cache: Allow proactive caching of tree packs

This commit adds a function to the cache which can decide to proactively
load the complete pack file and store it in the cache. This is helpful
for pack files containing only tree blobs, as it is likely that the same
file is accessed again in the future.

internal/cache/backend.go

@@ -82,6 +82,25 @@ var autoCacheFiles = map[restic.FileType]bool{
 	restic.SnapshotFile: true,
 }
 
+func (b *Backend) cacheFile(ctx context.Context, h restic.Handle) error {
+	rd, err := b.Backend.Load(ctx, h, 0, 0)
+	if err != nil {
+		return err
+	}
+
+	if err = b.Cache.Save(h, rd); err != nil {
+		return err
+	}
+
+	if err = rd.Close(); err != nil {
+		// try to remove from the cache, ignore errors
+		_ = b.Cache.Remove(h)
+		return err
+	}
+
+	return nil
+}
+
 // Load loads a file from the cache or the backend.
 func (b *Backend) Load(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
 	if b.Cache.Has(h) {
@@ -91,39 +110,36 @@ func (b *Backend) Load(ctx context.Context, h restic.Handle, length int, offset
 
 	// partial file requested
 	if offset != 0 || length != 0 {
+		if b.Cache.PerformReadahead(h) {
+			debug.Log("performing readahead for %v", h)
+			err := b.cacheFile(ctx, h)
+			if err == nil {
+				return b.Cache.Load(h, length, offset)
+			}
+
+			debug.Log("error caching %v: %v", h, err)
+		}
+
 		debug.Log("Load(%v, %v, %v): partial file requested, delegating to backend", h, length, offset)
 		return b.Backend.Load(ctx, h, length, offset)
 	}
 
-	rd, err := b.Backend.Load(ctx, h, length, offset)
-	if err != nil {
-		if b.Backend.IsNotExist(err) {
-			// try to remove from the cache, ignore errors
-			_ = b.Cache.Remove(h)
-		}
-
-		return nil, err
-	}
-
+	// if we don't automatically cache this file type, fall back to the backend
 	if _, ok := autoCacheFiles[h.Type]; !ok {
-		return rd, nil
+		debug.Log("Load(%v, %v, %v): delegating to backend", h, length, offset)
+		return b.Backend.Load(ctx, h, length, offset)
 	}
 
 	debug.Log("auto-store %v in the cache", h)
+	err := b.cacheFile(ctx, h)
 
-	// cache the file, then return cached copy
-	if err = b.Cache.Save(h, rd); err != nil {
-		return nil, err
-	}
-
-	if err = rd.Close(); err != nil {
-		// try to remove from the cache, ignore errors
-		_ = b.Cache.Remove(h)
-		return nil, err
-	}
-
-	// load from the cache and save in the backend
+	if err == nil {
+		// load the cached version
 		return b.Cache.Load(h, 0, 0)
+	}
+
+	debug.Log("error caching %v: %v, falling back to backend", h, err)
+	return b.Backend.Load(ctx, h, length, offset)
 }
 
 // Stat tests whether the backend has a file. If it does not exist but still