Adapted non-force fetching with locking

options.go

@@ -95,6 +95,9 @@ type PullOptions struct {
 	// stored, if nil nothing is stored and the capability (if supported)
 	// no-progress, is sent to the server to avoid send this information.
 	Progress sideband.Progress
+	// Force allows the pull to update a local branch even when the remote
+	// branch does not descend from it.
+	Force bool
 }
 
 // Validate validates the fields and sets the default values.
@@ -142,6 +145,9 @@ type FetchOptions struct {
 	// Tags describe how the tags will be fetched from the remote repository,
 	// by default is TagFollowing.
 	Tags TagMode
+	// Force allows the fetch to update a local branch even when the remote
+	// branch does not descend from it.
+	Force bool
 }
 
 // Validate validates the fields and sets the default values.

remote.go

@@ -25,6 +25,7 @@ import (
 var (
 	NoErrAlreadyUpToDate     = errors.New("already up-to-date")
 	ErrDeleteRefNotSupported = errors.New("server does not support delete-refs")
+	ErrForceNeeded           = errors.New("some refs were not updated")
 )
 
 // Remote represents a connection to a remote repository.
@@ -294,7 +295,7 @@ func (r *Remote) fetch(ctx context.Context, o *FetchOptions) (storer.ReferenceSt
 		}
 	}
 
-	updated, err := r.updateLocalReferenceStorage(o.RefSpecs, refs, remoteRefs, o.Tags)
+	updated, err := r.updateLocalReferenceStorage(o.RefSpecs, refs, remoteRefs, o.Tags, o.Force)
 	if err != nil {
 		return nil, err
 	}
@@ -682,8 +683,11 @@ func (r *Remote) updateLocalReferenceStorage(
 	specs []config.RefSpec,
 	fetchedRefs, remoteRefs memory.ReferenceStorage,
 	tagMode TagMode,
+	force bool,
 ) (updated bool, err error) {
 	isWildcard := true
+	forceNeeded := false
+
 	for _, spec := range specs {
 		if !spec.IsWildcard() {
 			isWildcard = false
@@ -698,7 +702,23 @@ func (r *Remote) updateLocalReferenceStorage(
 				continue
 			}
 
-			new := plumbing.NewHashReference(spec.Dst(ref.Name()), ref.Hash())
+			localName := spec.Dst(ref.Name())
+			old, _ := storer.ResolveReference(r.s, localName)
+			new := plumbing.NewHashReference(localName, ref.Hash())
+
+			// If the ref exists locally as a branch and force is not specified,
+			// only update if the new ref is an ancestor of the old
+			if old != nil && old.Name().IsBranch() && !force {
+				ff, err := isFastForward(r.s, old.Hash(), new.Hash())
+				if err != nil {
+					return updated, err
+				}
+
+				if !ff {
+					forceNeeded = true
+					continue
+				}
+			}
 
 			refUpdated, err := updateReferenceStorerIfNeeded(r.s, new)
 			if err != nil {
@@ -728,6 +748,10 @@ func (r *Remote) updateLocalReferenceStorage(
 		updated = true
 	}
 
+	if err == nil && forceNeeded {
+		err = ErrForceNeeded
+	}
+
 	return
 }
 

worktree.go

@@ -69,6 +69,7 @@ func (w *Worktree) PullContext(ctx context.Context, o *PullOptions) error {
 		Depth:      o.Depth,
 		Auth:       o.Auth,
 		Progress:   o.Progress,
+		Force:      o.Force,
 	})
 
 	updated := true