support conditionally building artifacts in an OSBuild stage

With coreos/coreos-assembler#3930 we can
now build multiple artifacts/platforms in a single OSBuild call.
Let's add support here for:

1. Detecting if an artifact supports building via OSBuild by
   looking to see if the buildextend-{artifact} API points
   to cmd-osbuild. i.e. is OSBuild the default for this artifact?
2. Allowing the pipecfg to specify additional artifacts to build
   using OSBuild. This is useful if OSBuild is not yet the default
   way to build this particular artifact.

Author: dustymabe

config.yaml

@@ -16,6 +16,7 @@ streams:
     type: mechanical
     env:
       COSA_TESTISO_DEBUG: true
+    osbuild_experimental: true
   # branched:
   #  type: mechanical
   #  env:

docs/config.yaml

@@ -98,6 +98,9 @@ streams:
       check_kernel_rt_mismatch_rhcos: true
       # OPTIONAL: list of kola tags to skip for this stream
       skip_kola_tags: [openshift, needs-internet, luks]
+      # OPTIONAL: allow building using OSBuild for artifacts where OSBuild is
+      # not yet the default in CoreOS Assembler.
+      osbuild_experimental: true
 
 # REQUIRED: architectures to build for other than x86_64
 additional_arches: [aarch64, ppc64le, s390x]

utils.groovy

@@ -427,6 +427,41 @@ def build_artifacts(pipecfg, stream, basearch) {
     // First get the list of artifacts to build from the config
     def artifacts = get_artifacts_to_build(pipecfg, stream, basearch)
 
+    // If `cosa osbuild` is supported then let's build what we can using OSBuild
+    if (shwrapRc("cosa shell -- test -e /usr/lib/coreos-assembler/cmd-osbuild") == 0) {
+        // Determine which platforms are OSBuild experimental versus
+        // stable (i.e. the default is to use OSBuild for them).
+        def experimental = shwrapCapture("cosa osbuild --supported-platforms").tokenize()
+        def stable = shwrapCapture('''
+            cosa shell -- bash -c '
+                for buildextend in /usr/lib/coreos-assembler/cmd-buildextend-*; do
+                    if [ "$(readlink -f ${buildextend})" == "/usr/lib/coreos-assembler/cmd-osbuild" ]; then
+                        echo "${buildextend:42}"
+                    fi
+                done
+            '
+        ''').tokenize('\n')
+        // Based on the pipeline configuration we'll either use OSBuild for as
+        // much as we can (experimental) or just what it is the default for (stable)
+        def osbuild_supported_artifacts = stable
+        if (pipecfg.streams[stream].osbuild_experimental) {
+            osbuild_supported_artifacts = experimental
+        }
+        // Let's build separately the artifacts that can be built directly with OSBuild.
+        def osbuild_artifacts = []
+        for (artifact in artifacts) {
+            if (artifact in osbuild_supported_artifacts) {
+                osbuild_artifacts += artifact
+            }
+        }
+        if (!osbuild_artifacts.isEmpty()) {
+            artifacts.remove(osbuild_artifacts)
+            stage('💽:OSBuild') {
+                shwrap("cosa osbuild ${osbuild_artifacts.join(' ')}")
+            }
+        }
+    }
+
     // Next let's do some massaging of the inputs based on two problems we
     // need to consider:
     //