cache usage, prefix-usage, and buckets for AccountInfo up to 10 secs (#18051)

AccountInfo is quite frequently called by the Console UI 
login attempts, when many users are logging in it is important
that we provide them with better responsiveness.

- ListBuckets information is cached every second
- Bucket usage info is cached for up to 10 seconds
- Prefix usage (optional) info is cached for up to 10 secs

Failure to update after cache expiration, would still
allow login which would end up providing information
previously cached.

This allows for seamless responsiveness for the Console UI
logins, and overall responsiveness on a heavily loaded
system.

cmd/data-usage.go

@@ -21,6 +21,7 @@ import (
 	"context"
 	"errors"
 	"strings"
+	"time"
 
 	jsoniter "github.com/json-iterator/go"
 	"github.com/minio/minio/internal/logger"
@@ -61,6 +62,8 @@ func storeDataUsageInBackend(ctx context.Context, objAPI ObjectLayer, dui <-chan
 	}
 }
 
+var prefixUsageCache timedValue
+
 // loadPrefixUsageFromBackend returns prefix usages found in passed buckets
 //
 //	e.g.:  /testbucket/prefix => 355601334
@@ -73,28 +76,45 @@ func loadPrefixUsageFromBackend(ctx context.Context, objAPI ObjectLayer, bucket
 
 	cache := dataUsageCache{}
 
-	m := make(map[string]uint64)
-	for _, pool := range z.serverPools {
-		for _, er := range pool.sets {
-			// Load bucket usage prefixes
-			if err := cache.load(ctx, er, bucket+slashSeparator+dataUsageCacheName); err == nil {
-				root := cache.find(bucket)
-				if root == nil {
-					// We dont have usage information for this bucket in this
-					// set, go to the next set
-					continue
-				}
+	prefixUsageCache.Once.Do(func() {
+		prefixUsageCache.TTL = 30 * time.Second
 
-				for id, usageInfo := range cache.flattenChildrens(*root) {
-					prefix := decodeDirObject(strings.TrimPrefix(id, bucket+slashSeparator))
-					// decodeDirObject to avoid any __XLDIR__ objects
-					m[prefix] += uint64(usageInfo.Size)
+		// No need to fail upon Update() error, fallback to old value.
+		prefixUsageCache.Relax = true
+		prefixUsageCache.Update = func() (interface{}, error) {
+			m := make(map[string]uint64)
+			for _, pool := range z.serverPools {
+				for _, er := range pool.sets {
+					// Load bucket usage prefixes
+					ctx, done := context.WithTimeout(context.Background(), 2*time.Second)
+					ok := cache.load(ctx, er, bucket+slashSeparator+dataUsageCacheName) == nil
+					done()
+					if ok {
+						root := cache.find(bucket)
+						if root == nil {
+							// We dont have usage information for this bucket in this
+							// set, go to the next set
+							continue
+						}
+
+						for id, usageInfo := range cache.flattenChildrens(*root) {
+							prefix := decodeDirObject(strings.TrimPrefix(id, bucket+slashSeparator))
+							// decodeDirObject to avoid any __XLDIR__ objects
+							m[prefix] += uint64(usageInfo.Size)
+						}
+					}
 				}
 			}
+			return m, nil
 		}
+	})
+
+	v, _ := prefixUsageCache.Get()
+	if v != nil {
+		return v.(map[string]uint64), nil
 	}
 
-	return m, nil
+	return map[string]uint64{}, nil
 }
 
 func loadDataUsageFromBackend(ctx context.Context, objAPI ObjectLayer) (DataUsageInfo, error) {