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.

diff --git a/integrations/pull_merge_test.go b/integrations/pull_merge_test.go
index 27f9fc6bb9a56c42d1e58a5414ecc8a15c4e8a99..a63e27e1494d979ba97637253b5ef7d59ce39cd6 100644 (file)
--- a/integrations/pull_merge_test.go
+++ b/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")
+       })
+}

diff --git a/models/error.go b/models/error.go
index 505df288685806fc1db6796dd965b3c55caba2f7..313c36354df3013280265ef1f59e9eaff748112d 100644 (file)
--- a/models/error.go
+++ b/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)
+}
+
 // _________                                       __
 // \_   ___ \  ____   _____   _____   ____   _____/  |_
 // /    \  \/ /  _ \ /     \ /     \_/ __ \ /    \   __\

diff --git a/modules/git/command.go b/modules/git/command.go
index 347dcfe39f884480e7a23e49d66d8a69e6fb1247..2b5288aeab72d0eba7d1ea565754ba46bb645a47 100644 (file)
--- a/modules/git/command.go
+++ b/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

diff --git a/options/locale/locale_en-US.ini b/options/locale/locale_en-US.ini
index 4433c5bb2abc4769594a47d423c6aa18dd4c01a2..43b56f4ec76478c9e2fa78f3ef0fb8e1cefaace7 100644 (file)
--- a/options/locale/locale_en-US.ini
+++ b/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

diff --git a/routers/api/v1/repo/pull.go b/routers/api/v1/repo/pull.go
index 6d86105a15d131c0c03356d4be780fcc2fa82bdf..6af1ba1b04b0c1a3e57ee536cb27fbbf843cd747 100644 (file)
--- a/routers/api/v1/repo/pull.go
+++ b/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

diff --git a/routers/repo/pull.go b/routers/repo/pull.go
index cb9c7c116406e6de2a1309d8ec468fdf147ca5fa..0eea5fcbe6051e574f7ad0d23a1203edb9cfb437 100644 (file)
--- a/routers/repo/pull.go
+++ b/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

diff --git a/services/pull/merge.go b/services/pull/merge.go
index c6607910a25d68871b756aa6db60027d8ba68193..5fbf550ad2cb79ff8b7e49b8368320782f10c9df 100644 (file)
--- a/services/pull/merge.go
+++ b/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)
-               }
-
-               if version.Compare(binVersion, "1.8.0", ">=") {
-                       return func() *git.Command {
-                               return git.NewCommand("config", "--local")
-                       }
+       var gitConfigCommand func() *git.Command
+       if version.Compare(binVersion, "1.8.0", ">=") {
+               gitConfigCommand = 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.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())
        }
-       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())
+       outbuf.Reset()
+       errbuf.Reset()
+
+       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())
        }
-       if err := gitConfigCommand().AddArguments("filter.lfs.clean", "").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-               return fmt.Errorf("git config [filter.lfs.clean -> <> ]: %v", 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())
        }
-       if err := gitConfigCommand().AddArguments("filter.lfs.smudge", "").RunInDirPipeline(tmpBasePath, nil, &errbuf); err != nil {
-               return fmt.Errorf("git config [filter.lfs.smudge -> <> ]: %v", 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, nil, &errbuf); err != nil {
-               return fmt.Errorf("git config [core.sparsecheckout -> true]: %v", errbuf.String())
+       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) {