api: Introduce metadata update APIs to update only metadata (#11962)

Current implementation heavily relies on readAllFileInfo
but with the advent of xl.meta inlined with data, we cannot
easily avoid reading data when we are only interested is
updating metadata, this leads to invariably write
amplification during metadata updates, repeatedly reading
data when we are only interested in updating metadata.

This PR ensures that we implement a metadata only update
API at storage layer, that handles updates to metadata alone
for any given version - given the version is valid and
present.

This helps reduce the chattiness for following calls..

- PutObjectTags
- DeleteObjectTags
- PutObjectLegalHold
- PutObjectRetention
- ReplicateObject (updates metadata on replication status)

cmd/config/storageclass/storage-class.go

@@ -22,6 +22,7 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
+	"sync"
 
 	"github.com/minio/minio/cmd/config"
 	"github.com/minio/minio/pkg/env"
@@ -90,6 +91,9 @@ type StorageClass struct {
 	Parity int
 }
 
+// ConfigLock is a global lock for storage-class config
+var ConfigLock = sync.RWMutex{}
+
 // Config storage class configuration
 type Config struct {
 	Standard StorageClass `json:"standard"`
@@ -232,6 +236,8 @@ func validateParity(ssParity, rrsParity, setDriveCount int) (err error) {
 //    is returned, the caller is expected to choose the right parity
 //    at that point.
 func (sCfg Config) GetParityForSC(sc string) (parity int) {
+	ConfigLock.RLock()
+	defer ConfigLock.RUnlock()
 	switch strings.TrimSpace(sc) {
 	case RRS:
 		// set the rrs parity if available
@@ -244,8 +250,19 @@ func (sCfg Config) GetParityForSC(sc string) (parity int) {
 	}
 }
 
+// Update update storage-class with new config
+func (sCfg Config) Update(newCfg Config) {
+	ConfigLock.Lock()
+	defer ConfigLock.Unlock()
+	sCfg.RRS = newCfg.RRS
+	sCfg.DMA = newCfg.DMA
+	sCfg.Standard = newCfg.Standard
+}
+
 // GetDMA - returns DMA configuration.
 func (sCfg Config) GetDMA() string {
+	ConfigLock.RLock()
+	defer ConfigLock.RUnlock()
 	return sCfg.DMA
 }