Change target branch for pull request (#6488)

* Adds functionality to change target branch of created pull requests

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Use const instead of var in JavaScript additions

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Check if branches are equal and if PR already exists before changing target branch

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Make sure to check all commits

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Print error messages for user as error flash message

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Disallow changing target branch of closed or merged pull requests

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Resolve conflicts after merge of upstream/master

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Change order of branch select fields

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Removes duplicate check

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Use ctx.Tr for translations

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Recompile JS

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Use correct translation namespace

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Remove redundant if condition

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Moves most change branch logic into pull service

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Completes comment

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Add Ref to ChangesPayload for logging changed target branches
instead of creating a new struct

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Revert changes to go.mod

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Directly use createComment method

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Return 404 if pull request is not found. Move written check up

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Remove variable declaration

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Return client errors on change pull request target errors

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Return error in commit.HasPreviousCommit

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Adds blank line

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Test patch before persisting new target branch

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Update patch before testing (not working)

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Removes patch calls when changeing pull request target

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Removes unneeded check for base name

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Moves ChangeTargetBranch completely to pull service. Update patch status.

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Set webhook mode after errors were validated

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Update PR in one transaction

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Move logic for check if head is equal with branch to pull model

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Adds missing comment and simplify return

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Adjust CreateComment method call

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

19 files changed, 461 insertions, 19 deletions

diff --git a/models/error.go b/models/error.go
index a17ff5f9d0..396d7594c8 100644
--- a/models/error.go
+++ b/models/error.go
@@ -953,6 +953,22 @@ func (err ErrBranchNameConflict) Error() string {
 	return fmt.Sprintf("branch conflicts with existing branch [name: %s]", err.BranchName)
 }
 
+// ErrBranchesEqual represents an error that branch name conflicts with other branch.
+type ErrBranchesEqual struct {
+	BaseBranchName string
+	HeadBranchName string
+}
+
+// IsErrBranchesEqual checks if an error is an ErrBranchesEqual.
+func IsErrBranchesEqual(err error) bool {
+	_, ok := err.(ErrBranchesEqual)
+	return ok
+}
+
+func (err ErrBranchesEqual) Error() string {
+	return fmt.Sprintf("branches are equal [head: %sm base: %s]", err.HeadBranchName, err.BaseBranchName)
+}
+
 // ErrNotAllowedToMerge represents an error that a branch is protected and the current user is not allowed to modify it.
 type ErrNotAllowedToMerge struct {
 	Reason string
@@ -1090,6 +1106,23 @@ func (err ErrIssueNotExist) Error() string {
 	return fmt.Sprintf("issue does not exist [id: %d, repo_id: %d, index: %d]", err.ID, err.RepoID, err.Index)
 }
 
+// ErrIssueIsClosed represents a "IssueIsClosed" kind of error.
+type ErrIssueIsClosed struct {
+	ID     int64
+	RepoID int64
+	Index  int64
+}
+
+// IsErrIssueIsClosed checks if an error is a ErrIssueNotExist.
+func IsErrIssueIsClosed(err error) bool {
+	_, ok := err.(ErrIssueIsClosed)
+	return ok
+}
+
+func (err ErrIssueIsClosed) Error() string {
+	return fmt.Sprintf("issue is closed [id: %d, repo_id: %d, index: %d]", err.ID, err.RepoID, err.Index)
+}
+
 // ErrIssueLabelTemplateLoad represents a "ErrIssueLabelTemplateLoad" kind of error.
 type ErrIssueLabelTemplateLoad struct {
 	TemplateFile  string
@@ -1326,6 +1359,28 @@ func (err ErrRebaseConflicts) Error() string {
 	return fmt.Sprintf("Rebase Error: %v: Whilst Rebasing: %s\n%s\n%s", err.Err, err.CommitSHA, err.StdErr, err.StdOut)
 }
 
+// ErrPullRequestHasMerged represents a "PullRequestHasMerged"-error
+type ErrPullRequestHasMerged struct {
+	ID         int64
+	IssueID    int64
+	HeadRepoID int64
+	BaseRepoID int64
+	HeadBranch string
+	BaseBranch string
+}
+
+// IsErrPullRequestHasMerged checks if an error is a ErrPullRequestHasMerged.
+func IsErrPullRequestHasMerged(err error) bool {
+	_, ok := err.(ErrPullRequestHasMerged)
+	return ok
+}
+
+// Error does pretty-printing :D
+func (err ErrPullRequestHasMerged) Error() string {
+	return fmt.Sprintf("pull request has merged [id: %d, issue_id: %d, head_repo_id: %d, base_repo_id: %d, head_branch: %s, base_branch: %s]",
+		err.ID, err.IssueID, err.HeadRepoID, err.BaseRepoID, err.HeadBranch, err.BaseBranch)
+}
+
 // _________                                       __
 // \_   ___ \  ____   _____   _____   ____   _____/  |_
 // /    \  \/ /  _ \ /     \ /     \_/ __ \ /    \   __\
diff --git a/models/issue_comment.go b/models/issue_comment.go
index 9f87c049df..aeaee68003 100644
--- a/models/issue_comment.go
+++ b/models/issue_comment.go
@@ -82,6 +82,8 @@ const (
 	CommentTypeLock
 	// Unlocks a previously locked issue
 	CommentTypeUnlock
+	// Change pull request's target branch
+	CommentTypeChangeTargetBranch
 )
 
 // CommentTag defines comment tag type
@@ -116,6 +118,8 @@ type Comment struct {
 	Assignee         *User `xorm:"-"`
 	OldTitle         string
 	NewTitle         string
+	OldRef           string
+	NewRef           string
 	DependentIssueID int64
 	DependentIssue   *Issue `xorm:"-"`
 
@@ -517,6 +521,8 @@ func createComment(e *xorm.Session, opts *CreateCommentOptions) (_ *Comment, err
 		Content:          opts.Content,
 		OldTitle:         opts.OldTitle,
 		NewTitle:         opts.NewTitle,
+		OldRef:           opts.OldRef,
+		NewRef:           opts.NewRef,
 		DependentIssueID: opts.DependentIssueID,
 		TreePath:         opts.TreePath,
 		ReviewID:         opts.ReviewID,
@@ -673,6 +679,8 @@ type CreateCommentOptions struct {
 	RemovedAssignee  bool
 	OldTitle         string
 	NewTitle         string
+	OldRef           string
+	NewRef           string
 	CommitID         int64
 	CommitSHA        string
 	Patch            string
diff --git a/models/migrations/migrations.go b/models/migrations/migrations.go
index 78fbc18ca5..cbea5a95dd 100644
--- a/models/migrations/migrations.go
+++ b/models/migrations/migrations.go
@@ -280,6 +280,8 @@ var migrations = []Migration{
 	NewMigration("update branch protection for can push and whitelist enable", addBranchProtectionCanPushAndEnableWhitelist),
 	// v112 -> v113
 	NewMigration("remove release attachments which repository deleted", removeAttachmentMissedRepo),
+	// v113 -> v114
+	NewMigration("new feature: change target branch of pull requests", featureChangeTargetBranch),
 }
 
 // Migrate database to current version
diff --git a/models/migrations/v113.go b/models/migrations/v113.go
new file mode 100644
index 0000000000..199c02ac98
--- /dev/null
+++ b/models/migrations/v113.go
@@ -0,0 +1,23 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"fmt"
+
+	"xorm.io/xorm"
+)
+
+func featureChangeTargetBranch(x *xorm.Engine) error {
+	type Comment struct {
+		OldRef string
+		NewRef string
+	}
+
+	if err := x.Sync2(new(Comment)); err != nil {
+		return fmt.Errorf("Sync2: %v", err)
+	}
+	return nil
+}
diff --git a/models/pull.go b/models/pull.go
index d703c9a2ee..ba9c575775 100644
--- a/models/pull.go
+++ b/models/pull.go
@@ -664,3 +664,32 @@ func (pr *PullRequest) GetWorkInProgressPrefix() string {
 	}
 	return ""
 }
+
+// IsHeadEqualWithBranch returns if the commits of branchName are available in pull request head
+func (pr *PullRequest) IsHeadEqualWithBranch(branchName string) (bool, error) {
+	var err error
+	if err = pr.GetBaseRepo(); err != nil {
+		return false, err
+	}
+	baseGitRepo, err := git.OpenRepository(pr.BaseRepo.RepoPath())
+	if err != nil {
+		return false, err
+	}
+	baseCommit, err := baseGitRepo.GetBranchCommit(branchName)
+	if err != nil {
+		return false, err
+	}
+
+	if err = pr.GetHeadRepo(); err != nil {
+		return false, err
+	}
+	headGitRepo, err := git.OpenRepository(pr.HeadRepo.RepoPath())
+	if err != nil {
+		return false, err
+	}
+	headCommit, err := headGitRepo.GetBranchCommit(pr.HeadBranch)
+	if err != nil {
+		return false, err
+	}
+	return baseCommit.HasPreviousCommit(headCommit.ID)
+}
diff --git a/modules/git/commit.go b/modules/git/commit.go
index 0388d5e9be..dfb7adcd1a 100644
--- a/modules/git/commit.go
+++ b/modules/git/commit.go
@@ -306,6 +306,27 @@ func (c *Commit) CommitsBefore() (*list.List, error) {
 	return c.repo.getCommitsBefore(c.ID)
 }
 
+// HasPreviousCommit returns true if a given commitHash is contained in commit's parents
+func (c *Commit) HasPreviousCommit(commitHash SHA1) (bool, error) {
+	for i := 0; i < c.ParentCount(); i++ {
+		commit, err := c.Parent(i)
+		if err != nil {
+			return false, err
+		}
+		if commit.ID == commitHash {
+			return true, nil
+		}
+		commitInParentCommit, err := commit.HasPreviousCommit(commitHash)
+		if err != nil {
+			return false, err
+		}
+		if commitInParentCommit {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
 // CommitsBeforeLimit returns num commits before current revision
 func (c *Commit) CommitsBeforeLimit(num int) (*list.List, error) {
 	return c.repo.getCommitsBeforeLimit(c.ID, num)
diff --git a/modules/notification/base/notifier.go b/modules/notification/base/notifier.go
index 934ee80aa7..48846b3446 100644
--- a/modules/notification/base/notifier.go
+++ b/modules/notification/base/notifier.go
@@ -34,6 +34,7 @@ type Notifier interface {
 	NotifyMergePullRequest(*models.PullRequest, *models.User, *git.Repository)
 	NotifyPullRequestSynchronized(doer *models.User, pr *models.PullRequest)
 	NotifyPullRequestReview(*models.PullRequest, *models.Review, *models.Comment)
+	NotifyPullRequestChangeTargetBranch(doer *models.User, pr *models.PullRequest, oldBranch string)
 
 	NotifyCreateIssueComment(*models.User, *models.Repository,
 		*models.Issue, *models.Comment)
diff --git a/modules/notification/base/null.go b/modules/notification/base/null.go
index a04d0e8caa..bea4e55277 100644
--- a/modules/notification/base/null.go
+++ b/modules/notification/base/null.go
@@ -50,6 +50,10 @@ func (*NullNotifier) NotifyMergePullRequest(pr *models.PullRequest, doer *models
 func (*NullNotifier) NotifyPullRequestSynchronized(doer *models.User, pr *models.PullRequest) {
 }
 
+// NotifyPullRequestChangeTargetBranch places a place holder function
+func (*NullNotifier) NotifyPullRequestChangeTargetBranch(doer *models.User, pr *models.PullRequest, oldBranch string) {
+}
+
 // NotifyUpdateComment places a place holder function
 func (*NullNotifier) NotifyUpdateComment(doer *models.User, c *models.Comment, oldContent string) {
 }
diff --git a/modules/notification/notification.go b/modules/notification/notification.go
index ab671fa291..f567552df5 100644
--- a/modules/notification/notification.go
+++ b/modules/notification/notification.go
@@ -87,6 +87,13 @@ func NotifyPullRequestReview(pr *models.PullRequest, review *models.Review, comm
 	}
 }
 
+// NotifyPullRequestChangeTargetBranch notifies when a pull request's target branch was changed
+func NotifyPullRequestChangeTargetBranch(doer *models.User, pr *models.PullRequest, oldBranch string) {
+	for _, notifier := range notifiers {
+		notifier.NotifyPullRequestChangeTargetBranch(doer, pr, oldBranch)
+	}
+}
+
 // NotifyUpdateComment notifies update comment to notifiers
 func NotifyUpdateComment(doer *models.User, c *models.Comment, oldContent string) {
 	for _, notifier := range notifiers {
diff --git a/modules/notification/webhook/webhook.go b/modules/notification/webhook/webhook.go
index ee91a29f02..e0801445d8 100644
--- a/modules/notification/webhook/webhook.go
+++ b/modules/notification/webhook/webhook.go
@@ -559,6 +559,37 @@ func (*webhookNotifier) NotifyMergePullRequest(pr *models.PullRequest, doer *mod
 	}
 }
 
+func (m *webhookNotifier) NotifyPullRequestChangeTargetBranch(doer *models.User, pr *models.PullRequest, oldBranch string) {
+	issue := pr.Issue
+	if !issue.IsPull {
+		return
+	}
+	var err error
+
+	if err = issue.LoadPullRequest(); err != nil {
+		log.Error("LoadPullRequest failed: %v", err)
+		return
+	}
+	issue.PullRequest.Issue = issue
+	mode, _ := models.AccessLevel(issue.Poster, issue.Repo)
+	err = webhook_module.PrepareWebhooks(issue.Repo, models.HookEventPullRequest, &api.PullRequestPayload{
+		Action: api.HookIssueEdited,
+		Index:  issue.Index,
+		Changes: &api.ChangesPayload{
+			Ref: &api.ChangesFromPayload{
+				From: oldBranch,
+			},
+		},
+		PullRequest: issue.PullRequest.APIFormat(),
+		Repository:  issue.Repo.APIFormat(mode),
+		Sender:      doer.APIFormat(),
+	})
+
+	if err != nil {
+		log.Error("PrepareWebhooks [is_pull: %v]: %v", issue.IsPull, err)
+	}
+}
+
 func (m *webhookNotifier) NotifyPullRequestReview(pr *models.PullRequest, review *models.Review, comment *models.Comment) {
 	var reviewHookType models.HookEventType
 
diff --git a/modules/structs/hook.go b/modules/structs/hook.go
index e036442904..d62c900f42 100644
--- a/modules/structs/hook.go
+++ b/modules/structs/hook.go
@@ -398,10 +398,11 @@ type ChangesFromPayload struct {
 	From string `json:"from"`
 }
 
-// ChangesPayload FIXME
+// ChangesPayload represents the payload information of issue change
 type ChangesPayload struct {
 	Title *ChangesFromPayload `json:"title,omitempty"`
 	Body  *ChangesFromPayload `json:"body,omitempty"`
+	Ref   *ChangesFromPayload `json:"ref,omitempty"`
 }
 
 // __________      .__  .__    __________                                     __
diff --git a/options/locale/locale_en-US.ini b/options/locale/locale_en-US.ini
index c5cfb1663f..97596451bc 100644
--- a/options/locale/locale_en-US.ini
+++ b/options/locale/locale_en-US.ini
@@ -1033,8 +1033,9 @@ pulls.no_results = No results found.
 pulls.nothing_to_compare = These branches are equal. There is no need to create a pull request.
 pulls.has_pull_request = `A pull request between these branches already exists: <a href="%[1]s/pulls/%[3]d">%[2]s#%[3]d</a>`
 pulls.create = Create Pull Request
-pulls.title_desc = wants to merge %[1]d commits from <code>%[2]s</code> into <code>%[3]s</code>
+pulls.title_desc = wants to merge %[1]d commits from <code>%[2]s</code> into <code id="branch_target">%[3]s</code>
 pulls.merged_title_desc = merged %[1]d commits from <code>%[2]s</code> into <code>%[3]s</code> %[4]s
+pulls.change_target_branch_at = `changed target branch from <b>%s</b> to <b>%s</b> %s`
 pulls.tab_conversation = Conversation
 pulls.tab_commits = Commits
 pulls.tab_files = Files Changed
@@ -1042,6 +1043,7 @@ pulls.reopen_to_merge = Please reopen this pull request to perform a merge.
 pulls.cant_reopen_deleted_branch = This pull request cannot be reopened because the branch was deleted.
 pulls.merged = Merged
 pulls.merged_as = The pull request has been merged as <a rel="nofollow" class="ui sha" href="%[1]s"><code>%[2]s</code></a>.
+pulls.is_closed = The pull request has been closed.
 pulls.has_merged = The pull request has been merged.
 pulls.title_wip_desc = `<a href="#">Start the title with <strong>%s</strong></a> to prevent the pull request from being merged accidentally.`
 pulls.cannot_merge_work_in_progress = This pull request is marked as a work in progress. Remove the <strong>%s</strong> prefix from the title when it's ready
diff --git a/routers/repo/issue.go b/routers/repo/issue.go
index 7d11ed3537..07292c1993 100644
--- a/routers/repo/issue.go
+++ b/routers/repo/issue.go
@@ -605,6 +605,19 @@ func commentTag(repo *models.Repository, poster *models.User, issue *models.Issu
 	return models.CommentTagNone, nil
 }
 
+func getBranchData(ctx *context.Context, issue *models.Issue) {
+	ctx.Data["BaseBranch"] = nil
+	ctx.Data["HeadBranch"] = nil
+	ctx.Data["HeadUserName"] = nil
+	ctx.Data["BaseName"] = ctx.Repo.Repository.OwnerName
+	if issue.IsPull {
+		pull := issue.PullRequest
+		ctx.Data["BaseBranch"] = pull.BaseBranch
+		ctx.Data["HeadBranch"] = pull.HeadBranch
+		ctx.Data["HeadUserName"] = pull.MustHeadUserName()
+	}
+}
+
 // ViewIssue render issue view page
 func ViewIssue(ctx *context.Context) {
 	if ctx.Params(":type") == "issues" {
@@ -885,6 +898,7 @@ func ViewIssue(ctx *context.Context) {
 		}
 	}
 
+	getBranchData(ctx, issue)
 	if issue.IsPull {
 		pull := issue.PullRequest
 		pull.Issue = issue
diff --git a/routers/repo/pull.go b/routers/repo/pull.go
index 559d9b267a..f8612d2b3b 100644
--- a/routers/repo/pull.go
+++ b/routers/repo/pull.go
@@ -11,6 +11,7 @@ import (
 	"crypto/subtle"
 	"fmt"
 	"html"
+	"net/http"
 	"path"
 	"strings"
 
@@ -467,6 +468,7 @@ func ViewPullCommits(ctx *context.Context) {
 	ctx.Data["Commits"] = commits
 	ctx.Data["CommitCount"] = commits.Len()
 
+	getBranchData(ctx, issue)
 	ctx.HTML(200, tplPullCommits)
 }
 
@@ -596,6 +598,7 @@ func ViewPullFiles(ctx *context.Context) {
 		ctx.ServerError("GetCurrentReview", err)
 		return
 	}
+	getBranchData(ctx, issue)
 	ctx.HTML(200, tplPullFiles)
 }
 
@@ -1010,3 +1013,74 @@ func DownloadPullDiffOrPatch(ctx *context.Context, patch bool) {
 		return
 	}
 }
+
+// UpdatePullRequestTarget change pull request's target branch
+func UpdatePullRequestTarget(ctx *context.Context) {
+	issue := GetActionIssue(ctx)
+	pr := issue.PullRequest
+	if ctx.Written() {
+		return
+	}
+	if !issue.IsPull {
+		ctx.Error(http.StatusNotFound)
+		return
+	}
+
+	if !ctx.IsSigned || (!issue.IsPoster(ctx.User.ID) && !ctx.Repo.CanWriteIssuesOrPulls(issue.IsPull)) {
+		ctx.Error(http.StatusForbidden)
+		return
+	}
+
+	targetBranch := ctx.QueryTrim("target_branch")
+	if len(targetBranch) == 0 {
+		ctx.Error(http.StatusNoContent)
+		return
+	}
+
+	if err := pull_service.ChangeTargetBranch(pr, ctx.User, targetBranch); err != nil {
+		if models.IsErrPullRequestAlreadyExists(err) {
+			err := err.(models.ErrPullRequestAlreadyExists)
+
+			RepoRelPath := ctx.Repo.Owner.Name + "/" + ctx.Repo.Repository.Name
+			errorMessage := ctx.Tr("repo.pulls.has_pull_request", ctx.Repo.RepoLink, RepoRelPath, err.IssueID)
+
+			ctx.Flash.Error(errorMessage)
+			ctx.JSON(http.StatusConflict, map[string]interface{}{
+				"error":      err.Error(),
+				"user_error": errorMessage,
+			})
+		} else if models.IsErrIssueIsClosed(err) {
+			errorMessage := ctx.Tr("repo.pulls.is_closed")
+
+			ctx.Flash.Error(errorMessage)
+			ctx.JSON(http.StatusConflict, map[string]interface{}{
+				"error":      err.Error(),
+				"user_error": errorMessage,
+			})
+		} else if models.IsErrPullRequestHasMerged(err) {
+			errorMessage := ctx.Tr("repo.pulls.has_merged")
+
+			ctx.Flash.Error(errorMessage)
+			ctx.JSON(http.StatusConflict, map[string]interface{}{
+				"error":      err.Error(),
+				"user_error": errorMessage,
+			})
+		} else if models.IsErrBranchesEqual(err) {
+			errorMessage := ctx.Tr("repo.pulls.nothing_to_compare")
+
+			ctx.Flash.Error(errorMessage)
+			ctx.JSON(http.StatusBadRequest, map[string]interface{}{
+				"error":      err.Error(),
+				"user_error": errorMessage,
+			})
+		} else {
+			ctx.ServerError("UpdatePullRequestTarget", err)
+		}
+		return
+	}
+	notification.NotifyPullRequestChangeTargetBranch(ctx.User, pr, targetBranch)
+
+	ctx.JSON(http.StatusOK, map[string]interface{}{
+		"base_branch": pr.BaseBranch,
+	})
+}
diff --git a/routers/routes/routes.go b/routers/routes/routes.go
index 60fd93df9c..cb4fadbcdb 100644
--- a/routers/routes/routes.go
+++ b/routers/routes/routes.go
@@ -770,6 +770,9 @@ func RegisterRoutes(m *macaron.Macaron) {
 		m.Combo("/compare/*", repo.MustBeNotEmpty, reqRepoCodeReader, repo.SetEditorconfigIfExists).
 			Get(repo.SetDiffViewStyle, repo.CompareDiff).
 			Post(context.RepoMustNotBeArchived(), reqRepoPullsReader, repo.MustAllowPulls, bindIgnErr(auth.CreateIssueForm{}), repo.CompareAndPullRequestPost)
+		m.Group("/pull", func() {
+			m.Post("/:index/target_branch", repo.UpdatePullRequestTarget)
+		}, context.RepoMustNotBeArchived())
 
 		m.Group("", func() {
 			m.Group("", func() {
diff --git a/services/pull/pull.go b/services/pull/pull.go
index e7f4e4eede..fb47df1c3a 100644
--- a/services/pull/pull.go
+++ b/services/pull/pull.go
@@ -46,6 +46,94 @@ func NewPullRequest(repo *models.Repository, pull *models.Issue, labelIDs []int6
 	return nil
 }
 
+// ChangeTargetBranch changes the target branch of this pull request, as the given user.
+func ChangeTargetBranch(pr *models.PullRequest, doer *models.User, targetBranch string) (err error) {
+	// Current target branch is already the same
+	if pr.BaseBranch == targetBranch {
+		return nil
+	}
+
+	if pr.Issue.IsClosed {
+		return models.ErrIssueIsClosed{
+			ID:     pr.Issue.ID,
+			RepoID: pr.Issue.RepoID,
+			Index:  pr.Issue.Index,
+		}
+	}
+
+	if pr.HasMerged {
+		return models.ErrPullRequestHasMerged{
+			ID:         pr.ID,
+			IssueID:    pr.Index,
+			HeadRepoID: pr.HeadRepoID,
+			BaseRepoID: pr.BaseRepoID,
+			HeadBranch: pr.HeadBranch,
+			BaseBranch: pr.BaseBranch,
+		}
+	}
+
+	// Check if branches are equal
+	branchesEqual, err := pr.IsHeadEqualWithBranch(targetBranch)
+	if err != nil {
+		return err
+	}
+	if branchesEqual {
+		return models.ErrBranchesEqual{
+			HeadBranchName: pr.HeadBranch,
+			BaseBranchName: targetBranch,
+		}
+	}
+
+	// Check if pull request for the new target branch already exists
+	existingPr, err := models.GetUnmergedPullRequest(pr.HeadRepoID, pr.BaseRepoID, pr.HeadBranch, targetBranch)
+	if existingPr != nil {
+		return models.ErrPullRequestAlreadyExists{
+			ID:         existingPr.ID,
+			IssueID:    existingPr.Index,
+			HeadRepoID: existingPr.HeadRepoID,
+			BaseRepoID: existingPr.BaseRepoID,
+			HeadBranch: existingPr.HeadBranch,
+			BaseBranch: existingPr.BaseBranch,
+		}
+	}
+	if err != nil && !models.IsErrPullRequestNotExist(err) {
+		return err
+	}
+
+	// Set new target branch
+	oldBranch := pr.BaseBranch
+	pr.BaseBranch = targetBranch
+
+	// Refresh patch
+	if err := TestPatch(pr); err != nil {
+		return err
+	}
+
+	// Update target branch, PR diff and status
+	// This is the same as checkAndUpdateStatus in check service, but also updates base_branch
+	if pr.Status == models.PullRequestStatusChecking {
+		pr.Status = models.PullRequestStatusMergeable
+	}
+	if err := pr.UpdateCols("status, conflicted_files, base_branch"); err != nil {
+		return err
+	}
+
+	// Create comment
+	options := &models.CreateCommentOptions{
+		Type:   models.CommentTypeChangeTargetBranch,
+		Doer:   doer,
+		Repo:   pr.Issue.Repo,
+		Issue:  pr.Issue,
+		OldRef: oldBranch,
+		NewRef: targetBranch,
+	}
+	if _, err = models.CreateComment(options); err != nil {
+		return fmt.Errorf("CreateChangeTargetBranchComment: %v", err)
+	}
+
+	return nil
+}
+
 func checkForInvalidation(requests models.PullRequestList, repoID int64, doer *models.User, branch string) error {
 	repo, err := models.GetRepositoryByID(repoID)
 	if err != nil {
diff --git a/templates/repo/issue/view_content/comments.tmpl b/templates/repo/issue/view_content/comments.tmpl
index 49d2e9bc5f..d8a90caa85 100644
--- a/templates/repo/issue/view_content/comments.tmpl
+++ b/templates/repo/issue/view_content/comments.tmpl
@@ -6,7 +6,7 @@
 	 5 = COMMENT_REF, 6 = PULL_REF, 7 = COMMENT_LABEL, 12 = START_TRACKING,
 	 13 = STOP_TRACKING, 14 = ADD_TIME_MANUAL, 16 = ADDED_DEADLINE, 17 = MODIFIED_DEADLINE,
 	 18 = REMOVED_DEADLINE, 19 = ADD_DEPENDENCY, 20 = REMOVE_DEPENDENCY, 21 = CODE,
-	 22 = REVIEW, 23 = ISSUE_LOCKED, 24 = ISSUE_UNLOCKED -->
+	 22 = REVIEW, 23 = ISSUE_LOCKED, 24 = ISSUE_UNLOCKED, 25 = TARGET_BRANCH_CHANGED -->
 	{{if eq .Type 0}}
 		<div class="comment" id="{{.HashTag}}">
 		{{if .OriginalAuthor }}
@@ -411,5 +411,15 @@
 					{{$.i18n.Tr "repo.issues.unlock_comment" $createdStr | Safe}}
 				</span>
 		</div>
+	{{else if eq .Type 25}}
+		<div class="event">
+			<span class="octicon octicon-primitive-dot"></span>
+			<a class="ui avatar image" href="{{.Poster.HomeLink}}">
+				<img src="{{.Poster.RelAvatarLink}}">
+			</a>
+			<span class="text grey"><a href="{{.Poster.HomeLink}}">{{.Poster.Name}}</a>
+			{{$.i18n.Tr "repo.pulls.change_target_branch_at" (.OldRef|Escape) (.NewRef|Escape) $createdStr | Safe}}
+			</span>
+		</div>
 	{{end}}
 {{end}}
diff --git a/templates/repo/issue/view_title.tmpl b/templates/repo/issue/view_title.tmpl
index e97ca3265c..635638f325 100644
--- a/templates/repo/issue/view_title.tmpl
+++ b/templates/repo/issue/view_title.tmpl
@@ -11,7 +11,7 @@
 				<div class="edit-zone text right">
 					<div id="edit-title" class="ui basic green not-in-edit button">{{.i18n.Tr "repo.issues.edit"}}</div>
 					<div id="cancel-edit-title" class="ui basic blue in-edit button" style="display: none">{{.i18n.Tr "repo.issues.cancel"}}</div>
-					<div id="save-edit-title" class="ui green in-edit button" style="display: none" data-update-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/title">{{.i18n.Tr "repo.issues.save"}}</div>
+					<div id="save-edit-title" class="ui green in-edit button" style="display: none" data-update-url="{{$.RepoLink}}/issues/{{.Issue.Index}}/title" {{if .Issue.IsPull}}data-target-update-url="{{$.RepoLink}}/pull/{{.Issue.Index}}/target_branch"{{end}}>{{.i18n.Tr "repo.issues.save"}}</div>
 				</div>
 			</div>
 		{{end}}
@@ -36,12 +36,42 @@
 			{{end}}
 		{{else}}
 			{{if .Issue.OriginalAuthor }}
-				{{.Issue.OriginalAuthor}}
-				<span class="pull-desc">{{$.i18n.Tr "repo.pulls.title_desc" .NumCommits .HeadTarget .BaseTarget | Str2html}}</span>
+				<span id="pull-desc" class="pull-desc">{{.Issue.OriginalAuthor}} {{$.i18n.Tr "repo.pulls.title_desc" .NumCommits .HeadTarget .BaseTarget | Str2html}}</span>
 			{{else}}
-				<a {{if gt .Issue.Poster.ID 0}}href="{{.Issue.Poster.HomeLink}}"{{end}}>{{.Issue.Poster.GetDisplayName}}</a>
-				<span class="pull-desc">{{$.i18n.Tr "repo.pulls.title_desc" .NumCommits .HeadTarget .BaseTarget | Str2html}}</span>
+				<span id="pull-desc" class="pull-desc">
+				 <a {{if gt .Issue.Poster.ID 0}}href="{{.Issue.Poster.HomeLink}}"{{end}}>{{.Issue.Poster.GetDisplayName}}</a>
+				 {{$.i18n.Tr "repo.pulls.title_desc" .NumCommits .HeadTarget .BaseTarget | Str2html}}
+			    </span>
 			{{end}}
+			<span id="pull-desc-edit" style="display: none">
+                <div class="ui floating filter dropdown">
+                	<div class="ui basic small button">
+                		<span class="text">{{.i18n.Tr "repo.pulls.compare_compare"}}: {{$.HeadTarget}}</span>
+                	</div>
+                </div>
+                <i class="octicon octicon-arrow-right"></i>
+                 <div class="ui floating filter dropdown" data-no-results="{{.i18n.Tr "repo.pulls.no_results"}}">
+                	<div class="ui basic small button">
+                		<span class="text" id="pull-target-branch" data-basename="{{$.BaseName}}" data-branch="{{$.BaseBranch}}">{{.i18n.Tr "repo.pulls.compare_base"}}: {{$.BaseName}}:{{$.BaseBranch}}</span>
+                		<i class="dropdown icon"></i>
+                	</div>
+                	<div class="menu">
+                		<div class="ui icon search input">
+                			<i class="filter icon"></i>
+                			<input name="search" placeholder="{{.i18n.Tr "repo.pulls.filter_branch"}}...">
+                		</div>
+                		<div class="scrolling menu" id="branch-select">
+                			{{range .Branches}}
+                				{{ $sameBase := ne $.BaseName $.HeadUserName }}
+                				{{ $differentBranch := ne . $.HeadBranch }}
+                				{{ if or $sameBase $differentBranch }}
+                					<div class="item {{if eq $.BaseBranch .}}selected{{end}}" data-branch="{{.}}">{{$.BaseName}}{{if $.HeadRepo}}/{{$.HeadRepo}}{{end}}:{{.}}</div>
+                				{{ end }}
+                			{{end}}
+                		</div>
+                	</div>
+                </div>
+           </span>
 		{{end}}
 	{{else}}
 		{{ $createdStr:= TimeSinceUnix .Issue.CreatedUnix $.Lang }}
diff --git a/web_src/js/index.js b/web_src/js/index.js
index a7aa0d52c4..a310243c08 100644
--- a/web_src/js/index.js
+++ b/web_src/js/index.js
@@ -731,27 +731,66 @@ function initRepository() {
       $issueTitle.toggle();
       $('.not-in-edit').toggle();
       $('#edit-title-input').toggle();
+      $('#pull-desc').toggle();
+      $('#pull-desc-edit').toggle();
       $('.in-edit').toggle();
       $editInput.focus();
       return false;
     };
+
+    const changeBranchSelect = function () {
+      const selectionTextField = $('#pull-target-branch');
+
+      const baseName = selectionTextField.data('basename');
+      const branchNameNew = $(this).data('branch');
+      const branchNameOld = selectionTextField.data('branch');
+
+      // Replace branch name to keep translation from HTML template
+      selectionTextField.html(selectionTextField.html().replace(
+        `${baseName}:${branchNameOld}`,
+        `${baseName}:${branchNameNew}`
+      ));
+      selectionTextField.data('branch', branchNameNew); // update branch name in setting
+    };
+    $('#branch-select > .item').click(changeBranchSelect);
+
     $('#edit-title').click(editTitleToggle);
     $('#cancel-edit-title').click(editTitleToggle);
     $('#save-edit-title').click(editTitleToggle).click(function () {
+      const pullrequest_targetbranch_change = function (update_url) {
+        const targetBranch = $('#pull-target-branch').data('branch');
+        const $branchTarget = $('#branch_target');
+        if (targetBranch === $branchTarget.text()) {
+          return false;
+        }
+        $.post(update_url, {
+          _csrf: csrf,
+          target_branch: targetBranch
+        })
+          .success((data) => {
+            $branchTarget.text(data.base_branch);
+          })
+          .always(() => {
+            reload();
+          });
+      };
+
+      const pullrequest_target_update_url = $(this).data('target-update-url');
       if ($editInput.val().length === 0 || $editInput.val() === $issueTitle.text()) {
         $editInput.val($issueTitle.text());
-        return false;
+        pullrequest_targetbranch_change(pullrequest_target_update_url);
+      } else {
+        $.post($(this).data('update-url'), {
+          _csrf: csrf,
+          title: $editInput.val()
+        },
+        (data) => {
+          $editInput.val(data.title);
+          $issueTitle.text(data.title);
+          pullrequest_targetbranch_change(pullrequest_target_update_url);
+          reload();
+        });
       }
-
-      $.post($(this).data('update-url'), {
-        _csrf: csrf,
-        title: $editInput.val()
-      },
-      (data) => {
-        $editInput.val(data.title);
-        $issueTitle.text(data.title);
-        reload();
-      });
       return false;
     });