
+            .yarn
+            app/util/termsOfUse/termsOfUseContent.ts
+          key: unit-test-deps-${{ github.sha }}
+
+  unit-tests:
+    name: Unit tests (${{ matrix.shard }})
+    runs-on: ubuntu-latest
+    if: github.event_name != 'merge_group' || needs.check-skip-merge-queue.outputs.skip-merge-queue != 'true'
+    needs:
+      - check-skip-merge-queue
+      - prepare-unit-test-deps
+    strategy:
+      matrix:
+        shard: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-node@v3
+        with:
+          node-version-file: '.nvmrc'
+      - name: Restore prepared workspace
+        uses: actions/cache/restore@v4
+        with:
+          path: |
+            node_modules
+            .yarn
+            app/util/termsOfUse/termsOfUseContent.ts
+          key: unit-test-deps-${{ github.sha }}
+          fail-on-cache-miss: true
+      - name: Restore Jest transform cache
+        uses: actions/cache@v4
+        with:
+          path: /tmp/jest_rs
+          key: jest-transform-${{ hashFiles('yarn.lock', 'babel.config.tests.js') }}-${{ matrix.shard }}
+          restore-keys: |
+            jest-transform-${{ hashFiles('yarn.lock', 'babel.config.tests.js') }}-
       - name: Prepare results directory
         run: mkdir -p tests/results
       # The "10" in this command is the total number of shards. It must be kept
       # in sync with the length of matrix.shard
       - run: yarn test:unit --shard=${{ matrix.shard }}/10 --forceExit --silent --coverageReporters=json --json --outputFile=tests/results/unit-test-results-${{ matrix.shard }}.json
         env:
           NODE_OPTIONS: --max_old_space_size=20480
+          JEST_CACHE_DIRECTORY: /tmp/jest_rs
       - name: Rename coverage report and extract test count for this shard
         shell: bash
         run: |
@@ -317,23 +357,22 @@ jobs:
   # threshold calculation is accurate.
   merge-unit-and-component-view-tests:
     runs-on: ubuntu-latest
-    needs: [unit-tests, component-view-tests]
+    needs: [unit-tests, component-view-tests, prepare-unit-test-deps]
     if: ${{ !cancelled() && github.event_name != 'merge_group' }}
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
           node-version-file: '.nvmrc'
-          cache: yarn
-      - name: Install Yarn dependencies with retry
-        uses: nick-fields/retry@ce71cc2ab81d554ebbe88c79ab5975992d79ba08 #v3.0.2
-        with:
-          timeout_minutes: 10
-          max_attempts: 3
-          retry_wait_seconds: 30
-          command: yarn install --immutable
-      - name: Clean state and following up dependencies installation
-        run: yarn setup:github-ci --node
+      - name: Restore prepared workspace
+        uses: actions/cache/restore@v4
+        with:
+          path: |
+            node_modules
+            .yarn
+            app/util/termsOfUse/termsOfUseContent.ts
+          key: unit-test-deps-${{ github.sha }}
+          fail-on-cache-miss: true
       - uses: actions/download-artifact@v4
         with:
           pattern: coverage-*
@@ -428,6 +467,7 @@ jobs:
     if: github.event_name != 'merge_group' || needs.check-skip-merge-queue.outputs.skip-merge-queue != 'true'
     needs:
       - check-skip-merge-queue
+      - prepare-unit-test-deps
     strategy:
       matrix:
         shard: [1, 2]
@@ -436,16 +476,15 @@ jobs:
       - uses: actions/setup-node@v3
         with:
           node-version-file: '.nvmrc'
-          cache: yarn
-      - name: Install Yarn dependencies with retry
-        uses: nick-fields/retry@ce71cc2ab81d554ebbe88c79ab5975992d79ba08 #v3.0.2
-        with:
-          timeout_minutes: 10
-          max_attempts: 3
-          retry_wait_seconds: 30
-          command: yarn install --immutable
-      - name: Clean state and following up dependencies installation
-        run: yarn setup:github-ci --node
+      - name: Restore prepared workspace
+        uses: actions/cache/restore@v4
+        with:
+          path: |
+            node_modules
+            .yarn
+            app/util/termsOfUse/termsOfUseContent.ts
+          key: unit-test-deps-${{ github.sha }}
+          fail-on-cache-miss: true
       - name: Prepare results directory
         run: mkdir -p tests/results
       - run: |

jest.config.js

@@ -60,7 +60,7 @@ const config = {
   ],
   coverageReporters: ['text-summary', 'lcov'],
   coverageDirectory: '<rootDir>/tests/coverage',
-  maxWorkers: process.env.NODE_ENV === 'production' ? '50%' : '20%',
+  maxWorkers: process.env.CI ? '50%' : '20%',
   moduleNameMapper: {
     '\\.(svg)$': '<rootDir>/app/__mocks__/svgMock.js',
     '\\.(png)$': '<rootDir>/app/__mocks__/pngMock.js',
@@ -89,8 +89,10 @@ const config = {
       '<rootDir>/app/__mocks__/spinnerMock.js',
     '^rive-react-native$': '<rootDir>/app/__mocks__/rive-react-native.tsx',
   },
-  // Disable jest cache
-  cache: false,
+  cache: true,
+  ...(process.env.JEST_CACHE_DIRECTORY && {
+    cacheDirectory: process.env.JEST_CACHE_DIRECTORY,
+  }),
 };
 
 // eslint-disable-next-line import-x/no-commonjs