more flexible perf test check (#5891)

This PR separates the "last known valid perf test" commit from the
"baseline speedy implementation" commit. It is important for the perf
test to be meaningful that the changes between those two commits are
benign, say minor API adjustments, so that the perf measurement remains
meaningful.

This also adds a check on merging to master that tells Slack if the perf
test has changed and the `test_sha` file needs updating. The Slack
message is conditional on the current commit to avoid excessive noise.

CHANGELOG_BEGIN
CHANGELOG_END

azure-pipelines.yml

@@ -260,6 +260,30 @@ jobs:
           fi
         name: out
 
+  - job: check_perf_test
+    pool:
+      name: linux-pool
+    condition: eq(variables['Build.Reason'], 'IndividualCI')
+    steps:
+    - bash: |
+        TEST_SHA=$(cat ci/cron/perf/test_sha)
+        LAST_CHANGES=$(git log -n1 --format=%H daml-lf/scenario-interpreter/src/perf)
+        CURRENT_SHA=$(git rev-parse HEAD)
+        if [ "$TEST_SHA" != "$LAST_CHANGES" ]; then
+            if [ "$LAST_CHANGES" = "$CURRENT_SHA" ]; then
+                curl -XPOST \
+                     -i \
+                     -H 'Content-Type: application/json' \
+                     --data "{\"text\":\"<!here> Perf tests seem to have changed. Please manually check:\n\`\`\`\ngit diff $TEST_SHA $LAST_CHANGES -- daml-lf/scenario-interpreter/src/perf\n\`\`\`\nand update accordingly. If the change is benign, update \`ci/cron/perf/test_sha\` to \`$LAST_CHANGES\`. With no intervention, you will no longer get performance reports.\"}" \
+                     $(Slack.team-daml)
+            else
+                echo "Changes detected, but not from this commit."
+            fi
+        else
+            echo "No change detected."
+        fi
+      displayName: check perf changes
+
   - job: release
     dependsOn: [ "check_for_release", "Linux", "macOS", "Windows" ]
     condition: and(succeeded(),

ci/cron/daily-compat.yml

@@ -98,10 +98,11 @@ jobs:
           set -euo pipefail
           eval "$(dev-env/bin/dade assist)"
           BASELINE="cebc26af88efef4a7c81c62b0c14353f829b755e"
+          TEST_SHA=$(cat ci/cron/perf/test_sha)
           OUT="$(Build.StagingDirectory)/perf-results.json"
-          if git diff --exit-code $BASELINE -- daml-lf/scenario-interpreter/src/perf >&2; then
+          if git diff --exit-code $TEST_SHA -- daml-lf/scenario-interpreter/src/perf >&2; then
               # no changes, all good
-              ci/cron/compare-perf.sh $BASELINE > $OUT
+              ci/cron/perf/compare.sh $BASELINE > $OUT
           else
               # the tests have changed, we need to figure out what to do with
               # the baseline.

ci/cron/perf/test_sha

@@ -0,0 +1 @@
+53b24793f4a90b777198c7e6dd4e509b4085c140