pkg/aflow: cache disk usage in Cache

Calculating total disk usage of all cache entries can take very long time
for large caches (needs to stat all files). This is especially problematic
for tools/syz-aflow. Cache disk usage in the meta file.

Author: dvyukov

pkg/aflow/cache.go

@@ -4,6 +4,7 @@
 package aflow
 
 import (
+	"encoding/json"
 	"fmt"
 	"maps"
 	"os"
@@ -88,7 +89,11 @@ func (c *Cache) Create(typ, desc string, populate func(string) error) (string, e
 		if err != nil {
 			return "", err
 		}
-		if err := osutil.WriteFile(metaFile, []byte(desc)); err != nil {
+		meta := cacheMeta{
+			Description: desc,
+			DiskUsage:   size,
+		}
+		if err := osutil.WriteJSON(metaFile, meta); err != nil {
 			os.RemoveAll(dir)
 			return "", err
 		}
@@ -135,26 +140,42 @@ func (c *Cache) init() error {
 	}
 	for _, dir := range dirs {
 		metaFile := filepath.Join(dir, cacheMetaFile)
-		if !osutil.IsExist(metaFile) {
-			if err := osutil.RemoveAll(dir); err != nil {
+		data, err := os.ReadFile(metaFile)
+		if err != nil {
+			if os.IsNotExist(err) {
+				// Incomplete cache dir.
+				if err := osutil.RemoveAll(dir); err != nil {
+					return err
+				}
+				continue
+			}
+			return err
+		}
+		var meta cacheMeta
+		if err := json.Unmarshal(data, &meta); err != nil {
+			// Assume the old format that contained just the description.
+			// This code can be removed after 2027-06-01,
+			// and the code above can use osutil.ReadJSON.
+			size, err := osutil.DiskUsage(dir)
+			if err != nil {
+				return err
+			}
+			meta.Description = string(data)
+			meta.DiskUsage = size
+			if err := osutil.WriteJSON(metaFile, meta); err != nil {
 				return err
 			}
-			continue
 		}
 		stat, err := os.Stat(metaFile)
 		if err != nil {
 			return err
 		}
-		size, err := osutil.DiskUsage(dir)
-		if err != nil {
-			return err
-		}
 		c.entries[dir] = &cacheEntry{
 			dir:      dir,
-			size:     size,
+			size:     meta.DiskUsage,
 			lastUsed: stat.ModTime(),
 		}
-		c.currentSize += size
+		c.currentSize += meta.DiskUsage
 	}
 	c.mu.Lock()
 	defer c.mu.Unlock()
@@ -198,4 +219,9 @@ func (c *Cache) logf(msg string, args ...any) {
 	}
 }
 
+type cacheMeta struct {
+	Description string
+	DiskUsage   uint64
+}
+
 const cacheMetaFile = "aflow-meta"