jobs/build: rerun build-arch if previous build is incomplete

Currently, if the x86_64 build succeeds, but not e.g. the aarch64 build,
a rerun of the `build` job will no-op. Salvaging the build requires
rerunning just the `build-arch` job for aarch64, and then rerunning the
`release` job. This is relatively easy for a human, but isn't very
automation-friendly.

With this patch, if we no-op but the latest build is missing builds for
some of the architectures, we automatically rerun the `build-arch` job
for those missing arches and then rerun the `release` job. This allows
the interface for automation to solely be the `build` job, and allows
transparently salvaging up-to-date but incomplete builds rather than
paying for a much costlier full rebuild on all arches.

Author: jlebon

jobs/build.Jenkinsfile

@@ -261,6 +261,27 @@ lock(resource: "build-${params.STREAM}") {
         if (prevBuildID == buildID) {
             currentBuild.result = 'SUCCESS'
             currentBuild.description = "${build_description} 💤 (no new build)"
+
+            // Nothing changed since the latest build. Check if it's missing
+            // some arches and retrigger `build-arch` only for the missing
+            // arches, and the follow-up `release` job. Match the exact src
+            // config commit that was used.
+            def builds = readJSON file: "builds/builds.json"
+            assert buildID == builds.builds[0].id
+            def missing_arches = additional_arches - builds.builds[0].arches
+            if (missing_arches) {
+                def meta = readJSON(text: shwrapCapture("cosa meta --build=${buildID} --dump"))
+                def rev = meta["coreos-assembler.config-gitrev"]
+                currentBuild.description = "${build_description} 🔨 ${buildID}"
+                if (uploading) {
+                    run_multiarch_jobs(missing_arches, rev, buildID, cosa_img)
+                    if (stream_info.type != "production") {
+                        run_release_job(buildID)
+                    }
+                }
+            }
+
+            // And we're done!
             return
         }