Add code coverage README (#7613)

Author: granluo

README.md

@@ -187,6 +187,10 @@ files without real values, but can be replaced with real plist files. To get you
 identifier (e.g. `com.google.Database-Example`)
 4. Download the resulting `GoogleService-Info.plist` and add it to the Xcode project.
 
+### Coverage Report Generation
+
+See [scripts/code_coverage_report/README.md](scripts/code_coverage_report/README.md).
+
 ## Specific Component Instructions
 See the sections below for any special instructions for those components.
 

scripts/code_coverage_report/README.md

@@ -0,0 +1,38 @@
+# Code Coverage Report Generation
+
+This tool is to help generate coverage reports for pull requests. It's defined by the [test_coverage  workflow](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/code_coverage_report/code_coverage_file_list.json).
+
+Coverage reports of SDK frameworks will be displayed in a pull request if the change is under corresponding SDK file patterns.
+
+[UpdatedFilesCollector](https://github.com/firebase/firebase-ios-sdk/tree/master/scripts/code_coverage_report/generate_code_coverage_report/Sources/UpdatedFilesCollector) will detect file changes and compare file paths to file patterns in [code_coverage_file_list.json](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/code_coverage_report/code_coverage_file_list.json). If updated file paths fit any patterns, corresponding SDK coverage job will be triggered.
+
+## Add a new coverage workflow
+
+To create a code coverage workflow for a new SDK,
+1. Add `newsdk` and its patterns in [code_coverage_file_list.json](https://github.com/firebase/firebase-ios-sdk/blob/master/scripts/code_coverage_report/code_coverage_file_list.json).
+2. Add a new output flag, e.g. `newsdk_run_job`, in the [coverage workflow](https://github.com/firebase/firebase-ios-sdk/blob/64d50a7f7b3af104a88f9c9203285ae20ea309d4/.github/workflows/test_coverage.yml#L17). `newsdk_run_job` should be aligned with the name of SDK `newsdk` in code_coverage_file_list.json.
+3. Add a newsdk coverage job in the [workflow](https://github.com/firebase/firebase-ios-sdk/blob/master/.github/workflows/test_coverage.yml):
+```
+pod-lib-lint-newsdk:
+    needs: check
+    # Don't run on private repo unless it is a PR.
+    if: always() && github.repository == 'Firebase/firebase-ios-sdk' && (needs.check.outputs.newsdk_run_job == 'true'|| github.event.pull_request.merged)
+    runs-on: macOS-latest
+    strategy:
+      matrix:
+        target: [iOS]
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup Bundler
+      run: scripts/setup_bundler.sh
+    - name: Build and test
+      run: ./scripts/code_coverage_report/pod_test_code_coverage_report.sh FirebaseNewSDK "${{ matrix.target }}"
+    - uses: actions/upload-artifact@v2
+      with:
+        name: codecoverage
+        path: /Users/runner/*.xcresult
+```
+Add the job name to the [`needs` of `create_report` job](https://github.com/firebase/firebase-ios-sdk/blob/64d50a7f7b3af104a88f9c9203285ae20ea309d4/.github/workflows/test_coverage.yml#L277).
+
+4. If this newsdk podspec has unit test setup, e.g. [database](https://github.com/firebase/firebase-ios-sdk/blob/64d50a7f7b3af104a88f9c9203285ae20ea309d4/FirebaseDatabase.podspec#L44-L57), with `unit_tests.scheme = { :code_coverage => true }`, the code coverage workflow should run unit tests through podspecs and utilize those coverage data and nothing is needed to update here. Otherwise, add [another way of running tests](https://github.com/firebase/firebase-ios-sdk/blob/64d50a7f7b3af104a88f9c9203285ae20ea309d4/scripts/code_coverage_report/pod_test_code_coverage_report.sh#L26) and generating xcresult bundles with code coverage data in pod_test_code_coverage_report.sh.
+