get coverage hsitory from git notes

.github/workflows/test.yml

@@ -643,6 +643,8 @@ jobs:
       - name: ⛙ Merge coverage reports
         run: |
           coverage combine test-logs/*/.coverage
+      - name: ± Compute change in coverage
+        run: ./utils/coverage-log.sh
       - name: 📃 Produce coverage reports
         run: |
           coverage html -d lenskit-coverage

lkdev/workflows/test.py

@@ -462,6 +462,7 @@ def jobs_result(deps: list[str]) -> GHJob:
                     coverage combine test-logs/*/.coverage
                 """),
             },
+            {"name": "± Compute change in coverage", "run": "./utils/coverage-log.sh"},
             {
                 "name": "📃 Produce coverage reports",
                 "run": script("""

utils/coverage-log.sh

@@ -0,0 +1,40 @@
+#!/bin/zsh
+
+git fetch --all || exit 2
+coverage json || exit 2
+
+if [[ -n $GITHUB_HEAD_REF ]]; then
+    # PR run — save difference to output
+    git log --pretty=ref $GITHUB_HEAD_REF | while read commit msg; do
+        prev_data="$(git notes --ref=coverage show $log)"
+        if [[ "$?" -eq 0 ]]; then
+            break
+        fi
+    done
+    if [[ -z "$prev_data" ]]; then
+        echo "no previous coverage found" >&2
+        exit 2
+    fi
+
+    prev_cov="$(echo "prev_data" |jq .totals.percent_covered)"
+    if [[ $? -ne 0 ]]; then
+        echo "jq failed" >&2
+        exit 2
+    fi
+    cur_cov="$(jq .totals.percent_covered coverage.json)"
+    if [[ $? -ne 0 ]]; then
+        echo "jq failed" >&2
+        exit 2
+    fi
+
+    cov_change=$(( $cur_cov - $prev_cov ))
+    echo "coverage change: $cov_change"
+    cat >>"$GITHUB_JOB_SUMMARY" <<EOM
+Coverage change **$cov_change** (from $prev_cov to $cur_cov).
+
+EOM
+elif [[ $GITHUB_EVENT_NAME = push && -n $GITHUB_TOKEN ]]; then
+    jq '{meta: .meta, totals: .totals}' coverage.json >cov-summary.json || exit 2
+    git notes --ref=coverage add -F cov-summary.json HEAD || exit 2
+    git push origin refs/notes/coverage || exit 2
+fi