Support non-force fetches

1 files changed, 26 insertions, 2 deletions

diff --git a/remote.go b/remote.go
index 2416152..91ae2fa 100644
--- a/remote.go
+++ b/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")
 )
 
 const (
@@ -302,7 +303,7 @@ 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
 	}
@@ -773,8 +774,11 @@ 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
@@ -789,7 +793,23 @@ 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 {
@@ -819,6 +839,10 @@ func (r *Remote) updateLocalReferenceStorage(
 		updated = true
 	}
 
+	if err == nil && forceNeeded {
+		err = ErrForceNeeded
+	}
+
 	return
 }