devtool: Build A and B binaries if $do_build is true

tests/README.md

@@ -142,6 +142,21 @@ above when run on a PR will fail iff a newly added dependency has a known open
 RustSec advisory. If run outside a PR, it will fail if any existing dependency
 has an open RustSec advisory).
 
+### Functional A/B-Tests
+
+Firecracker has some functional A/B-tests (for example, in
+`test_vulnerabilities.py`), which generally compare the state of the pull
+request target branch (e.g. `main`), with the PR head. However, when running
+these locally, pytest does not know anything about potential PRs that the commit
+the tests are being run on are contained in, and as such cannot do this
+A/B-Test. To run functional A/B-Tests locally, you need to create a "fake" PR
+environment by setting the `BUILDKITE_PULL_REQUEST` and
+`BUILDKITE_PULL_REQUEST_BASE_BRANCH` environment variables:
+
+```
+BUILDKITE_PULL_REQUEST=true BUILDKITE_PULL_REQUEST_BASE_BRANCH=main ./tools/devtool test -- integration_tests/security/test_vulnerabilities.py
+```
+
 ### Performance A/B-Tests
 
 Firecracker has a special framework for orchestrating long-running A/B-tests

tools/devtool

@@ -722,7 +722,12 @@ cmd_test() {
     ensure_devctr
     ensure_build_dir
     ensure_ci_artifacts
-    [ $do_build != 0 ] && cmd_build --release
+    if [ $do_build != 0 ]; then
+      cmd_build --release
+      if [ -n "$BUILDKITE_PULL_REQUEST_BASE_BRANCH" ]; then
+        cmd_build --release --rev "$BUILDKITE_PULL_REQUEST_BASE_BRANCH"
+      fi
+    fi
 
     apply_linux_61_tweaks