Abstract hash function usage (#28138)

Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.

This forms the "no-op" part of the SHA256 enablement patch.

modules/git/object_id.go

@@ -0,0 +1,143 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"bytes"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"hash"
+	"strconv"
+	"strings"
+)
+
+type ObjectID interface {
+	String() string
+	IsZero() bool
+	RawValue() []byte
+	Type() ObjectFormat
+}
+
+/* SHA1 */
+type Sha1Hash [20]byte
+
+func (h *Sha1Hash) String() string {
+	return hex.EncodeToString(h[:])
+}
+
+func (h *Sha1Hash) IsZero() bool {
+	empty := Sha1Hash{}
+	return bytes.Equal(empty[:], h[:])
+}
+func (h *Sha1Hash) RawValue() []byte { return h[:] }
+func (*Sha1Hash) Type() ObjectFormat { return &Sha1ObjectFormat{} }
+
+func NewSha1() *Sha1Hash {
+	return &Sha1Hash{}
+}
+
+// generic implementations
+func NewHash(hash string) (ObjectID, error) {
+	hash = strings.ToLower(hash)
+	switch hash {
+	case "sha1":
+		return &Sha1Hash{}, nil
+	}
+
+	return nil, errors.New("unsupported hash type")
+}
+
+func IDFromRaw(h ObjectFormat, b []byte) (ObjectID, error) {
+	if len(b) != h.FullLength()/2 {
+		return h.Empty(), fmt.Errorf("length must be %d: %v", h.FullLength(), b)
+	}
+	return h.MustID(b), nil
+}
+
+func MustIDFromString(h ObjectFormat, s string) ObjectID {
+	b, _ := hex.DecodeString(s)
+	return h.MustID(b)
+}
+
+func genericIDFromString(h ObjectFormat, s string) (ObjectID, error) {
+	s = strings.TrimSpace(s)
+	if len(s) != h.FullLength() {
+		return h.Empty(), fmt.Errorf("length must be %d: %s", h.FullLength(), s)
+	}
+	b, err := hex.DecodeString(s)
+	if err != nil {
+		return h.Empty(), err
+	}
+	return h.NewID(b)
+}
+
+// utils
+func IDFromString(hexHash string) (ObjectID, error) {
+	switch len(hexHash) {
+	case 40:
+		hashType := Sha1ObjectFormat{}
+		h, err := hashType.NewIDFromString(hexHash)
+		if err != nil {
+			return nil, err
+		}
+		return h, nil
+	}
+
+	return nil, fmt.Errorf("invalid hash hex string: '%s' len: %d", hexHash, len(hexHash))
+}
+
+func IsEmptyCommitID(commitID string) bool {
+	if commitID == "" {
+		return true
+	}
+
+	id, err := IDFromString(commitID)
+	if err != nil {
+		return false
+	}
+
+	return id.IsZero()
+}
+
+// HashInterface is a struct that will generate a Hash
+type HasherInterface interface {
+	hash.Hash
+
+	HashSum() ObjectID
+}
+
+type Sha1Hasher struct {
+	hash.Hash
+}
+
+// ComputeBlobHash compute the hash for a given blob content
+func ComputeBlobHash(hashType ObjectFormat, content []byte) ObjectID {
+	return ComputeHash(hashType, ObjectBlob, content)
+}
+
+// ComputeHash compute the hash for a given ObjectType and content
+func ComputeHash(hashType ObjectFormat, t ObjectType, content []byte) ObjectID {
+	h := hashType.NewHasher()
+	_, _ = h.Write(t.Bytes())
+	_, _ = h.Write([]byte(" "))
+	_, _ = h.Write([]byte(strconv.FormatInt(int64(len(content)), 10)))
+	_, _ = h.Write([]byte{0})
+	return h.HashSum()
+}
+
+// Sum generates a SHA1 for the provided hash
+func (h *Sha1Hasher) HashSum() ObjectID {
+	var sha1 Sha1Hash
+	copy(sha1[:], h.Hash.Sum(nil))
+	return &sha1
+}
+
+type ErrInvalidSHA struct {
+	SHA string
+}
+
+func (err ErrInvalidSHA) Error() string {
+	return fmt.Sprintf("invalid sha: %s", err.SHA)
+}