Use random file name for write check (#8563)

Since there may be multiple writes going on concurrently
Use a random file name for the write check to avoid collisions.

cmd/posix.go

@@ -19,6 +19,8 @@ package cmd
 import (
 	"bufio"
 	"context"
+	"crypto/rand"
+	"encoding/hex"
 	"errors"
 	"io"
 	"io/ioutil"
@@ -154,12 +156,15 @@ func getValidPath(path string) (string, error) {
 	}
 
 	// check if backend is writable.
-	file, err := os.Create(pathJoin(path, ".writable-check.tmp"))
+	var rnd [8]byte
+	_, _ = rand.Read(rnd[:])
+	fn := pathJoin(path, ".writable-check-"+hex.EncodeToString(rnd[:])+".tmp")
+	file, err := os.Create(fn)
 	if err != nil {
 		return path, err
 	}
-	defer os.Remove(pathJoin(path, ".writable-check.tmp"))
 	file.Close()
+	os.Remove(fn)
 
 	return path, nil
 }