lock: Moving locking to handler layer. (#3381)

This is implemented so that the issues like in the
following flow don't affect the behavior of operation.

```
GetObjectInfo()
.... --> Time window for mutation (no lock held)
.... --> Time window for mutation (no lock held)
GetObject()
```

This happens when two simultaneous uploads are made
to the same object the object has returned wrong
info to the client.

Another classic example is "CopyObject" API itself
which reads from a source object and copies to
destination object.

Fixes #3370
Fixes #2912

cmd/bucket-handlers.go

@@ -334,6 +334,10 @@ func (api objectAPIHandlers) PutBucketHandler(w http.ResponseWriter, r *http.Req
 		return
 	}
 
+	bucketLock := globalNSMutex.NewNSLock(bucket, "")
+	bucketLock.Lock()
+	defer bucketLock.Unlock()
+
 	// Proceed to creating a bucket.
 	err := objectAPI.MakeBucket(bucket)
 	if err != nil {
@@ -431,6 +435,10 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 
 	sha256sum := ""
 
+	objectLock := globalNSMutex.NewNSLock(bucket, object)
+	objectLock.Lock()
+	defer objectLock.Unlock()
+
 	objInfo, err := objectAPI.PutObject(bucket, object, -1, fileBody, metadata, sha256sum)
 	if err != nil {
 		errorIf(err, "Unable to create object.")
@@ -478,6 +486,10 @@ func (api objectAPIHandlers) HeadBucketHandler(w http.ResponseWriter, r *http.Re
 		return
 	}
 
+	bucketLock := globalNSMutex.NewNSLock(bucket, "")
+	bucketLock.RLock()
+	defer bucketLock.RUnlock()
+
 	if _, err := objectAPI.GetBucketInfo(bucket); err != nil {
 		errorIf(err, "Unable to fetch bucket info.")
 		writeErrorResponse(w, r, toAPIErrorCode(err), r.URL.Path)
@@ -503,6 +515,10 @@ func (api objectAPIHandlers) DeleteBucketHandler(w http.ResponseWriter, r *http.
 	vars := mux.Vars(r)
 	bucket := vars["bucket"]
 
+	bucketLock := globalNSMutex.NewNSLock(bucket, "")
+	bucketLock.Lock()
+	defer bucketLock.Unlock()
+
 	// Attempt to delete bucket.
 	if err := objectAPI.DeleteBucket(bucket); err != nil {
 		errorIf(err, "Unable to delete a bucket.")