repository: Alloc zstd encoder/decoder on demand

internal/repository/repository.go

@@ -42,8 +42,10 @@ type Repository struct {
 	treePM *packerManager
 	dataPM *packerManager
 
-	enc *zstd.Encoder
-	dec *zstd.Decoder
+	allocEnc sync.Once
+	allocDec sync.Once
+	enc      *zstd.Encoder
+	dec      *zstd.Decoder
 }
 
 // New returns a new repository with backend be.
@@ -55,16 +57,6 @@ func New(be restic.Backend) *Repository {
 		treePM: newPackerManager(be, nil),
 	}
 
-	enc, err := zstd.NewWriter(nil)
-	if err != nil {
-		panic(err)
-	}
-	repo.enc = enc
-	dec, err := zstd.NewReader(nil)
-	if err != nil {
-		panic(err)
-	}
-	repo.dec = dec
 	return repo
 }
 
@@ -236,7 +228,7 @@ func (r *Repository) LoadBlob(ctx context.Context, t restic.BlobType, id restic.
 		}
 
 		if blob.IsCompressed() {
-			plaintext, err = r.dec.DecodeAll(plaintext, make([]byte, 0, blob.DataLength()))
+			plaintext, err = r.getZstdDecoder().DecodeAll(plaintext, make([]byte, 0, blob.DataLength()))
 			if err != nil {
 				lastError = errors.Errorf("decompressing blob %v failed: %v", id, err)
 				continue
@@ -280,6 +272,28 @@ func (r *Repository) LookupBlobSize(id restic.ID, tpe restic.BlobType) (uint, bo
 	return r.idx.LookupSize(restic.BlobHandle{ID: id, Type: tpe})
 }
 
+func (r *Repository) getZstdEncoder() *zstd.Encoder {
+	r.allocEnc.Do(func() {
+		enc, err := zstd.NewWriter(nil)
+		if err != nil {
+			panic(err)
+		}
+		r.enc = enc
+	})
+	return r.enc
+}
+
+func (r *Repository) getZstdDecoder() *zstd.Decoder {
+	r.allocDec.Do(func() {
+		dec, err := zstd.NewReader(nil)
+		if err != nil {
+			panic(err)
+		}
+		r.dec = dec
+	})
+	return r.dec
+}
+
 // saveAndEncrypt encrypts data and stores it to the backend as type t. If data
 // is small enough, it will be packed together with other small blobs.
 // The caller must ensure that the id matches the data.
@@ -289,7 +303,7 @@ func (r *Repository) saveAndEncrypt(ctx context.Context, t restic.BlobType, data
 	uncompressedLength := 0
 	if r.cfg.Version > 1 {
 		uncompressedLength = len(data)
-		data = r.enc.EncodeAll(data, nil)
+		data = r.getZstdEncoder().EncodeAll(data, nil)
 	}
 
 	nonce := crypto.NewRandomNonce()
@@ -354,7 +368,7 @@ func (r *Repository) compressUnpacked(p []byte) ([]byte, error) {
 
 	// version byte
 	out := []byte{2}
-	out = r.enc.EncodeAll(p, out)
+	out = r.getZstdEncoder().EncodeAll(p, out)
 	return out, nil
 }
 
@@ -377,7 +391,7 @@ func (r *Repository) decompressUnpacked(p []byte) ([]byte, error) {
 		return nil, errors.New("not supported encoding format")
 	}
 
-	return r.dec.DecodeAll(p[1:], nil)
+	return r.getZstdDecoder().DecodeAll(p[1:], nil)
 }
 
 // SaveUnpacked encrypts data and stores it in the backend. Returned is the