refac: atomic merge, lost-update race, RunPurge cost, duplicated walks

internal/infrastructure/useCases/siteImageUsage.go

@@ -20,6 +20,10 @@ func siteImageUsage(sites site.Repository) (map[uint32]int, error) {
 }
 
 func imageUsageCount(sites site.Repository, authors author.Repository, entries *entryMediaScanner) (map[uint32]int, error) {
+	return combinedImageUsage(sites, authors, entries.usageCounts)
+}
+
+func combinedImageUsage(sites site.Repository, authors author.Repository, entryCounts func() (map[uint32]int, error)) (map[uint32]int, error) {
 	usage, err := siteImageUsage(sites)
 	if err != nil {
 		return nil, err
@@ -31,11 +35,11 @@ func imageUsageCount(sites site.Repository, authors author.Repository, entries *
 	for imageID, total := range avatars {
 		usage[imageID] += total
 	}
-	entryCounts, err := entries.usageCounts()
+	counts, err := entryCounts()
 	if err != nil {
 		return nil, err
 	}
-	for imageID, total := range entryCounts {
+	for imageID, total := range counts {
 		usage[imageID] += total
 	}
 	return usage, nil