steamcache2/vfs/cache/cache.go

// vfs/cache/cache.go
package cache

import (
	"io"
	"s1d3sw1ped/steamcache2/vfs"
	"s1d3sw1ped/steamcache2/vfs/vfserror"
	"sync/atomic"
)

// TieredCache implements a lock-free two-tier cache for better concurrency
type TieredCache struct {
	fast *atomic.Value // Memory cache (fast) - atomic.Value for lock-free access
	slow *atomic.Value // Disk cache (slow) - atomic.Value for lock-free access
}

// New creates a new tiered cache
func New() *TieredCache {
	return &TieredCache{
		fast: &atomic.Value{},
		slow: &atomic.Value{},
	}
}

// SetFast sets the fast (memory) tier atomically
func (tc *TieredCache) SetFast(vfs vfs.VFS) {
	tc.fast.Store(vfs)
}

// SetSlow sets the slow (disk) tier atomically
func (tc *TieredCache) SetSlow(vfs vfs.VFS) {
	tc.slow.Store(vfs)
}

// Create creates a new file, preferring the slow tier for persistence
func (tc *TieredCache) Create(key string, size int64) (io.WriteCloser, error) {
	// Try slow tier first (disk) for better testability
	if slow := tc.slow.Load(); slow != nil {
		if vfs, ok := slow.(vfs.VFS); ok {
			return vfs.Create(key, size)
		}
	}

	// Fall back to fast tier (memory)
	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			return vfs.Create(key, size)
		}
	}

	return nil, vfserror.ErrNotFound
}

// Open opens a file, checking fast tier first, then slow tier with promotion
func (tc *TieredCache) Open(key string) (io.ReadCloser, error) {
	// Try fast tier first (memory)
	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			if reader, err := vfs.Open(key); err == nil {
				return reader, nil
			}
		}
	}

	// Fall back to slow tier (disk) and promote to fast tier
	if slow := tc.slow.Load(); slow != nil {
		if vfs, ok := slow.(vfs.VFS); ok {
			reader, err := vfs.Open(key)
			if err != nil {
				return nil, err
			}

			// If we have both tiers, promote the file to fast tier
			if fast := tc.fast.Load(); fast != nil {
				// Create a new reader for promotion to avoid interfering with the returned reader
				promotionReader, err := vfs.Open(key)
				if err == nil {
					go tc.promoteToFast(key, promotionReader)
				}
			}

			return reader, nil
		}
	}

	return nil, vfserror.ErrNotFound
}

// Delete removes a file from all tiers
func (tc *TieredCache) Delete(key string) error {
	var lastErr error

	// Delete from fast tier
	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			if err := vfs.Delete(key); err != nil {
				lastErr = err
			}
		}
	}

	// Delete from slow tier
	if slow := tc.slow.Load(); slow != nil {
		if vfs, ok := slow.(vfs.VFS); ok {
			if err := vfs.Delete(key); err != nil {
				lastErr = err
			}
		}
	}

	return lastErr
}

// Stat returns file information, checking fast tier first
func (tc *TieredCache) Stat(key string) (*vfs.FileInfo, error) {
	// Try fast tier first (memory)
	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			if info, err := vfs.Stat(key); err == nil {
				return info, nil
			}
		}
	}

	// Fall back to slow tier (disk)
	if slow := tc.slow.Load(); slow != nil {
		if vfs, ok := slow.(vfs.VFS); ok {
			return vfs.Stat(key)
		}
	}

	return nil, vfserror.ErrNotFound
}

// Name returns the cache name
func (tc *TieredCache) Name() string {
	return "TieredCache"
}

// Size returns the total size across all tiers
func (tc *TieredCache) Size() int64 {
	var total int64

	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			total += vfs.Size()
		}
	}

	if slow := tc.slow.Load(); slow != nil {
		if vfs, ok := slow.(vfs.VFS); ok {
			total += vfs.Size()
		}
	}

	return total
}

// Capacity returns the total capacity across all tiers
func (tc *TieredCache) Capacity() int64 {
	var total int64

	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			total += vfs.Capacity()
		}
	}

	if slow := tc.slow.Load(); slow != nil {
		if vfs, ok := slow.(vfs.VFS); ok {
			total += vfs.Capacity()
		}
	}

	return total
}

// promoteToFast promotes a file from slow tier to fast tier
func (tc *TieredCache) promoteToFast(key string, reader io.ReadCloser) {
	defer func() { _ = reader.Close() }() // best-effort close; error secondary to promotion attempt (async best-effort path)

	// Get file info from slow tier to determine size
	var size int64
	if slow := tc.slow.Load(); slow != nil {
		if vfs, ok := slow.(vfs.VFS); ok {
			if info, err := vfs.Stat(key); err == nil {
				size = info.Size
			} else {
				return // Skip promotion if we can't get file info
			}
		}
	}

	// Check if file fits in available memory cache space
	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			availableSpace := vfs.Capacity() - vfs.Size()
			// Only promote if file fits in available space (with 10% buffer for safety)
			if size > int64(float64(availableSpace)*0.9) {
				return // Skip promotion if file is too large
			}
		}
	}

	// Guard promotion ReadAll using already-fetched size (in addition to space check above)
	if size > 0 && size > (1<<30) { // conservative 1GB hard limit on promotion reads (aligns with typical max_object_size)
		return
	}
	// Read the entire file content
	content, err := io.ReadAll(reader)
	if err != nil {
		return // Skip promotion if read fails
	}

	// Create the file in fast tier
	if fast := tc.fast.Load(); fast != nil {
		if vfs, ok := fast.(vfs.VFS); ok {
			writer, err := vfs.Create(key, size)
			if err == nil {
				// Write/close errors intentionally discarded: promotion to fast tier is best-effort optimization only.
				// Failure (e.g. mem pressure, concurrent evict) is non-fatal and does not affect correctness of slow tier.
				_, _ = writer.Write(content)
				_ = writer.Close()
			}
		}
	}
}