Allow skipping an entry expansion in tree.Walk()

It is now possible to skip expanding an entry in `tree.Walk()` by
returning `TreeWalkSkip`, in addition to stopping altogether by
returning a non-nil error.

Fixes: libgit2#837

tree.go

@@ -8,6 +8,7 @@ extern int _go_git_treewalk(git_tree *tree, git_treewalk_mode mode, void *ptr);
 import "C"
 
 import (
+	"errors"
 	"runtime"
 	"unsafe"
 )
@@ -134,14 +135,29 @@ func treeWalkCallback(_root *C.char, entry *C.git_tree_entry, ptr unsafe.Pointer
 	}
 
 	err := data.callback(C.GoString(_root), newTreeEntry(entry))
-	if err != nil {
+	if err == TreeWalkSkip {
+		return C.int(1)
+	} else if err != nil {
 		*data.errorTarget = err
 		return C.int(ErrorCodeUser)
 	}
 
 	return C.int(ErrorCodeOK)
 }
 
+// TreeWalkSkip is an error that can be returned form TreeWalkCallback to skip
+// a subtree from being expanded.
+var TreeWalkSkip = errors.New("skip")
+
+// Walk traverses the entries in a tree and its subtrees in pre order.
+//
+// The entries will be traversed in the pre order, children subtrees will be
+// automatically loaded as required, and the callback will be called once per
+// entry with the current (relative) root for the entry and the entry data
+// itself.
+//
+// If the callback returns TreeWalkSkip, the passed entry will be skipped on
+// the traversal. Any other non-nil error stops the walk.
 func (t *Tree) Walk(callback TreeWalkCallback) error {
 	var err error
 	data := treeWalkCallbackData{

tree_test.go

@@ -1,6 +1,9 @@
 package git
 
-import "testing"
+import (
+	"errors"
+	"testing"
+)
 
 func TestTreeEntryById(t *testing.T) {
 	t.Parallel()
@@ -63,3 +66,72 @@ func TestTreeBuilderInsert(t *testing.T) {
 		t.Fatalf("got oid %v, want %v", entry.Id, blobId)
 	}
 }
+
+func TestTreeWalk(t *testing.T) {
+	t.Parallel()
+	repo, err := OpenRepository("testdata/TestGitRepository.git")
+	checkFatal(t, err)
+	treeID, err := NewOid("6020a3b8d5d636e549ccbd0c53e2764684bb3125")
+	checkFatal(t, err)
+
+	tree, err := repo.LookupTree(treeID)
+	checkFatal(t, err)
+
+	var callCount int
+	err = tree.Walk(func(name string, entry *TreeEntry) error {
+		callCount++
+
+		return nil
+	})
+	checkFatal(t, err)
+	if callCount != 11 {
+		t.Fatalf("got called %v times, want %v", callCount, 11)
+	}
+}
+
+func TestTreeWalkSkip(t *testing.T) {
+	t.Parallel()
+	repo, err := OpenRepository("testdata/TestGitRepository.git")
+	checkFatal(t, err)
+	treeID, err := NewOid("6020a3b8d5d636e549ccbd0c53e2764684bb3125")
+	checkFatal(t, err)
+
+	tree, err := repo.LookupTree(treeID)
+	checkFatal(t, err)
+
+	var callCount int
+	err = tree.Walk(func(name string, entry *TreeEntry) error {
+		callCount++
+
+		return TreeWalkSkip
+	})
+	checkFatal(t, err)
+	if callCount != 4 {
+		t.Fatalf("got called %v times, want %v", callCount, 4)
+	}
+}
+
+func TestTreeWalkStop(t *testing.T) {
+	t.Parallel()
+	repo, err := OpenRepository("testdata/TestGitRepository.git")
+	checkFatal(t, err)
+	treeID, err := NewOid("6020a3b8d5d636e549ccbd0c53e2764684bb3125")
+	checkFatal(t, err)
+
+	tree, err := repo.LookupTree(treeID)
+	checkFatal(t, err)
+
+	var callCount int
+	stopError := errors.New("stop")
+	err = tree.Walk(func(name string, entry *TreeEntry) error {
+		callCount++
+
+		return stopError
+	})
+	if err != stopError {
+		t.Fatalf("got error %v, want %v", err, stopError)
+	}
+	if callCount != 1 {
+		t.Fatalf("got called %v times, want %v", callCount, 1)
+	}
+}