cmd/osbuild-pipeline: find dnf-json binary

Search for (and set) the path for dnf-json by checking a few known
locations:
- ./dnf-json: for situations when the tool is ran from the source tree.
  This is checked first to prioritise local changes.
- /usr/libexec/osbuild-composer/dnf-json: the default install location
  of the script when osbuild-composer is installed.
- /usr/lib/osbuild-composer/dnf-json: the default install location of
  the script for distributions which don't use /usr/libexec.

The function panics with an informative error message when it fails to
find dnf-json.

cmd/osbuild-pipeline/main.go

@@ -42,6 +42,23 @@ type composeRequest struct {
 	OSTree       ostreeOptions       `json:"ostree"`
 }
 
+// osbuild-pipeline is a utility command and is often run from within the
+// source tree.  Find the dnf-json binary in case the osbuild-composer package
+// isn't installed.  This prioritises the local source version over the system
+// version if run from within the source tree.
+func findDnfJsonBin() string {
+	locations := []string{"./dnf-json", "/usr/libexec/osbuild-composer/dnf-json", "/usr/lib/osbuild-composer/dnf-json"}
+	for _, djPath := range locations {
+		_, err := os.Stat(djPath)
+		if !os.IsNotExist(err) {
+			return djPath
+		}
+	}
+
+	// can't run: panic
+	panic(fmt.Sprintf("could not find 'dnf-json' in any of the known paths: %+v", locations))
+}
+
 func main() {
 	var rpmmdArg bool
 	flag.BoolVar(&rpmmdArg, "rpmmd", false, "output rpmmd struct instead of pipeline manifest")
@@ -126,6 +143,7 @@ func main() {
 	packageSets := imageType.PackageSets(composeRequest.Blueprint)
 
 	solver := dnfjson.NewSolver(d.ModulePlatformID(), d.Releasever(), arch.Name(), path.Join(home, ".cache/osbuild-composer/rpmmd"))
+	solver.SetDNFJSONPath(findDnfJsonBin())
 	depsolvedSets := make(map[string][]rpmmd.PackageSpec)
 	// first depsolve package sets that are part of a chain
 	for specName, setNames := range imageType.PackageSetsChains() {