Reject a -sites list with no site keys instead of panicking

cmd/doc-scraper/exec_test.go

@@ -164,6 +164,17 @@ func TestResolveSiteKeys(t *testing.T) {
 
 	_, _, ok = resolveSiteKeys("", "", false)
 	assert.False(t, ok)
+
+	// A -sites list with no keys selects nothing, so it is not a selector.
+	for _, sites := range []string{",", " , ,", " "} {
+		_, _, ok = resolveSiteKeys("", sites, false)
+		assert.False(t, ok, "-sites %q", sites)
+	}
+
+	keys, warning, ok = resolveSiteKeys("a", ",", false)
+	assert.True(t, ok)
+	assert.Equal(t, []string{"a"}, keys)
+	assert.Equal(t, "-sites has no site keys; using -site", warning)
 }
 
 func TestLogHelpers(t *testing.T) {

cmd/doc-scraper/main.go

@@ -167,27 +167,32 @@ func extraArgsError(fs *flag.FlagSet) error {
 // resolveSiteKeys picks the crawl target in precedence --all-sites > -sites >
 // -site. warning is non-empty when a lower-precedence selector was also set and
 // silently ignored, so a typo'd -site next to a valid -sites is not missed.
+// A -sites list with no keys in it (e.g. ",") counts as not given; if -site
+// is also set it is used, with a warning.
 func resolveSiteKeys(siteKey, sites string, allSites bool) (siteKeys []string, warning string, ok bool) {
 	if allSites {
 		if sites != "" || siteKey != "" {
 			warning = "--all-sites is set; ignoring -site/-sites"
 		}
 		return nil, warning, true
 	}
-	if sites != "" {
+	for _, s := range strings.Split(sites, ",") {
+		s = strings.TrimSpace(s)
+		if s != "" {
+			siteKeys = append(siteKeys, s)
+		}
+	}
+	if len(siteKeys) > 0 {
 		if siteKey != "" {
 			warning = "both -site and -sites given; using -sites and ignoring -site"
 		}
-		for _, s := range strings.Split(sites, ",") {
-			s = strings.TrimSpace(s)
-			if s != "" {
-				siteKeys = append(siteKeys, s)
-			}
-		}
 		return siteKeys, warning, true
 	}
 	if siteKey != "" {
-		return []string{siteKey}, "", true
+		if sites != "" {
+			warning = "-sites has no site keys; using -site"
+		}
+		return []string{siteKey}, warning, true
 	}
 	return nil, "", false
 }