limit number of buckets to 500k (#15668)

500k is a reasonable limit for any single MinIO
cluster deployment, in future we may increase this
value.

However for now we are going to keep this limit.

cmd/api-errors.go

@@ -176,6 +176,7 @@ const (
 	ErrBucketAlreadyOwnedByYou
 	ErrInvalidDuration
 	ErrBucketAlreadyExists
+	ErrTooManyBuckets
 	ErrMetadataTooLarge
 	ErrUnsupportedMetadata
 	ErrMaximumExpires
@@ -684,6 +685,11 @@ var errorCodes = errorCodeMap{
 		Description:    "The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrTooManyBuckets: {
+		Code:           "TooManyBuckets",
+		Description:    "You have attempted to create more buckets than allowed",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrBucketNotEmpty: {
 		Code:           "BucketNotEmpty",
 		Description:    "The bucket you tried to delete is not empty",

cmd/bucket-handlers.go

@@ -769,6 +769,14 @@ func (api objectAPIHandlers) PutBucketHandler(w http.ResponseWriter, r *http.Req
 		return
 	}
 
+	// check if client is attempting to create more buckets than allowed maximum.
+	if currBuckets := globalBucketMetadataSys.Count(); currBuckets+1 > maxBuckets {
+		apiErr := errorCodes.ToAPIErr(ErrTooManyBuckets)
+		apiErr.Description = fmt.Sprintf("You have attempted to create %d buckets than allowed %d", currBuckets+1, maxBuckets)
+		writeErrorResponse(ctx, w, apiErr, r.URL)
+		return
+	}
+
 	opts := MakeBucketOptions{
 		Location:    location,
 		LockEnabled: objectLockEnabled,

cmd/bucket-metadata-sys.go

@@ -45,6 +45,14 @@ type BucketMetadataSys struct {
 	metadataMap map[string]BucketMetadata
 }
 
+// Count returns number of bucket metadata map entries.
+func (sys *BucketMetadataSys) Count() int {
+	sys.RLock()
+	defer sys.RUnlock()
+
+	return len(sys.metadataMap)
+}
+
 // Remove bucket metadata from memory.
 func (sys *BucketMetadataSys) Remove(bucket string) {
 	if globalIsGateway {

cmd/generic-handlers.go

@@ -53,6 +53,9 @@ const (
 
 	// Maximum size for user-defined metadata - See: https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html
 	maxUserDataSize = 2 * 1024
+
+	// maxBuckets upto 500000 for any MinIO deployment.
+	maxBuckets = 500 * 1000
 )
 
 // ReservedMetadataPrefix is the prefix of a metadata key which