remote: add the last 100 commits for each ref in haves list

If the local ref is not an ancestor of the remote ref being fetched,
then when we send an UploadPack request with that local ref as one of
the Haves, the remote will not recognize it, and will think we are
asking for the entire history of the repo, even if there's a common
ancestor.

To do this right, we need to support the multi-ack protocol so we can
negotiate a common commit.  That's hard though; this is a quick fix
just to include the previous 100 commits for each local ref in the
Haves list, and hope that one of them is the common commit.

1 files changed, 92 insertions, 3 deletions

diff --git a/remote.go b/remote.go
index 7cf4b65..3718813 100644
--- a/remote.go
+++ b/remote.go
@@ -27,6 +27,14 @@ var (
 	ErrDeleteRefNotSupported = errors.New("server does not support delete-refs")
 )
 
+const (
+	// This describes the maximum number of commits to walk when
+	// computing the haves to send to a server, for each ref in the
+	// repo containing this remote, when not using the multi-ack
+	// protocol.  Setting this to 0 means there is no limit.
+	maxHavesToVisitPerRef = 100
+)
+
 // Remote represents a connection to a remote repository.
 type Remote struct {
 	c *config.RemoteConfig
@@ -284,7 +292,7 @@ func (r *Remote) fetch(ctx context.Context, o *FetchOptions) (storer.ReferenceSt
 
 	req.Wants, err = getWants(r.s, refs)
 	if len(req.Wants) > 0 {
-		req.Haves, err = getHaves(localRefs)
+		req.Haves, err = getHaves(localRefs, remoteRefs, r.s)
 		if err != nil {
 			return nil, err
 		}
@@ -490,8 +498,86 @@ func (r *Remote) references() ([]*plumbing.Reference, error) {
 	return localRefs, nil
 }
 
-func getHaves(localRefs []*plumbing.Reference) ([]plumbing.Hash, error) {
+func getRemoteRefsFromStorer(remoteRefStorer storer.ReferenceStorer) (
+	map[plumbing.Hash]bool, error) {
+	remoteRefs := map[plumbing.Hash]bool{}
+	iter, err := remoteRefStorer.IterReferences()
+	if err != nil {
+		return nil, err
+	}
+	err = iter.ForEach(func(ref *plumbing.Reference) error {
+		if ref.Type() != plumbing.HashReference {
+			return nil
+		}
+		remoteRefs[ref.Hash()] = true
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return remoteRefs, nil
+}
+
+// getHavesFromRef populates the given `haves` map with the given
+// reference, and up to `maxHavesToVisitPerRef` ancestor commits.
+func getHavesFromRef(
+	ref *plumbing.Reference,
+	remoteRefs map[plumbing.Hash]bool,
+	s storage.Storer,
+	haves map[plumbing.Hash]bool,
+) error {
+	h := ref.Hash()
+	if haves[h] {
+		return nil
+	}
+
+	// No need to load the commit if we know the remote already
+	// has this hash.
+	if remoteRefs[h] {
+		haves[h] = true
+		return nil
+	}
+
+	commit, err := object.GetCommit(s, h)
+	if err != nil {
+		// Ignore the error if this isn't a commit.
+		haves[ref.Hash()] = true
+		return nil
+	}
+
+	// Until go-git supports proper commit negotiation during an
+	// upload pack request, include up to `maxHavesToVisitPerRef`
+	// commits from the history of each ref.
+	walker := object.NewCommitPreorderIter(commit, haves, nil)
+	toVisit := maxHavesToVisitPerRef
+	return walker.ForEach(func(c *object.Commit) error {
+		haves[c.Hash] = true
+		toVisit--
+		// If toVisit starts out at 0 (indicating there is no
+		// max), then it will be negative here and we won't stop
+		// early.
+		if toVisit == 0 || remoteRefs[c.Hash] {
+			return storer.ErrStop
+		}
+		return nil
+	})
+}
+
+func getHaves(
+	localRefs []*plumbing.Reference,
+	remoteRefStorer storer.ReferenceStorer,
+	s storage.Storer,
+) ([]plumbing.Hash, error) {
 	haves := map[plumbing.Hash]bool{}
+
+	// Build a map of all the remote references, to avoid loading too
+	// many parent commits for references we know don't need to be
+	// transferred.
+	remoteRefs, err := getRemoteRefsFromStorer(remoteRefStorer)
+	if err != nil {
+		return nil, err
+	}
+
 	for _, ref := range localRefs {
 		if haves[ref.Hash()] == true {
 			continue
@@ -501,7 +587,10 @@ func getHaves(localRefs []*plumbing.Reference) ([]plumbing.Hash, error) {
 			continue
 		}
 
-		haves[ref.Hash()] = true
+		err = getHavesFromRef(ref, remoteRefs, s, haves)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	var result []plumbing.Hash