ci: improve caching (#7943)

.github/actions/setup-go/action.yaml

@@ -8,8 +8,32 @@ runs:
       with:
         cache: true
         go-version: "1.20.5"
+
+    - name: Cache go
+      uses: buildjet/cache@v3
+      with:
+        # ~/go/pkg is the same across operating systems.
+        path: |
+          ~/go/pkg
+          ~/.cache/go-build
+          ~/AppData/Local/go-build
+          ~/Library/Caches/go-build
+        # Job name must be included in the key for effective
+        # test cache reuse.
+        key: go-${{ runner.os }}-${{ github.job }}-${{ hashFiles('**/*.go', 'go.**') }}
+        restore-keys: |
+          go-${{ runner.os }}-${{ github.job }}-
+          go-${{ runner.os }}-
+          go-
+
     - name: Install gotestsum
       uses: jaxxstorm/action-install-gh-release@v1.10.0
       with:
         repo: gotestyourself/gotestsum
         tag: v1.9.0
+
+    # It isn't necessary that we ever do this, but it helps
+    # separate the "setup" from the "run" times.
+    - name: go mod download
+      shell: bash
+      run: go mod download -x

.github/workflows/ci.yaml

@@ -218,8 +218,18 @@ jobs:
             echo "cover=false" >> $GITHUB_OUTPUT
           fi
 
+          # By default Go will use the number of logical CPUs, which
+          # is a fine default.
+          PARALLEL_FLAG=""
+          if [ "${{ matrix.os }}" == "windows-2019" ]; then
+            # Windows appears more I/O bound, so we increase parallelism
+            # to make better use of CPU.
+            PARALLEL_FLAG="-parallel=16"
+          fi
+
           export TS_DEBUG_DISCO=true
-          gotestsum --junitfile="gotests.xml" --jsonfile="gotests.json" --packages="./..." -- -parallel=8 -timeout=7m -short -failfast $COVERAGE_FLAGS
+          gotestsum --junitfile="gotests.xml" --jsonfile="gotests.json" \
+            --packages="./..." -- $PARALLEL_FLAG -short -failfast $COVERAGE_FLAGS
 
       - name: Print test stats
         if: success() || failure()
@@ -228,13 +238,6 @@ jobs:
           # so we need to print the test stats to the log.
           go run ./scripts/ci-report/main.go gotests.json | tee gotests_stats.json
 
-      - uses: actions/upload-artifact@v3
-        if: success() || failure()
-        with:
-          name: gotests-${{ matrix.os }}.xml
-          path: ./gotests.xml
-          retention-days: 30
-
       - uses: ./.github/actions/upload-datadog
         if: always()
         with: