feat: sync forks (#2364)

This allows syncing a branch of a fork with a branch of the base repo. It looks like this:
![grafik](/attachments/4508920c-7d0b-4330-9083-e3048733e38d)
This is only possible, if the fork don't have commits that are not in the main repo.

The feature is already working, but it is missing Finetuning, a better API, translations and tests, so this is currently WIP. It is also not tested with go-git.

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/2364): <!--number 2364 --><!--line 0 --><!--description c3luYyBmb3Jrcw==-->sync forks<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/2364
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: JakobDev <jakobdev@gmx.de>
Co-committed-by: JakobDev <jakobdev@gmx.de>

routers/api/v1/repo/sync_fork.go

@@ -0,0 +1,185 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"net/http"
+
+	git_model "forgejo.org/models/git"
+	"forgejo.org/services/context"
+	repo_service "forgejo.org/services/repository"
+)
+
+func getSyncForkInfo(ctx *context.APIContext, branch string) {
+	if !ctx.Repo.Repository.IsFork {
+		ctx.Error(http.StatusBadRequest, "NoFork", "The Repo must be a fork")
+		return
+	}
+
+	syncForkInfo, err := repo_service.GetSyncForkInfo(ctx, ctx.Repo.Repository, branch)
+	if err != nil {
+		if git_model.IsErrBranchNotExist(err) {
+			ctx.NotFound(err, branch)
+			return
+		}
+
+		ctx.Error(http.StatusInternalServerError, "GetSyncForkInfo", err)
+		return
+	}
+
+	ctx.JSON(http.StatusOK, syncForkInfo)
+}
+
+// SyncForkBranchInfo returns information about syncing the default fork branch with the base branch
+func SyncForkDefaultInfo(ctx *context.APIContext) {
+	// swagger:operation GET /repos/{owner}/{repo}/sync_fork repository repoSyncForkDefaultInfo
+	// ---
+	// summary: Gets information about syncing the fork default branch with the base branch
+	// produces:
+	// - application/json
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// responses:
+	//   "200":
+	//     "$ref": "#/responses/SyncForkInfo"
+	//   "400":
+	//     "$ref": "#/responses/error"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
+	getSyncForkInfo(ctx, ctx.Repo.Repository.DefaultBranch)
+}
+
+// SyncForkBranchInfo returns information about syncing a fork branch with the base branch
+func SyncForkBranchInfo(ctx *context.APIContext) {
+	// swagger:operation GET /repos/{owner}/{repo}/sync_fork/{branch} repository repoSyncForkBranchInfo
+	// ---
+	// summary: Gets information about syncing a fork branch with the base branch
+	// produces:
+	// - application/json
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// - name: branch
+	//   in: path
+	//   description: The branch
+	//   type: string
+	//   required: true
+	// responses:
+	//   "200":
+	//     "$ref": "#/responses/SyncForkInfo"
+	//   "400":
+	//     "$ref": "#/responses/error"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
+	getSyncForkInfo(ctx, ctx.Params("branch"))
+}
+
+func syncForkBranch(ctx *context.APIContext, branch string) {
+	if !ctx.Repo.Repository.IsFork {
+		ctx.Error(http.StatusBadRequest, "NoFork", "The Repo must be a fork")
+		return
+	}
+
+	syncForkInfo, err := repo_service.GetSyncForkInfo(ctx, ctx.Repo.Repository, branch)
+	if err != nil {
+		if git_model.IsErrBranchNotExist(err) {
+			ctx.NotFound(err, branch)
+			return
+		}
+
+		ctx.Error(http.StatusInternalServerError, "GetSyncForkInfo", err)
+		return
+	}
+
+	if !syncForkInfo.Allowed {
+		ctx.Error(http.StatusBadRequest, "NotAllowed", "You can't sync this branch")
+		return
+	}
+
+	err = repo_service.SyncFork(ctx, ctx.Doer, ctx.Repo.Repository, branch)
+	if err != nil {
+		ctx.Error(http.StatusInternalServerError, "SyncFork", err)
+		return
+	}
+
+	ctx.Status(http.StatusNoContent)
+}
+
+// SyncForkBranch syncs the default of a fork with the base branch
+func SyncForkDefault(ctx *context.APIContext) {
+	// swagger:operation POST /repos/{owner}/{repo}/sync_fork repository repoSyncForkDefault
+	// ---
+	// summary: Syncs the default branch of a fork with the base branch
+	// produces:
+	// - application/json
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// responses:
+	//   "204":
+	//     "$ref": "#/responses/empty"
+	//   "400":
+	//     "$ref": "#/responses/error"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
+	syncForkBranch(ctx, ctx.Repo.Repository.DefaultBranch)
+}
+
+// SyncForkBranch syncs a fork branch with the base branch
+func SyncForkBranch(ctx *context.APIContext) {
+	// swagger:operation POST /repos/{owner}/{repo}/sync_fork/{branch} repository repoSyncForkBranch
+	// ---
+	// summary: Syncs a fork branch with the base branch
+	// produces:
+	// - application/json
+	// parameters:
+	// - name: owner
+	//   in: path
+	//   description: owner of the repo
+	//   type: string
+	//   required: true
+	// - name: repo
+	//   in: path
+	//   description: name of the repo
+	//   type: string
+	//   required: true
+	// - name: branch
+	//   in: path
+	//   description: The branch
+	//   type: string
+	//   required: true
+	// responses:
+	//   "204":
+	//     "$ref": "#/responses/empty"
+	//   "400":
+	//     "$ref": "#/responses/error"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
+	syncForkBranch(ctx, ctx.Params("branch"))
+}