Fix writing 'format.json' and make it atomic (#8296)

- Choose a unique uuid such that under situations of duplicate
  mounts we do not append to an existing json entry.
- Avoid AppendFile instead use WriteAll() to write the entire
  byte array atomically.

cmd/format-xl.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -291,16 +292,6 @@ func formatXLMigrateV2ToV3(export string) error {
 	return ioutil.WriteFile(formatPath, b, 0644)
 }
 
-// Returns true, if one of the errors is non-nil and is Unformatted disk.
-func hasAnyErrorsUnformatted(errs []error) bool {
-	for _, err := range errs {
-		if err != nil && err == errUnformattedDisk {
-			return true
-		}
-	}
-	return false
-}
-
 // countErrs - count a specific error.
 func countErrs(errs []error, err error) int {
 	var i = 0
@@ -367,16 +358,18 @@ func saveFormatXL(disk StorageAPI, format interface{}) error {
 		return err
 	}
 
+	tmpFormatJSON := mustGetUUID() + ".json"
+
 	// Purge any existing temporary file, okay to ignore errors here.
-	defer disk.DeleteFile(minioMetaBucket, formatConfigFileTmp)
+	defer disk.DeleteFile(minioMetaBucket, tmpFormatJSON)
 
 	// Append file `format.json.tmp`.
-	if err = disk.AppendFile(minioMetaBucket, formatConfigFileTmp, formatBytes); err != nil {
+	if err = disk.WriteAll(minioMetaBucket, tmpFormatJSON, bytes.NewReader(formatBytes)); err != nil {
 		return err
 	}
 
-	// Rename file `format.json.tmp` --> `format.json`.
-	return disk.RenameFile(minioMetaBucket, formatConfigFileTmp, minioMetaBucket, formatConfigFile)
+	// Rename file `uuid.json` --> `format.json`.
+	return disk.RenameFile(minioMetaBucket, tmpFormatJSON, minioMetaBucket, formatConfigFile)
 }
 
 var ignoredHiddenDirectories = []string{