1 files changed, 201 insertions, 0 deletions
diff --git a/pkg/aflow/cache.go b/pkg/aflow/cache.go
new file mode 100644
index 000000000..fe60e5358
--- /dev/null
+++ b/pkg/aflow/cache.go
@@ -0,0 +1,201 @@
+// Copyright 2025 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package aflow
+
+import (
+	"fmt"
+	"maps"
+	"os"
+	"path/filepath"
+	"slices"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/google/syzkaller/pkg/hash"
+	"github.com/google/syzkaller/pkg/osutil"
+)
+
+// Cache maintains on-disk cache with directories with arbitrary contents (kernel checkouts, builds, etc).
+// Create method is used to either create a new directory, if it's not cached yet, or returns a previously
+// cached directory. Old unused directories are incrementally removed if the total disk usage grows
+// over the specified limit.
+type Cache struct {
+	dir         string
+	maxSize     uint64
+	timeNow     func() time.Time
+	t           *testing.T
+	mu          sync.Mutex
+	currentSize uint64
+	entries     map[string]*cacheEntry
+}
+
+type cacheEntry struct {
+	dir        string
+	size       uint64
+	usageCount int
+	lastUsed   time.Time
+}
+
+func NewCache(dir string, maxSize uint64) (*Cache, error) {
+	return newTestCache(nil, dir, maxSize, time.Now)
+}
+
+func newTestCache(t *testing.T, dir string, maxSize uint64, timeNow func() time.Time) (*Cache, error) {
+	if dir == "" {
+		return nil, fmt.Errorf("cache workdir is empty")
+	}
+	c := &Cache{
+		dir:     osutil.Abs(dir),
+		maxSize: maxSize,
+		timeNow: timeNow,
+		t:       t,
+		entries: make(map[string]*cacheEntry),
+	}
+	if err := c.init(); err != nil {
+		return nil, err
+	}
+	return c, nil
+}
+
+// Create creates/returns a cached directory with contents created by the populate callback.
+// The populate callback receives a dir it needs to populate with cached files.
+// The typ must be a short descriptive name of the contents (e.g. "build", "source", etc).
+// The desc is used to identify cached entries and must fully describe the cached contents
+// (the second invocation with the same typ+desc will return dir created by the first
+// invocation with the same typ+desc).
+func (c *Cache) Create(typ, desc string, populate func(string) error) (string, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	// Note: we don't populate a temp dir and then atomically rename it to the final destination,
+	// because at least kernel builds encode the current path in debug info/compile commands,
+	// so moving the dir later would break all that. Instead we rely on the presence of the meta file
+	// to denote valid cache entries. Modification time of the file says when it was last used.
+	id := hash.String(desc)
+	dir := filepath.Join(c.dir, typ, id)
+	metaFile := filepath.Join(dir, cacheMetaFile)
+	if c.entries[dir] == nil {
+		os.RemoveAll(dir)
+		if err := osutil.MkdirAll(dir); err != nil {
+			return "", err
+		}
+		if err := populate(dir); err != nil {
+			os.RemoveAll(dir)
+			return "", err
+		}
+		size, err := osutil.DiskUsage(dir)
+		if err != nil {
+			return "", err
+		}
+		if err := osutil.WriteFile(metaFile, []byte(desc)); err != nil {
+			os.RemoveAll(dir)
+			return "", err
+		}
+		c.entries[dir] = &cacheEntry{
+			dir:  dir,
+			size: size,
+		}
+		c.currentSize += size
+		c.logf("created entry %v, size %v, current size %v", dir, size, c.currentSize)
+	}
+	// Note the entry was used now.
+	now := c.timeNow()
+	if err := os.Chtimes(metaFile, now, now); err != nil {
+		return "", err
+	}
+	entry := c.entries[dir]
+	entry.usageCount++
+	entry.lastUsed = now
+	c.logf("using entry %v, usage count %v", dir, entry.usageCount)
+	if err := c.purge(); err != nil {
+		entry.usageCount--
+		return "", err
+	}
+	return dir, nil
+}
+
+// Release must be called for every directory returned by Create method when the directory is not used anymore.
+func (c *Cache) Release(dir string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	entry := c.entries[dir]
+	entry.usageCount--
+	c.logf("release entry %v, usage count %v", dir, entry.usageCount)
+	if entry.usageCount < 0 {
+		panic("negative usageCount")
+	}
+}
+
+// init reads the cached dirs (disk usage, last use time) from disk when the cache is created.
+func (c *Cache) init() error {
+	dirs, err := filepath.Glob(filepath.Join(c.dir, "*", "*"))
+	if err != nil {
+		return err
+	}
+	for _, dir := range dirs {
+		metaFile := filepath.Join(dir, cacheMetaFile)
+		if !osutil.IsExist(metaFile) {
+			if err := osutil.RemoveAll(dir); 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,
+			lastUsed: stat.ModTime(),
+		}
+		c.currentSize += size
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.purge()
+}
+
+// purge removes oldest unused directories if the cache is over maxSize.
+func (c *Cache) purge() error {
+	if c.mu.TryLock() {
+		panic("c.mu is not locked")
+	}
+	if c.currentSize < c.maxSize {
+		return nil
+	}
+	list := slices.Collect(maps.Values(c.entries))
+	slices.SortFunc(list, func(a, b *cacheEntry) int {
+		if a.usageCount != b.usageCount {
+			return a.usageCount - b.usageCount
+		}
+		return a.lastUsed.Compare(b.lastUsed)
+	})
+	for _, entry := range list {
+		if entry.usageCount != 0 || c.currentSize < c.maxSize {
+			break
+		}
+		if err := os.RemoveAll(entry.dir); err != nil {
+			return err
+		}
+		delete(c.entries, entry.dir)
+		if c.currentSize < entry.size {
+			panic(fmt.Sprintf("negative currentSize: %v %v", c.currentSize, entry.size))
+		}
+		c.currentSize -= entry.size
+	}
+	return nil
+}
+
+func (c *Cache) logf(msg string, args ...any) {
+	if c.t != nil {
+		c.t.Logf("cache: "+msg, args...)
+	}
+}
+
+const cacheMetaFile = "aflow-meta"