1 files changed, 148 insertions, 0 deletions
diff --git a/models/db/index.go b/models/db/index.go
new file mode 100644
index 0000000..259ddd6
--- /dev/null
+++ b/models/db/index.go
@@ -0,0 +1,148 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package db
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strconv"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// ResourceIndex represents a resource index which could be used as issue/release and others
+// We can create different tables i.e. issue_index, release_index, etc.
+type ResourceIndex struct {
+	GroupID  int64 `xorm:"pk"`
+	MaxIndex int64 `xorm:"index"`
+}
+
+var (
+	// ErrResouceOutdated represents an error when request resource outdated
+	ErrResouceOutdated = errors.New("resource outdated")
+	// ErrGetResourceIndexFailed represents an error when resource index retries 3 times
+	ErrGetResourceIndexFailed = errors.New("get resource index failed")
+)
+
+// SyncMaxResourceIndex sync the max index with the resource
+func SyncMaxResourceIndex(ctx context.Context, tableName string, groupID, maxIndex int64) (err error) {
+	e := GetEngine(ctx)
+
+	// try to update the max_index and acquire the write-lock for the record
+	res, err := e.Exec(fmt.Sprintf("UPDATE %s SET max_index=? WHERE group_id=? AND max_index<?", tableName), maxIndex, groupID, maxIndex)
+	if err != nil {
+		return err
+	}
+	affected, err := res.RowsAffected()
+	if err != nil {
+		return err
+	}
+	if affected == 0 {
+		// if nothing is updated, the record might not exist or might be larger, it's safe to try to insert it again and then check whether the record exists
+		_, errIns := e.Exec(fmt.Sprintf("INSERT INTO %s (group_id, max_index) VALUES (?, ?)", tableName), groupID, maxIndex)
+		var savedIdx int64
+		has, err := e.SQL(fmt.Sprintf("SELECT max_index FROM %s WHERE group_id=?", tableName), groupID).Get(&savedIdx)
+		if err != nil {
+			return err
+		}
+		// if the record still doesn't exist, there must be some errors (insert error)
+		if !has {
+			if errIns == nil {
+				return errors.New("impossible error when SyncMaxResourceIndex, insert succeeded but no record is saved")
+			}
+			return errIns
+		}
+	}
+	return nil
+}
+
+func postgresGetNextResourceIndex(ctx context.Context, tableName string, groupID int64) (int64, error) {
+	res, err := GetEngine(ctx).Query(fmt.Sprintf("INSERT INTO %s (group_id, max_index) "+
+		"VALUES (?,1) ON CONFLICT (group_id) DO UPDATE SET max_index = %s.max_index+1 RETURNING max_index",
+		tableName, tableName), groupID)
+	if err != nil {
+		return 0, err
+	}
+	if len(res) == 0 {
+		return 0, ErrGetResourceIndexFailed
+	}
+	return strconv.ParseInt(string(res[0]["max_index"]), 10, 64)
+}
+
+func mysqlGetNextResourceIndex(ctx context.Context, tableName string, groupID int64) (int64, error) {
+	if _, err := GetEngine(ctx).Exec(fmt.Sprintf("INSERT INTO %s (group_id, max_index) "+
+		"VALUES (?,1) ON DUPLICATE KEY UPDATE max_index = max_index+1",
+		tableName), groupID); err != nil {
+		return 0, err
+	}
+
+	var idx int64
+	_, err := GetEngine(ctx).SQL(fmt.Sprintf("SELECT max_index FROM %s WHERE group_id = ?", tableName), groupID).Get(&idx)
+	if err != nil {
+		return 0, err
+	}
+	if idx == 0 {
+		return 0, errors.New("cannot get the correct index")
+	}
+	return idx, nil
+}
+
+// GetNextResourceIndex generates a resource index, it must run in the same transaction where the resource is created
+func GetNextResourceIndex(ctx context.Context, tableName string, groupID int64) (int64, error) {
+	switch {
+	case setting.Database.Type.IsPostgreSQL():
+		return postgresGetNextResourceIndex(ctx, tableName, groupID)
+	case setting.Database.Type.IsMySQL():
+		return mysqlGetNextResourceIndex(ctx, tableName, groupID)
+	}
+
+	e := GetEngine(ctx)
+
+	// try to update the max_index to next value, and acquire the write-lock for the record
+	res, err := e.Exec(fmt.Sprintf("UPDATE %s SET max_index=max_index+1 WHERE group_id=?", tableName), groupID)
+	if err != nil {
+		return 0, err
+	}
+	affected, err := res.RowsAffected()
+	if err != nil {
+		return 0, err
+	}
+	if affected == 0 {
+		// this slow path is only for the first time of creating a resource index
+		_, errIns := e.Exec(fmt.Sprintf("INSERT INTO %s (group_id, max_index) VALUES (?, 0)", tableName), groupID)
+		res, err = e.Exec(fmt.Sprintf("UPDATE %s SET max_index=max_index+1 WHERE group_id=?", tableName), groupID)
+		if err != nil {
+			return 0, err
+		}
+		affected, err = res.RowsAffected()
+		if err != nil {
+			return 0, err
+		}
+		// if the update still can not update any records, the record must not exist and there must be some errors (insert error)
+		if affected == 0 {
+			if errIns == nil {
+				return 0, errors.New("impossible error when GetNextResourceIndex, insert and update both succeeded but no record is updated")
+			}
+			return 0, errIns
+		}
+	}
+
+	// now, the new index is in database (protected by the transaction and write-lock)
+	var newIdx int64
+	has, err := e.SQL(fmt.Sprintf("SELECT max_index FROM %s WHERE group_id=?", tableName), groupID).Get(&newIdx)
+	if err != nil {
+		return 0, err
+	}
+	if !has {
+		return 0, errors.New("impossible error when GetNextResourceIndex, upsert succeeded but no record can be selected")
+	}
+	return newIdx, nil
+}
+
+// DeleteResourceIndex delete resource index
+func DeleteResourceIndex(ctx context.Context, tableName string, groupID int64) error {
+	_, err := Exec(ctx, fmt.Sprintf("DELETE FROM %s WHERE group_id=?", tableName), groupID)
+	return err
+}