feat: [CODE-2385]: add fast-forward merge (#2718)

* add fast-forward merge

app/api/controller/pullreq/merge.go

@@ -71,8 +71,10 @@ func (in *MergeInput) sanitize() error {
 	in.Title = strings.TrimSpace(in.Title)
 	in.Message = strings.TrimSpace(in.Message)
 
-	if in.Method == enum.MergeMethodRebase && (in.Title != "" || in.Message != "") {
+	if (in.Method == enum.MergeMethodRebase || in.Method == enum.MergeMethodFastForward) &&
-		return usererror.BadRequest("rebase doesn't support customizing commit title and message")
+		(in.Title != "" || in.Message != "") {
+		return usererror.BadRequestf(
+			"merge method %q doesn't support customizing commit title and message", in.Method)
 	}
 
 	return nil
@@ -338,8 +340,8 @@ func (c *Controller) Merge(
 		author = identityFromPrincipalInfo(*session.Principal.ToPrincipalInfo())
 	case enum.MergeMethodSquash:
 		author = identityFromPrincipalInfo(pr.Author)
-	case enum.MergeMethodRebase:
+	case enum.MergeMethodRebase, enum.MergeMethodFastForward:
-		author = nil // Not important for the rebase merge: the author info in the commits will be preserved.
+		author = nil // Not important for these merge methods: the author info in the commits will be preserved.
 	}
 
 	var committer *git.Identity
@@ -349,6 +351,8 @@ func (c *Controller) Merge(
 		committer = identityFromPrincipalInfo(*bootstrap.NewSystemServiceSession().Principal.ToPrincipalInfo())
 	case enum.MergeMethodRebase:
 		committer = identityFromPrincipalInfo(*session.Principal.ToPrincipalInfo())
+	case enum.MergeMethodFastForward:
+		committer = nil // Not important for fast-forward merge
 	}
 
 	// backfill commit title if none provided
@@ -358,7 +362,7 @@ func (c *Controller) Merge(
 			in.Title = fmt.Sprintf("Merge branch '%s' of %s (#%d)", pr.SourceBranch, sourceRepo.Path, pr.Number)
 		case enum.MergeMethodSquash:
 			in.Title = fmt.Sprintf("%s (#%d)", pr.Title, pr.Number)
-		case enum.MergeMethodRebase:
+		case enum.MergeMethodRebase, enum.MergeMethodFastForward:
 			// Not used.
 		}
 	}

app/services/protection/rule_branch_test.go

@@ -228,6 +228,7 @@ func TestBranch_MergeVerify(t *testing.T) {
 			},
 			expOut: MergeVerifyOutput{
 				AllowedMethods: []enum.MergeMethod{
+					enum.MergeMethodFastForward,
 					enum.MergeMethodMerge,
 					enum.MergeMethodRebase,
 					enum.MergeMethodSquash,

git/enum/merge.go

@@ -24,17 +24,20 @@ const (
 	MergeMethodSquash MergeMethod = "squash"
 	// MergeMethodRebase rebase before merging.
 	MergeMethodRebase MergeMethod = "rebase"
+	// MergeMethodFastForward fast-forward merging.
+	MergeMethodFastForward MergeMethod = "fast-forward"
 )
 
 var MergeMethods = []MergeMethod{
 	MergeMethodMerge,
 	MergeMethodSquash,
 	MergeMethodRebase,
+	MergeMethodFastForward,
 }
 
 func (m MergeMethod) Sanitize() (MergeMethod, bool) {
 	switch m {
-	case MergeMethodMerge, MergeMethodSquash, MergeMethodRebase:
+	case MergeMethodMerge, MergeMethodSquash, MergeMethodRebase, MergeMethodFastForward:
 		return m, true
 	default:
 		return MergeMethodMerge, false

git/merge.go

@@ -144,6 +144,8 @@ func (s *Service) Merge(ctx context.Context, params *MergeParams) (MergeOutput,
 		mergeFunc = merge.Squash
 	case enum.MergeMethodRebase:
 		mergeFunc = merge.Rebase
+	case enum.MergeMethodFastForward:
+		mergeFunc = merge.FastForward
 	default:
 		// should not happen, the call to Sanitize above should handle this case.
 		panic("unsupported merge method")
@@ -295,6 +297,10 @@ func (s *Service) Merge(ctx context.Context, params *MergeParams) (MergeOutput,
 		&author, &committer,
 		mergeMsg,
 		mergeBaseCommitSHA, baseCommitSHA, headCommitSHA)
+	if errors.IsConflict(err) {
+		return MergeOutput{}, fmt.Errorf("failed to merge %q to %q in %q using the %q merge method: %w",
+			params.HeadBranch, params.BaseBranch, params.RepoUID, mergeMethod, err)
+	}
 	if err != nil {
 		return MergeOutput{}, errors.Internal(err, "failed to merge %q to %q in %q using the %q merge method",
 			params.HeadBranch, params.BaseBranch, params.RepoUID, mergeMethod)

git/merge/merge.go

@@ -16,9 +16,9 @@ package merge
 
 import (
 	"context"
-	"errors"
 	"fmt"
 
+	"github.com/harness/gitness/errors"
 	"github.com/harness/gitness/git/api"
 	"github.com/harness/gitness/git/hook"
 	"github.com/harness/gitness/git/sha"
@@ -215,3 +215,27 @@ func Rebase(
 
 	return mergeSHA, conflicts, nil
 }
+
+// FastForward points the  is internal implementation of merge used for Merge and Squash methods.
+func FastForward(
+	ctx context.Context,
+	refUpdater *hook.RefUpdater,
+	repoPath, tmpDir string,
+	_, _ *api.Signature, // commit author and committer aren't used here
+	_ string, // commit message isn't used here
+	mergeBaseSHA, targetSHA, sourceSHA sha.SHA,
+) (mergeSHA sha.SHA, conflicts []string, err error) {
+	if targetSHA != mergeBaseSHA {
+		return sha.None, nil,
+			errors.Conflict("Target branch has diverged from the source branch. Fast-forward not possible.")
+	}
+
+	err = sharedrepo.Run(ctx, refUpdater, tmpDir, repoPath, func(*sharedrepo.SharedRepo) error {
+		return refUpdater.InitNew(ctx, sourceSHA)
+	})
+	if err != nil {
+		return sha.None, nil, fmt.Errorf("merge method=fast-forward: %w", err)
+	}
+
+	return sourceSHA, nil, nil
+}

types/enum/pullreq.go

@@ -219,15 +219,17 @@ type MergeMethod gitenum.MergeMethod
 
 // MergeMethod enumeration.
 const (
-	MergeMethodMerge  = MergeMethod(gitenum.MergeMethodMerge)
+	MergeMethodMerge       = MergeMethod(gitenum.MergeMethodMerge)
-	MergeMethodSquash = MergeMethod(gitenum.MergeMethodSquash)
+	MergeMethodSquash      = MergeMethod(gitenum.MergeMethodSquash)
-	MergeMethodRebase = MergeMethod(gitenum.MergeMethodRebase)
+	MergeMethodRebase      = MergeMethod(gitenum.MergeMethodRebase)
+	MergeMethodFastForward = MergeMethod(gitenum.MergeMethodFastForward)
 )
 
 var MergeMethods = sortEnum([]MergeMethod{
 	MergeMethodMerge,
 	MergeMethodSquash,
 	MergeMethodRebase,
+	MergeMethodFastForward,
 })
 
 func (MergeMethod) Enum() []interface{} { return toInterfaceSlice(MergeMethods) }