Fix listing fallback re-using disks (#14576)

When more than 2 disks are unavailable for listing, the same disk will be used for fallback.

This makes quorum calculations incorrect since the same disk will have multiple entries.

This PR keeps track of which fallback disks have been handed out and only every returns a disk once.

cmd/metacache-set.go

@@ -789,14 +789,30 @@ func listPathRaw(ctx context.Context, opts listPathRawOptions) (err error) {
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
-	fallback := func(err error) bool {
+	// Keep track of fallback disks
+	var fdMu sync.Mutex
+	fds := opts.fallbackDisks
+	fallback := func(err error) StorageAPI {
 		switch err.(type) {
 		case StorageErr:
-			// all supported disk errors
-			// attempt a fallback.
-			return true
+			// Attempt to grab a fallback disk
+			fdMu.Lock()
+			defer fdMu.Unlock()
+			if len(fds) == 0 {
+				return nil
+			}
+			fdsCopy := fds
+			for _, fd := range fdsCopy {
+				// Grab a fallback disk
+				fds = fds[1:]
+				if fd != nil {
+					return fd
+				}
+			}
 		}
-		return false
+		// Either no more disks for fallback or
+		// not a storage error.
+		return nil
 	}
 	askDisks := len(disks)
 	readers := make([]*metacacheReader, askDisks)
@@ -825,25 +841,20 @@ func listPathRaw(ctx context.Context, opts listPathRawOptions) (err error) {
 			}
 
 			// fallback only when set.
-			if len(opts.fallbackDisks) > 0 && fallback(werr) {
+			for fd := fallback(werr); fd != nil; {
 				// This fallback is only set when
 				// askDisks is less than total
 				// number of disks per set.
-				for _, fd := range opts.fallbackDisks {
-					if fd == nil {
-						continue
-					}
-					werr = fd.WalkDir(ctx, WalkDirOptions{
-						Bucket:         opts.bucket,
-						BaseDir:        opts.path,
-						Recursive:      opts.recursive,
-						ReportNotFound: opts.reportNotFound,
-						FilterPrefix:   opts.filterPrefix,
-						ForwardTo:      opts.forwardTo,
-					}, w)
-					if werr == nil {
-						break
-					}
+				werr = fd.WalkDir(ctx, WalkDirOptions{
+					Bucket:         opts.bucket,
+					BaseDir:        opts.path,
+					Recursive:      opts.recursive,
+					ReportNotFound: opts.reportNotFound,
+					FilterPrefix:   opts.filterPrefix,
+					ForwardTo:      opts.forwardTo,
+				}, w)
+				if werr == nil {
+					break
 				}
 			}
 			w.CloseWithError(werr)