XL: ListVols should provide consistent view. (#1648)

Additionally get list of all volumes in parallel for aggregation
and quorum verification.

Fixes #1647

xl-erasure-v1.go

@@ -262,48 +262,58 @@ func (xl XL) DeleteVol(volume string) error {
 // ListVols - list volumes.
 func (xl XL) ListVols() (volsInfo []VolInfo, err error) {
 	emptyCount := 0
-	// Success vols map carries successful results of ListVols from
-	// each disks.
-	var successVolsMap = make(map[int][]VolInfo)
+
+	// Initialize sync waitgroup.
+	var wg = &sync.WaitGroup{}
+
+	// Success vols map carries successful results of ListVols from each disks.
+	var successVols = make([][]VolInfo, len(xl.storageDisks))
 	for index, disk := range xl.storageDisks {
-		var vlsInfo []VolInfo
-		vlsInfo, err = disk.ListVols()
-		if err == nil {
-			if len(vlsInfo) == 0 {
-				emptyCount++
-			} else {
-				successVolsMap[index] = vlsInfo
+		wg.Add(1) // Add each go-routine to wait for.
+		go func(index int, disk StorageAPI) {
+			// Indicate wait group as finished.
+			defer wg.Done()
+
+			// Initiate listing.
+			vlsInfo, lErr := disk.ListVols()
+			if lErr == nil {
+				if len(vlsInfo) == 0 {
+					emptyCount++ // Calculate empty count specially.
+				} else {
+					successVols[index] = vlsInfo
+				}
 			}
-		}
+		}(index, disk)
 	}
+	// For all the list volumes running in parallel to finish.
+	wg.Wait()
 
 	// If all list operations resulted in an empty count which is same
 	// as your total storage disks, then it is a valid case return
 	// success with empty vols.
 	if emptyCount == len(xl.storageDisks) {
 		return []VolInfo{}, nil
-	} else if len(successVolsMap) < xl.readQuorum {
+	} else if len(successVols) < xl.readQuorum {
 		// If there is data and not empty, then we attempt quorum verification.
 		// Verify if we have enough quorum to list vols.
 		return nil, errReadQuorum
 	}
 
 	var total, free int64
-	// Loop through success vols map and get aggregated usage values.
-	for index := range xl.storageDisks {
-		if _, ok := successVolsMap[index]; ok {
-			volsInfo = successVolsMap[index]
-			free += volsInfo[0].Free
-			total += volsInfo[0].Total
-		}
+	// Loop through success vols and get aggregated usage values.
+	for _, vlsInfo := range successVols {
+		free += vlsInfo[0].Free
+		total += vlsInfo[0].Total
 	}
+
 	// Save the updated usage values back into the vols.
-	for index := range volsInfo {
-		volsInfo[index].Free = free
-		volsInfo[index].Total = total
+	for _, volInfo := range successVols[0] {
+		volInfo.Free = free
+		volInfo.Total = total
+		volsInfo = append(volsInfo, volInfo)
 	}
 
-	// TODO: the assumption here is that volumes across all disks in
+	// NOTE: The assumption here is that volumes across all disks in
 	// readQuorum have consistent view i.e they all have same number
 	// of buckets. This is essentially not verified since healing
 	// should take care of this.