Improve handling of droidXML config

A number of calls are made to retrieve the default location of the
DROID signature file dynamically. These are turned into a single
call in the app init. Further, there is a bit more feedback during
harvest to provide the user with more info when a DROID signature
file does not exist.

cmd/roy/roy.go

@@ -116,11 +116,18 @@ Additional flags:
       Use a different siegfried home directory.
 `
 
+var discoveredDroidFile string
+
+func init() {
+	// Retrieve our dynamic defaults.
+	discoveredDroidFile = config.Droid()
+}
+
 var (
 	// BUILD, ADD flag sets
 	build         = flag.NewFlagSet("build | add", flag.ExitOnError)
 	home          = build.String("home", config.Home(), "override the default home directory")
-	droid         = build.String("droid", config.Droid(), "set name/path for DROID signature file")
+	droid         = build.String("droid", discoveredDroidFile, "set name/path for DROID signature file")
 	mi            = build.String("mi", "", "set name/path for MIMEInfo signature file")
 	fdd           = build.String("fdd", "", "set name/path for LOC FDD signature file")
 	locfdd        = build.Bool("loc", false, "build a LOC FDD signature file")
@@ -158,7 +165,7 @@ var (
 	// HARVEST
 	harvest                    = flag.NewFlagSet("harvest", flag.ExitOnError)
 	harvestHome                = harvest.String("home", config.Home(), "override the default home directory")
-	harvestDroid               = harvest.String("droid", config.Droid(), "set name/path for DROID signature file")
+	harvestDroid               = harvest.String("droid", discoveredDroidFile, "set name/path for DROID signature file")
 	harvestChanges             = harvest.Bool("changes", false, "harvest the latest PRONOM release-notes.xml file")
 	_, htimeout, _, _          = config.HarvestOptions()
 	timeout                    = harvest.Duration("timeout", htimeout, "set duration before timing-out harvesting requests e.g. 120s")
@@ -171,7 +178,7 @@ var (
 	// INSPECT (roy inspect | roy inspect fmt/121 | roy inspect usr/local/mysig.sig | roy inspect 10)
 	inspect         = flag.NewFlagSet("inspect", flag.ExitOnError)
 	inspectHome     = inspect.String("home", config.Home(), "override the default home directory")
-	inspectDroid    = inspect.String("droid", config.Droid(), "set name/path for DROID signature file")
+	inspectDroid    = inspect.String("droid", discoveredDroidFile, "set name/path for DROID signature file")
 	inspectReports  = inspect.Bool("reports", false, "build signatures from PRONOM reports (rather than DROID xml)")
 	inspectExtend   = inspect.String("extend", "", "comma separated list of additional signatures")
 	inspectExtendc  = inspect.String("extendc", "", "comma separated list of additional container signatures")
@@ -188,7 +195,7 @@ var (
 	// SETS
 	setsf       = flag.NewFlagSet("sets", flag.ExitOnError)
 	setsHome    = setsf.String("home", config.Home(), "override the default home directory")
-	setsDroid   = setsf.String("droid", config.Droid(), "set name/path for DROID signature file")
+	setsDroid   = setsf.String("droid", discoveredDroidFile, "set name/path for DROID signature file")
 	setsChanges = setsf.Bool("changes", false, "create a pronom-changes.json sets file")
 	setsList    = setsf.String("list", "", "expand comma separated list of format sets")
 
@@ -208,7 +215,7 @@ func savereps() error {
 	file.Close()
 	errs := pronom.Harvest()
 	if len(errs) > 0 {
-		return fmt.Errorf("roy: errors saving reports to disk %s", errs)
+		return fmt.Errorf("roy: error saving reports to disk %s", errs)
 	}
 	return nil
 }
@@ -332,7 +339,7 @@ func viewReleases() error {
 func getOptions() []config.Option {
 	opts := []config.Option{}
 	// build options
-	if *droid != config.Droid() {
+	if *droid != discoveredDroidFile {
 		opts = append(opts, config.SetDroid(*droid))
 	}
 	if *container != config.Container() {
@@ -442,7 +449,7 @@ the DROID signature file you should also include a regular signature extension
 		opts = append(opts, config.SetVerbose(!*quiet)) // do the opposite, because the flag is quiet and the setting is verbose!
 	}
 	// inspect options
-	if *inspectDroid != config.Droid() {
+	if *inspectDroid != discoveredDroidFile {
 		opts = append(opts, config.SetDroid(*inspectDroid))
 	}
 	if *inspectMI != "" {
@@ -485,7 +492,7 @@ the DROID signature file you should also include a regular signature extension
 }
 
 func setHarvestOptions() {
-	if *harvestDroid != config.Droid() {
+	if *harvestDroid != discoveredDroidFile {
 		config.SetDroid(*harvestDroid)()
 	}
 	if *harvestHome != config.Home() {
@@ -510,7 +517,7 @@ func setHarvestOptions() {
 }
 
 func setSetsOptions() {
-	if *setsDroid != config.Droid() {
+	if *setsDroid != discoveredDroidFile {
 		config.SetDroid(*setsDroid)()
 	}
 	if *setsHome != config.Home() {

pkg/pronom/pronom.go

@@ -20,6 +20,7 @@ import (
 	"encoding/xml"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -138,7 +139,7 @@ func (p *pronom) setParseables() error {
 func newDroid(path string) (*droid, error) {
 	d := &mappings.Droid{}
 	if err := openXML(path, d); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("%W: %s", err, path)
 	}
 	return &droid{d, identifier.Blank{}}, nil
 }
@@ -195,7 +196,12 @@ func (p *pronom) setContainers() error {
 // UTILS
 // Harvest fetches PRONOM reports listed in the DROID file
 func Harvest() []error {
-	d, err := newDroid(config.Droid())
+	droidXML := config.Droid()
+	if droidXML == "" {
+		return []error{fmt.Errorf("a DROID signature file needs to be downloaded to %s", config.Home())}
+	}
+	log.Println("roy: harvesting PRONOM reports for records listed in DROID XML:", droidXML)
+	d, err := newDroid(droidXML)
 	if err != nil {
 		return []error{err}
 	}