internal/repository: Fix LoadBlob + fuzz test

When given a buf that is big enough for a compressed blob but not its
decompressed contents, the copy at the end of LoadBlob would skip the
last part of the contents.

Fixes #3783.

.gitattributes

@@ -0,0 +1,2 @@
+# Workaround for https://github.com/golang/go/issues/52268.
+**/testdata/fuzz/*/* eol=lf

internal/repository/fuzz_test.go

@@ -0,0 +1,43 @@
+//go:build go1.18
+// +build go1.18
+
+package repository
+
+import (
+	"context"
+	"testing"
+
+	"github.com/restic/restic/internal/backend/mem"
+	"github.com/restic/restic/internal/restic"
+)
+
+// Test saving a blob and loading it again, with varying buffer sizes.
+// Also a regression test for #3783.
+func FuzzSaveLoadBlob(f *testing.F) {
+	f.Fuzz(func(t *testing.T, blob []byte, buflen uint) {
+		if buflen > 64<<20 {
+			// Don't allocate enormous buffers. We're not testing the allocator.
+			t.Skip()
+		}
+
+		id := restic.Hash(blob)
+		repo, _ := TestRepositoryWithBackend(t, mem.New(), 2)
+
+		_, _, err := repo.SaveBlob(context.TODO(), restic.DataBlob, blob, id, false)
+		if err != nil {
+			t.Fatal(err)
+		}
+		err = repo.Flush(context.TODO())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		buf, err := repo.LoadBlob(context.TODO(), restic.DataBlob, id, make([]byte, buflen))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if restic.Hash(buf) != id {
+			t.Fatal("mismatch")
+		}
+	})
+}

internal/repository/repository.go

@@ -303,8 +303,9 @@ func (r *Repository) LoadBlob(ctx context.Context, t restic.BlobType, id restic.
 			return plaintext, nil
 		}
 		// move decrypted data to the start of the buffer
+		buf = buf[:len(plaintext)]
 		copy(buf, plaintext)
-		return buf[:len(plaintext)], nil
+		return buf, nil
 	}
 
 	if lastError != nil {

internal/repository/testdata/fuzz/FuzzSaveLoadBlob/62d79435b9ad1777d0562c405e1ab2e1ef5d11d07c8aa4fe6814010294bffd33

@@ -0,0 +1,3 @@
+go test fuzz v1
+[]byte("\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6\xc6")
+uint(109)