restic: speed up repository config initialization in tests

Avoid redundant polynomial generation, which takes a millisecond or so.

internal/repository/repository.go

@@ -919,13 +919,10 @@ func (r *Repository) Init(ctx context.Context, version uint, password string, ch
 		return err
 	}
 
-	cfg, err := restic.CreateConfig(version)
+	cfg, err := restic.CreateConfig(version, chunkerPolynomial)
 	if err != nil {
 		return err
 	}
-	if chunkerPolynomial != nil {
-		cfg.ChunkerPolynomial = *chunkerPolynomial
-	}
 
 	return r.init(ctx, password, cfg)
 }

internal/restic/config.go

@@ -28,15 +28,19 @@ const StableRepoVersion = 2
 
 // CreateConfig creates a config file with a randomly selected polynomial and
 // ID.
-func CreateConfig(version uint) (Config, error) {
+func CreateConfig(version uint, pol *chunker.Pol) (Config, error) {
 	var (
 		err error
 		cfg Config
 	)
 
-	cfg.ChunkerPolynomial, err = chunker.RandomPolynomial()
-	if err != nil {
-		return Config{}, errors.Wrap(err, "chunker.RandomPolynomial")
+	if pol == nil {
+		cfg.ChunkerPolynomial, err = chunker.RandomPolynomial()
+		if err != nil {
+			return Config{}, errors.Wrap(err, "chunker.RandomPolynomial")
+		}
+	} else {
+		cfg.ChunkerPolynomial = *pol
 	}
 
 	cfg.ID = NewRandomID().String()

internal/restic/config_test.go

@@ -43,7 +43,7 @@ func TestConfig(t *testing.T) {
 		return restic.ID{}, nil
 	}
 
-	cfg1, err := restic.CreateConfig(restic.MaxRepoVersion)
+	cfg1, err := restic.CreateConfig(restic.MaxRepoVersion, nil)
 	rtest.OK(t, err)
 
 	err = restic.SaveConfig(context.TODO(), saver{save}, cfg1)