Adjust error reporting from merge failures and use LC_ALL=C for git (#8548)

There are two major components to this PR:

* This PR handles merge and rebase failures from merging a little more nicely with Flash errors rather a 500.
* All git commands are run in the LC_ALL="C" environment to ensure that error messages are in English. This DefaultLocale is defined in a way that if necessary (due to platform weirdness) it can be overridden at build time using LDFLAGS="-X "code.gitea.io/gitea/modules/git.DefaultLocale=C"" with C changed for the locale as necessary.

integrations/pull_merge_test.go

@@ -5,15 +5,22 @@
 package integrations
 
 import (
+	"bytes"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"os"
 	"path"
 	"strings"
 	"testing"
+	"time"
 
 	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/git"
+	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/test"
+	"code.gitea.io/gitea/services/pull"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/unknwon/i18n"
@@ -202,3 +209,133 @@ func TestCantMergeWorkInProgress(t *testing.T) {
 		assert.Equal(t, replacer.Replace(expected), text, "Unable to find WIP text")
 	})
 }
+
+func TestCantMergeConflict(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
+		prepareTestEnv(t)
+		session := loginUser(t, "user1")
+		testRepoFork(t, session, "user2", "repo1", "user1", "repo1")
+		testEditFileToNewBranch(t, session, "user1", "repo1", "master", "conflict", "README.md", "Hello, World (Edited Once)\n")
+		testEditFileToNewBranch(t, session, "user1", "repo1", "master", "base", "README.md", "Hello, World (Edited Twice)\n")
+
+		// Use API to create a conflicting pr
+		token := getTokenForLoggedInUser(t, session)
+		req := NewRequestWithJSON(t, http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/%s/pulls?token=%s", "user1", "repo1", token), &api.CreatePullRequestOption{
+			Head:  "conflict",
+			Base:  "base",
+			Title: "create a conflicting pr",
+		})
+		session.MakeRequest(t, req, 201)
+
+		// Now this PR will be marked conflict - or at least a race will do - so drop down to pure code at this point...
+		user1 := models.AssertExistsAndLoadBean(t, &models.User{
+			Name: "user1",
+		}).(*models.User)
+		repo1 := models.AssertExistsAndLoadBean(t, &models.Repository{
+			OwnerID: user1.ID,
+			Name:    "repo1",
+		}).(*models.Repository)
+
+		pr := models.AssertExistsAndLoadBean(t, &models.PullRequest{
+			HeadRepoID: repo1.ID,
+			BaseRepoID: repo1.ID,
+			HeadBranch: "conflict",
+			BaseBranch: "base",
+		}).(*models.PullRequest)
+
+		gitRepo, err := git.OpenRepository(models.RepoPath(user1.Name, repo1.Name))
+		assert.NoError(t, err)
+
+		err = pull.Merge(pr, user1, gitRepo, models.MergeStyleMerge, "CONFLICT")
+		assert.Error(t, err, "Merge should return an error due to conflict")
+		assert.True(t, models.IsErrMergeConflicts(err), "Merge error is not a conflict error")
+
+		err = pull.Merge(pr, user1, gitRepo, models.MergeStyleRebase, "CONFLICT")
+		assert.Error(t, err, "Merge should return an error due to conflict")
+		assert.True(t, models.IsErrRebaseConflicts(err), "Merge error is not a conflict error")
+	})
+}
+
+func TestCantMergeUnrelated(t *testing.T) {
+	onGiteaRun(t, func(t *testing.T, giteaURL *url.URL) {
+		prepareTestEnv(t)
+		session := loginUser(t, "user1")
+		testRepoFork(t, session, "user2", "repo1", "user1", "repo1")
+		testEditFileToNewBranch(t, session, "user1", "repo1", "master", "base", "README.md", "Hello, World (Edited Twice)\n")
+
+		// Now we want to create a commit on a branch that is totally unrelated to our current head
+		// Drop down to pure code at this point
+		user1 := models.AssertExistsAndLoadBean(t, &models.User{
+			Name: "user1",
+		}).(*models.User)
+		repo1 := models.AssertExistsAndLoadBean(t, &models.Repository{
+			OwnerID: user1.ID,
+			Name:    "repo1",
+		}).(*models.Repository)
+		path := models.RepoPath(user1.Name, repo1.Name)
+
+		_, err := git.NewCommand("read-tree", "--empty").RunInDir(path)
+		assert.NoError(t, err)
+
+		stdin := bytes.NewBufferString("Unrelated File")
+		var stdout strings.Builder
+		err = git.NewCommand("hash-object", "-w", "--stdin").RunInDirFullPipeline(path, &stdout, nil, stdin)
+		assert.NoError(t, err)
+		sha := strings.TrimSpace(stdout.String())
+
+		_, err = git.NewCommand("update-index", "--add", "--replace", "--cacheinfo", "100644", sha, "somewher-over-the-rainbow").RunInDir(path)
+		assert.NoError(t, err)
+
+		treeSha, err := git.NewCommand("write-tree").RunInDir(path)
+		assert.NoError(t, err)
+		treeSha = strings.TrimSpace(treeSha)
+
+		commitTimeStr := time.Now().Format(time.RFC3339)
+		doerSig := user1.NewGitSig()
+		env := append(os.Environ(),
+			"GIT_AUTHOR_NAME="+doerSig.Name,
+			"GIT_AUTHOR_EMAIL="+doerSig.Email,
+			"GIT_AUTHOR_DATE="+commitTimeStr,
+			"GIT_COMMITTER_NAME="+doerSig.Name,
+			"GIT_COMMITTER_EMAIL="+doerSig.Email,
+			"GIT_COMMITTER_DATE="+commitTimeStr,
+		)
+
+		messageBytes := new(bytes.Buffer)
+		_, _ = messageBytes.WriteString("Unrelated")
+		_, _ = messageBytes.WriteString("\n")
+
+		stdout.Reset()
+		err = git.NewCommand("commit-tree", treeSha).RunInDirTimeoutEnvFullPipeline(env, -1, path, &stdout, nil, messageBytes)
+		assert.NoError(t, err)
+		commitSha := strings.TrimSpace(stdout.String())
+
+		_, err = git.NewCommand("branch", "unrelated", commitSha).RunInDir(path)
+		assert.NoError(t, err)
+
+		testEditFileToNewBranch(t, session, "user1", "repo1", "master", "conflict", "README.md", "Hello, World (Edited Once)\n")
+
+		// Use API to create a conflicting pr
+		token := getTokenForLoggedInUser(t, session)
+		req := NewRequestWithJSON(t, http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/%s/pulls?token=%s", "user1", "repo1", token), &api.CreatePullRequestOption{
+			Head:  "unrelated",
+			Base:  "base",
+			Title: "create an unrelated pr",
+		})
+		session.MakeRequest(t, req, 201)
+
+		// Now this PR could be marked conflict - or at least a race may occur - so drop down to pure code at this point...
+		gitRepo, err := git.OpenRepository(path)
+		assert.NoError(t, err)
+		pr := models.AssertExistsAndLoadBean(t, &models.PullRequest{
+			HeadRepoID: repo1.ID,
+			BaseRepoID: repo1.ID,
+			HeadBranch: "unrelated",
+			BaseBranch: "base",
+		}).(*models.PullRequest)
+
+		err = pull.Merge(pr, user1, gitRepo, models.MergeStyleMerge, "UNRELATED")
+		assert.Error(t, err, "Merge should return an error due to unrelated")
+		assert.True(t, models.IsErrMergeUnrelatedHistories(err), "Merge error is not a unrelated histories error")
+	})
+}

models/error.go

@@ -1206,6 +1206,79 @@ func (err ErrInvalidMergeStyle) Error() string {
 		err.ID, err.Style)
 }
 
+// ErrMergeConflicts represents an error if merging fails with a conflict
+type ErrMergeConflicts struct {
+	Style  MergeStyle
+	StdOut string
+	StdErr string
+	Err    error
+}
+
+// IsErrMergeConflicts checks if an error is a ErrMergeConflicts.
+func IsErrMergeConflicts(err error) bool {
+	_, ok := err.(ErrMergeConflicts)
+	return ok
+}
+
+func (err ErrMergeConflicts) Error() string {
+	return fmt.Sprintf("Merge Conflict Error: %v: %s\n%s", err.Err, err.StdErr, err.StdOut)
+}
+
+// ErrMergeUnrelatedHistories represents an error if merging fails due to unrelated histories
+type ErrMergeUnrelatedHistories struct {
+	Style  MergeStyle
+	StdOut string
+	StdErr string
+	Err    error
+}
+
+// IsErrMergeUnrelatedHistories checks if an error is a ErrMergeUnrelatedHistories.
+func IsErrMergeUnrelatedHistories(err error) bool {
+	_, ok := err.(ErrMergeUnrelatedHistories)
+	return ok
+}
+
+func (err ErrMergeUnrelatedHistories) Error() string {
+	return fmt.Sprintf("Merge UnrelatedHistories Error: %v: %s\n%s", err.Err, err.StdErr, err.StdOut)
+}
+
+// ErrMergePushOutOfDate represents an error if merging fails due to unrelated histories
+type ErrMergePushOutOfDate struct {
+	Style  MergeStyle
+	StdOut string
+	StdErr string
+	Err    error
+}
+
+// IsErrMergePushOutOfDate checks if an error is a ErrMergePushOutOfDate.
+func IsErrMergePushOutOfDate(err error) bool {
+	_, ok := err.(ErrMergePushOutOfDate)
+	return ok
+}
+
+func (err ErrMergePushOutOfDate) Error() string {
+	return fmt.Sprintf("Merge PushOutOfDate Error: %v: %s\n%s", err.Err, err.StdErr, err.StdOut)
+}
+
+// ErrRebaseConflicts represents an error if rebase fails with a conflict
+type ErrRebaseConflicts struct {
+	Style     MergeStyle
+	CommitSHA string
+	StdOut    string
+	StdErr    string
+	Err       error
+}
+
+// IsErrRebaseConflicts checks if an error is a ErrRebaseConflicts.
+func IsErrRebaseConflicts(err error) bool {
+	_, ok := err.(ErrRebaseConflicts)
+	return ok
+}
+
+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)
+}
+
 // _________                                       __
 // \_   ___ \  ____   _____   _____   ____   _____/  |_
 // /    \  \/ /  _ \ /     \ /     \_/ __ \ /    \   __\

modules/git/command.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"os"
 	"os/exec"
 	"strings"
 	"time"
@@ -24,6 +25,9 @@ var (
 	DefaultCommandExecutionTimeout = 60 * time.Second
 )
 
+// DefaultLocale is the default LC_ALL to run git commands in.
+const DefaultLocale = "C"
+
 // Command represents a command with its subcommands or arguments.
 type Command struct {
 	name string
@@ -77,7 +81,12 @@ func (c *Command) RunInDirTimeoutEnvFullPipeline(env []string, timeout time.Dura
 	defer cancel()
 
 	cmd := exec.CommandContext(ctx, c.name, c.args...)
-	cmd.Env = env
+	if env == nil {
+		cmd.Env = append(os.Environ(), fmt.Sprintf("LC_ALL=%s", DefaultLocale))
+	} else {
+		cmd.Env = env
+		cmd.Env = append(cmd.Env, fmt.Sprintf("LC_ALL=%s", DefaultLocale))
+	}
 	cmd.Dir = dir
 	cmd.Stdout = stdout
 	cmd.Stderr = stderr

options/locale/locale_en-US.ini

@@ -1026,6 +1026,10 @@ pulls.rebase_merge_pull_request = Rebase and Merge
 pulls.rebase_merge_commit_pull_request = Rebase and Merge (--no-ff)
 pulls.squash_merge_pull_request = Squash and Merge
 pulls.invalid_merge_option = You cannot use this merge option for this pull request.
+pulls.merge_conflict = Merge Failed: There was a conflict whilst merging: %[1]s<br>%[2]s<br>Hint: Try a different strategy
+pulls.rebase_conflict = Merge Failed: There was a conflict whilst rebasing commit: %s[1]<br>%[1]s<br>%[2]s<br>Hint:Try a different strategy
+pulls.unrelated_histories = Merge Failed: The merge head and base do not share a common history. Hint: Try a different strategy
+pulls.merge_out_of_date = Merge Failed: Whilst generating the merge, the base was updated. Hint: Try again.
 pulls.open_unmerged_pull_exists = `You cannot perform a reopen operation because there is a pending pull request (#%d) with identical properties.`
 pulls.status_checking = Some checks are pending
 pulls.status_checks_success = All checks were successful

routers/api/v1/repo/pull.go

@@ -626,6 +626,18 @@ func MergePullRequest(ctx *context.APIContext, form auth.MergePullRequestForm) {
 		if models.IsErrInvalidMergeStyle(err) {
 			ctx.Status(405)
 			return
+		} else if models.IsErrMergeConflicts(err) {
+			conflictError := err.(models.ErrMergeConflicts)
+			ctx.JSON(http.StatusConflict, conflictError)
+		} else if models.IsErrRebaseConflicts(err) {
+			conflictError := err.(models.ErrRebaseConflicts)
+			ctx.JSON(http.StatusConflict, conflictError)
+		} else if models.IsErrMergeUnrelatedHistories(err) {
+			conflictError := err.(models.ErrMergeUnrelatedHistories)
+			ctx.JSON(http.StatusConflict, conflictError)
+		} else if models.IsErrMergePushOutOfDate(err) {
+			ctx.Status(http.StatusConflict)
+			return
 		}
 		ctx.Error(500, "Merge", err)
 		return

routers/repo/pull.go

@@ -10,6 +10,7 @@ import (
 	"container/list"
 	"crypto/subtle"
 	"fmt"
+	"html"
 	"io"
 	"path"
 	"strings"
@@ -660,10 +661,39 @@ func MergePullRequest(ctx *context.Context, form auth.MergePullRequestForm) {
 	}
 
 	if err = pull_service.Merge(pr, ctx.User, ctx.Repo.GitRepo, models.MergeStyle(form.Do), message); err != nil {
+		sanitize := func(x string) string {
+			runes := []rune(x)
+
+			if len(runes) > 512 {
+				x = "..." + string(runes[len(runes)-512:])
+			}
+
+			return strings.Replace(html.EscapeString(x), "\n", "<br>", -1)
+		}
 		if models.IsErrInvalidMergeStyle(err) {
 			ctx.Flash.Error(ctx.Tr("repo.pulls.invalid_merge_option"))
 			ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(pr.Index))
 			return
+		} else if models.IsErrMergeConflicts(err) {
+			conflictError := err.(models.ErrMergeConflicts)
+			ctx.Flash.Error(ctx.Tr("repo.pulls.merge_conflict", sanitize(conflictError.StdErr), sanitize(conflictError.StdOut)))
+			ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(pr.Index))
+			return
+		} else if models.IsErrRebaseConflicts(err) {
+			conflictError := err.(models.ErrRebaseConflicts)
+			ctx.Flash.Error(ctx.Tr("repo.pulls.rebase_conflict", sanitize(conflictError.CommitSHA), sanitize(conflictError.StdErr), sanitize(conflictError.StdOut)))
+			ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(pr.Index))
+			return
+		} else if models.IsErrMergeUnrelatedHistories(err) {
+			log.Debug("MergeUnrelatedHistories error: %v", err)
+			ctx.Flash.Error(ctx.Tr("repo.pulls.unrelated_histories"))
+			ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(pr.Index))
+			return
+		} else if models.IsErrMergePushOutOfDate(err) {
+			log.Debug("MergePushOutOfDate error: %v", err)
+			ctx.Flash.Error(ctx.Tr("repo.pulls.merge_out_of_date"))
+			ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(pr.Index))
+			return
 		}
 		ctx.ServerError("Merge", err)
 		return

services/pull/merge.go

@@ -32,22 +32,27 @@ import (
 func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repository, mergeStyle models.MergeStyle, message string) (err error) {
 	binVersion, err := git.BinVersion()
 	if err != nil {
+		log.Error("git.BinVersion: %v", err)
 		return fmt.Errorf("Unable to get git version: %v", err)
 	}
 
 	if err = pr.GetHeadRepo(); err != nil {
+		log.Error("GetHeadRepo: %v", err)
 		return fmt.Errorf("GetHeadRepo: %v", err)
 	} else if err = pr.GetBaseRepo(); err != nil {
+		log.Error("GetBaseRepo: %v", err)
 		return fmt.Errorf("GetBaseRepo: %v", err)
 	}
 
 	prUnit, err := pr.BaseRepo.GetUnit(models.UnitTypePullRequests)
 	if err != nil {
+		log.Error("pr.BaseRepo.GetUnit(models.UnitTypePullRequests): %v", err)
 		return err
 	}
 	prConfig := prUnit.PullRequestsConfig()
 
 	if err := pr.CheckUserAllowedToMerge(doer); err != nil {
+		log.Error("CheckUserAllowedToMerge(%v): %v", doer, err)
 		return fmt.Errorf("CheckUserAllowedToMerge: %v", err)
 	}
 
@@ -63,6 +68,7 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 	// Clone base repo.
 	tmpBasePath, err := models.CreateTemporaryPath("merge")
 	if err != nil {
+		log.Error("CreateTemporaryPath: %v", err)
 		return err
 	}
 
@@ -75,7 +81,8 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 	headRepoPath := pr.HeadRepo.RepoPath()
 
 	if err := git.InitRepository(tmpBasePath, false); err != nil {
-		return fmt.Errorf("git init: %v", err)
+		log.Error("git init tmpBasePath: %v", err)
+		return err
 	}
 
 	remoteRepoName := "head_repo"
@@ -86,102 +93,141 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 		p := filepath.Join(staging, ".git", "objects", "info", "alternates")
 		f, err := os.OpenFile(p, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0600)
 		if err != nil {
+			log.Error("Could not create .git/objects/info/alternates file in %s: %v", staging, err)
 			return err
 		}
 		defer f.Close()
 		data := filepath.Join(cache, "objects")
 		if _, err := fmt.Fprintln(f, data); err != nil {
+			log.Error("Could not write to .git/objects/info/alternates file in %s: %v", staging, err)
 			return err
 		}
 		return nil
 	}
 
 	if err := addCacheRepo(tmpBasePath, baseGitRepo.Path); err != nil {
-		return fmt.Errorf("addCacheRepo [%s -> %s]: %v", headRepoPath, tmpBasePath, err)
+		log.Error("Unable to add base repository to temporary repo [%s -> %s]: %v", pr.BaseRepo.FullName(), tmpBasePath, err)
+		return fmt.Errorf("Unable to add base repository to temporary repo [%s -> tmpBasePath]: %v", pr.BaseRepo.FullName(), err)
 	}
 
-	var errbuf strings.Builder
-	if err := git.NewCommand("remote", "add", "-t", pr.BaseBranch, "-m", pr.BaseBranch, "origin", baseGitRepo.Path).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git remote add [%s -> %s]: %s", baseGitRepo.Path, tmpBasePath, errbuf.String())
+	var outbuf, errbuf strings.Builder
+	if err := git.NewCommand("remote", "add", "-t", pr.BaseBranch, "-m", pr.BaseBranch, "origin", baseGitRepo.Path).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("Unable to add base repository as origin [%s -> %s]: %v\n%s\n%s", pr.BaseRepo.FullName(), tmpBasePath, err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("Unable to add base repository as origin [%s -> tmpBasePath]: %v\n%s\n%s", pr.BaseRepo.FullName(), err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
-	if err := git.NewCommand("fetch", "origin", "--no-tags", pr.BaseBranch+":"+baseBranch, pr.BaseBranch+":original_"+baseBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git fetch [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
+	if err := git.NewCommand("fetch", "origin", "--no-tags", pr.BaseBranch+":"+baseBranch, pr.BaseBranch+":original_"+baseBranch).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("Unable to fetch origin base branch [%s:%s -> base, original_base in %s]: %v:\n%s\n%s", pr.BaseRepo.FullName(), pr.BaseBranch, tmpBasePath, err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("Unable to fetch origin base branch [%s:%s -> base, original_base in tmpBasePath]: %v\n%s\n%s", pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
-	if err := git.NewCommand("symbolic-ref", "HEAD", git.BranchPrefix+baseBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git symbolic-ref HEAD base [%s]: %s", tmpBasePath, errbuf.String())
+	if err := git.NewCommand("symbolic-ref", "HEAD", git.BranchPrefix+baseBranch).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("Unable to set HEAD as base branch [%s]: %v\n%s\n%s", tmpBasePath, err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("Unable to set HEAD as base branch [tmpBasePath]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
 	if err := addCacheRepo(tmpBasePath, headRepoPath); err != nil {
-		return fmt.Errorf("addCacheRepo [%s -> %s]: %v", headRepoPath, tmpBasePath, err)
+		log.Error("Unable to add head repository to temporary repo [%s -> %s]: %v", pr.HeadRepo.FullName(), tmpBasePath, err)
+		return fmt.Errorf("Unable to head base repository to temporary repo [%s -> tmpBasePath]: %v", pr.HeadRepo.FullName(), err)
 	}
 
-	if err := git.NewCommand("remote", "add", remoteRepoName, headRepoPath).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git remote add [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
+	if err := git.NewCommand("remote", "add", remoteRepoName, headRepoPath).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("Unable to add head repository as head_repo [%s -> %s]: %v\n%s\n%s", pr.HeadRepo.FullName(), tmpBasePath, err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("Unable to add head repository as head_repo [%s -> tmpBasePath]: %v\n%s\n%s", pr.HeadRepo.FullName(), err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
 	trackingBranch := "tracking"
 	// Fetch head branch
-	if err := git.NewCommand("fetch", "--no-tags", remoteRepoName, pr.HeadBranch+":"+trackingBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git fetch [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
+	if err := git.NewCommand("fetch", "--no-tags", remoteRepoName, pr.HeadBranch+":"+trackingBranch).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("Unable to fetch head_repo head branch [%s:%s -> tracking in %s]: %v:\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, tmpBasePath, err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("Unable to fetch head_repo head branch [%s:%s -> tracking in tmpBasePath]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
 	stagingBranch := "staging"
 
 	// Enable sparse-checkout
 	sparseCheckoutList, err := getDiffTree(tmpBasePath, baseBranch, trackingBranch)
 	if err != nil {
+		log.Error("getDiffTree(%s, %s, %s): %v", tmpBasePath, baseBranch, trackingBranch, err)
 		return fmt.Errorf("getDiffTree: %v", err)
 	}
 
 	infoPath := filepath.Join(tmpBasePath, ".git", "info")
 	if err := os.MkdirAll(infoPath, 0700); err != nil {
-		return fmt.Errorf("creating directory failed [%s]: %v", infoPath, err)
+		log.Error("Unable to create .git/info in %s: %v", tmpBasePath, err)
+		return fmt.Errorf("Unable to create .git/info in tmpBasePath: %v", err)
 	}
+
 	sparseCheckoutListPath := filepath.Join(infoPath, "sparse-checkout")
 	if err := ioutil.WriteFile(sparseCheckoutListPath, []byte(sparseCheckoutList), 0600); err != nil {
-		return fmt.Errorf("Writing sparse-checkout file to %s: %v", sparseCheckoutListPath, err)
+		log.Error("Unable to write .git/info/sparse-checkout file in %s: %v", tmpBasePath, err)
+		return fmt.Errorf("Unable to write .git/info/sparse-checkout file in tmpBasePath: %v", err)
 	}
 
-	gitConfigCommand := func() func() *git.Command {
-		binVersion, err := git.BinVersion()
-		if err != nil {
-			log.Fatal("Error retrieving git version: %v", err)
+	var gitConfigCommand func() *git.Command
+	if version.Compare(binVersion, "1.8.0", ">=") {
+		gitConfigCommand = func() *git.Command {
+			return git.NewCommand("config", "--local")
 		}
-
-		if version.Compare(binVersion, "1.8.0", ">=") {
-			return func() *git.Command {
-				return git.NewCommand("config", "--local")
-			}
-		}
-		return func() *git.Command {
+	} else {
+		gitConfigCommand = func() *git.Command {
 			return git.NewCommand("config")
 		}
-	}()
+	}
 
 	// Switch off LFS process (set required, clean and smudge here also)
-	if err := gitConfigCommand().AddArguments("filter.lfs.process", "").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git config [filter.lfs.process -> <> ]: %v", errbuf.String())
-	}
-	if err := gitConfigCommand().AddArguments("filter.lfs.required", "false").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git config [filter.lfs.required -> <false> ]: %v", errbuf.String())
-	}
-	if err := gitConfigCommand().AddArguments("filter.lfs.clean", "").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git config [filter.lfs.clean -> <> ]: %v", errbuf.String())
-	}
-	if err := gitConfigCommand().AddArguments("filter.lfs.smudge", "").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git config [filter.lfs.smudge -> <> ]: %v", errbuf.String())
+	if err := gitConfigCommand().AddArguments("filter.lfs.process", "").RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("git config [filter.lfs.process -> <> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("git config [filter.lfs.process -> <> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
-	if err := gitConfigCommand().AddArguments("core.sparseCheckout", "true").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git config [core.sparsecheckout -> true]: %v", errbuf.String())
+	if err := gitConfigCommand().AddArguments("filter.lfs.required", "false").RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("git config [filter.lfs.required -> <false> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("git config [filter.lfs.required -> <false> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
+
+	if err := gitConfigCommand().AddArguments("filter.lfs.clean", "").RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("git config [filter.lfs.clean -> <> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("git config [filter.lfs.clean -> <> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+	}
+	outbuf.Reset()
+	errbuf.Reset()
+
+	if err := gitConfigCommand().AddArguments("filter.lfs.smudge", "").RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("git config [filter.lfs.smudge -> <> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("git config [filter.lfs.smudge -> <> ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+	}
+	outbuf.Reset()
+	errbuf.Reset()
+
+	if err := gitConfigCommand().AddArguments("core.sparseCheckout", "true").RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("git config [core.sparseCheckout -> true ]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("git config [core.sparsecheckout -> true]: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+	}
+	outbuf.Reset()
+	errbuf.Reset()
 
 	// Read base branch index
-	if err := git.NewCommand("read-tree", "HEAD").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-		return fmt.Errorf("git read-tree HEAD: %s", errbuf.String())
+	if err := git.NewCommand("read-tree", "HEAD").RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		log.Error("git read-tree HEAD: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("Unable to read base branch in to the index: %v\n%s\n%s", err, outbuf.String(), errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
 	// Determine if we should sign
 	signArg := ""
@@ -210,80 +256,102 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 	// Merge commits.
 	switch mergeStyle {
 	case models.MergeStyleMerge:
-		if err := git.NewCommand("merge", "--no-ff", "--no-commit", trackingBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git merge --no-ff --no-commit [%s]: %v - %s", tmpBasePath, err, errbuf.String())
+		cmd := git.NewCommand("merge", "--no-ff", "--no-commit", trackingBranch)
+		if err := runMergeCommand(pr, mergeStyle, cmd, tmpBasePath); err != nil {
+			log.Error("Unable to merge tracking into base: %v", err)
+			return err
 		}
 
-		if signArg == "" {
-			if err := git.NewCommand("commit", "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, nil, &errbuf); err != nil {
-				return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, errbuf.String())
-			}
-		} else {
-			if err := git.NewCommand("commit", signArg, "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, nil, &errbuf); err != nil {
-				return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, errbuf.String())
-			}
+		if err := commitAndSignNoAuthor(pr, message, signArg, tmpBasePath, env); err != nil {
+			log.Error("Unable to make final commit: %v", err)
+			return err
 		}
 	case models.MergeStyleRebase:
-		// Checkout head branch
-		if err := git.NewCommand("checkout", "-b", stagingBranch, trackingBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git checkout: %s", errbuf.String())
-		}
-		// Rebase before merging
-		if err := git.NewCommand("rebase", "-q", baseBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git rebase [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
-		}
-		// Checkout base branch again
-		if err := git.NewCommand("checkout", baseBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git checkout: %s", errbuf.String())
-		}
-		// Merge fast forward
-		if err := git.NewCommand("merge", "--ff-only", "-q", stagingBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git merge --ff-only [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
-		}
+		fallthrough
 	case models.MergeStyleRebaseMerge:
 		// Checkout head branch
-		if err := git.NewCommand("checkout", "-b", stagingBranch, trackingBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git checkout: %s", errbuf.String())
+		if err := git.NewCommand("checkout", "-b", stagingBranch, trackingBranch).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+			log.Error("git checkout base prior to merge post staging rebase [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+			return fmt.Errorf("git checkout base prior to merge post staging rebase  [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
 		}
+		outbuf.Reset()
+		errbuf.Reset()
+
 		// Rebase before merging
-		if err := git.NewCommand("rebase", "-q", baseBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git rebase [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
+		if err := git.NewCommand("rebase", baseBranch).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+			// Rebase will leave a REBASE_HEAD file in .git if there is a conflict
+			if _, statErr := os.Stat(filepath.Join(tmpBasePath, ".git", "REBASE_HEAD")); statErr == nil {
+				// The original commit SHA1 that is failing will be in .git/rebase-apply/original-commit
+				commitShaBytes, readErr := ioutil.ReadFile(filepath.Join(tmpBasePath, ".git", "rebase-apply", "original-commit"))
+				if readErr != nil {
+					// Abandon this attempt to handle the error
+					log.Error("git rebase staging on to base [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+					return fmt.Errorf("git rebase staging on to base [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+				}
+				log.Debug("RebaseConflict at %s [%s:%s -> %s:%s]: %v\n%s\n%s", strings.TrimSpace(string(commitShaBytes)), pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+				return models.ErrRebaseConflicts{
+					Style:     mergeStyle,
+					CommitSHA: strings.TrimSpace(string(commitShaBytes)),
+					StdOut:    outbuf.String(),
+					StdErr:    errbuf.String(),
+					Err:       err,
+				}
+			}
+			log.Error("git rebase staging on to base [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+			return fmt.Errorf("git rebase staging on to base [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
 		}
+		outbuf.Reset()
+		errbuf.Reset()
+
 		// Checkout base branch again
-		if err := git.NewCommand("checkout", baseBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git checkout: %s", errbuf.String())
-		}
-		// Prepare merge with commit
-		if err := git.NewCommand("merge", "--no-ff", "--no-commit", "-q", stagingBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git merge --no-ff [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
+		if err := git.NewCommand("checkout", baseBranch).RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+			log.Error("git checkout base prior to merge post staging rebase [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+			return fmt.Errorf("git checkout base prior to merge post staging rebase  [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
 		}
+		outbuf.Reset()
+		errbuf.Reset()
 
-		// Set custom message and author and create merge commit
-		if signArg == "" {
-			if err := git.NewCommand("commit", "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, nil, &errbuf); err != nil {
-				return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, errbuf.String())
-			}
+		cmd := git.NewCommand("merge")
+		if mergeStyle == models.MergeStyleRebase {
+			cmd.AddArguments("--ff-only")
 		} else {
-			if err := git.NewCommand("commit", signArg, "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, nil, &errbuf); err != nil {
-				return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, errbuf.String())
+			cmd.AddArguments("--no-ff", "--no-commit")
+		}
+		cmd.AddArguments(stagingBranch)
+
+		// Prepare merge with commit
+		if err := runMergeCommand(pr, mergeStyle, cmd, tmpBasePath); err != nil {
+			log.Error("Unable to merge staging into base: %v", err)
+			return err
+		}
+		if mergeStyle == models.MergeStyleRebaseMerge {
+			if err := commitAndSignNoAuthor(pr, message, signArg, tmpBasePath, env); err != nil {
+				log.Error("Unable to make final commit: %v", err)
+				return err
 			}
 		}
-
 	case models.MergeStyleSquash:
 		// Merge with squash
-		if err := git.NewCommand("merge", "-q", "--squash", trackingBranch).RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-			return fmt.Errorf("git merge --squash [%s -> %s]: %s", headRepoPath, tmpBasePath, errbuf.String())
+		cmd := git.NewCommand("merge", "--squash", trackingBranch)
+		if err := runMergeCommand(pr, mergeStyle, cmd, tmpBasePath); err != nil {
+			log.Error("Unable to merge --squash tracking into base: %v", err)
+			return err
 		}
+
 		sig := pr.Issue.Poster.NewGitSig()
 		if signArg == "" {
-			if err := git.NewCommand("commit", fmt.Sprintf("--author='%s <%s>'", sig.Name, sig.Email), "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, nil, &errbuf); err != nil {
-				return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, errbuf.String())
+			if err := git.NewCommand("commit", fmt.Sprintf("--author='%s <%s>'", sig.Name, sig.Email), "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
+				log.Error("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+				return fmt.Errorf("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
 			}
 		} else {
-			if err := git.NewCommand("commit", signArg, fmt.Sprintf("--author='%s <%s>'", sig.Name, sig.Email), "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, nil, &errbuf); err != nil {
-				return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, errbuf.String())
+			if err := git.NewCommand("commit", signArg, fmt.Sprintf("--author='%s <%s>'", sig.Name, sig.Email), "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
+				log.Error("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+				return fmt.Errorf("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
 			}
 		}
+		outbuf.Reset()
+		errbuf.Reset()
 	default:
 		return models.ErrInvalidMergeStyle{ID: pr.BaseRepo.ID, Style: mergeStyle}
 	}
@@ -329,9 +397,19 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 	)
 
 	// Push back to upstream.
-	if err := git.NewCommand("push", "origin", baseBranch+":"+pr.BaseBranch).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, nil, &errbuf); err != nil {
+	if err := git.NewCommand("push", "origin", baseBranch+":"+pr.BaseBranch).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
+		if strings.Contains(errbuf.String(), "non-fast-forward") {
+			return models.ErrMergePushOutOfDate{
+				Style:  mergeStyle,
+				StdOut: outbuf.String(),
+				StdErr: errbuf.String(),
+				Err:    err,
+			}
+		}
 		return fmt.Errorf("git push: %s", errbuf.String())
 	}
+	outbuf.Reset()
+	errbuf.Reset()
 
 	pr.MergedCommitID, err = baseGitRepo.GetBranchCommitID(pr.BaseBranch)
 	if err != nil {
@@ -364,6 +442,51 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 	return nil
 }
 
+func commitAndSignNoAuthor(pr *models.PullRequest, message, signArg, tmpBasePath string, env []string) error {
+	var outbuf, errbuf strings.Builder
+	if signArg == "" {
+		if err := git.NewCommand("commit", "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
+			log.Error("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+			return fmt.Errorf("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+		}
+	} else {
+		if err := git.NewCommand("commit", signArg, "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {
+			log.Error("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+			return fmt.Errorf("git commit [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+		}
+	}
+	return nil
+}
+
+func runMergeCommand(pr *models.PullRequest, mergeStyle models.MergeStyle, cmd *git.Command, tmpBasePath string) error {
+	var outbuf, errbuf strings.Builder
+	if err := cmd.RunInDirPipeline(tmpBasePath, &outbuf, &errbuf); err != nil {
+		// Merge will leave a MERGE_HEAD file in the .git folder if there is a conflict
+		if _, statErr := os.Stat(filepath.Join(tmpBasePath, ".git", "MERGE_HEAD")); statErr == nil {
+			// We have a merge conflict error
+			log.Debug("MergeConflict [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+			return models.ErrMergeConflicts{
+				Style:  mergeStyle,
+				StdOut: outbuf.String(),
+				StdErr: errbuf.String(),
+				Err:    err,
+			}
+		} else if strings.Contains(errbuf.String(), "refusing to merge unrelated histories") {
+			log.Debug("MergeUnrelatedHistories [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+			return models.ErrMergeUnrelatedHistories{
+				Style:  mergeStyle,
+				StdOut: outbuf.String(),
+				StdErr: errbuf.String(),
+				Err:    err,
+			}
+		}
+		log.Error("git merge [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+		return fmt.Errorf("git merge [%s:%s -> %s:%s]: %v\n%s\n%s", pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseRepo.FullName(), pr.BaseBranch, err, outbuf.String(), errbuf.String())
+	}
+
+	return nil
+}
+
 var escapedSymbols = regexp.MustCompile(`([*[?! \\])`)
 
 func getDiffTree(repoPath, baseBranch, headBranch string) (string, error) {