From 81c626ef6dbd9d26e4964d44037e9dc26dd3b3bd Mon Sep 17 00:00:00 2001
From: Eric Myhre <hash@exultant.us>
Date: Tue, 31 Mar 2020 18:26:44 +0200
Subject: [PATCH] traversal.SkipMe can now be used to control walks.

---
 traversal/fns.go  | 13 +++++++++++++
 traversal/walk.go | 11 +++++++++++
 2 files changed, 24 insertions(+)

diff --git a/traversal/fns.go b/traversal/fns.go
index 5c5ea844..b67ba6f2 100644
--- a/traversal/fns.go
+++ b/traversal/fns.go
@@ -54,3 +54,16 @@ type Config struct {
 // could decide what kind of native type is expected, and return a
 // `bind.NodeBuilder` for that specific concrete native type.
 type LinkTargetNodeStyleChooser func(ipld.Link, ipld.LinkContext) (ipld.NodeStyle, error)
+
+// SkipMe is a signalling "error" which can be used to tell traverse to skip some data.
+//
+// SkipMe can be returned by the Config.LinkLoader to skip entire blocks without aborting the walk.
+// (This can be useful if you know you don't have data on hand,
+// but want to continue the walk in other areas anyway;
+// or, if you're doing a way where you know that it's valid to memoize seen
+// areas based on Link alone.)
+type SkipMe struct{}
+
+func (SkipMe) Error() string {
+	return "skip"
+}
diff --git a/traversal/walk.go b/traversal/walk.go
index dde9937d..bcd173d0 100644
--- a/traversal/walk.go
+++ b/traversal/walk.go
@@ -57,6 +57,8 @@ func WalkTransforming(n ipld.Node, s selector.Selector, fn TransformFn) (ipld.No
 // This is important to note because when walking DAGs with Links,
 // it means you may visit the same node multiple times
 // due to having reached it via a different path.
+// (You can prevent this by using a LinkLoader function which memoizes a set of
+// already-visited Links, and returns a SkipMe when encountering them again.)
 //
 // WalkMatching (and the other traversal functions) can be used again again inside the VisitFn!
 // By using the traversal.Progress handed to the VisitFn,
@@ -122,6 +124,9 @@ func (prog Progress) walkAdv_iterateAll(n ipld.Node, s selector.Selector, fn Adv
 				progNext.LastBlock.Link = lnk
 				v, err = progNext.loadLink(v, n)
 				if err != nil {
+					if _, ok := err.(SkipMe); ok {
+						return nil
+					}
 					return err
 				}
 			}
@@ -151,6 +156,9 @@ func (prog Progress) walkAdv_iterateSelective(n ipld.Node, attn []ipld.PathSegme
 				progNext.LastBlock.Link = lnk
 				v, err = progNext.loadLink(v, n)
 				if err != nil {
+					if _, ok := err.(SkipMe); ok {
+						return nil
+					}
 					return err
 				}
 			}
@@ -189,6 +197,9 @@ func (prog Progress) loadLink(v ipld.Node, parent ipld.Node) (ipld.Node, error)
 		prog.Cfg.LinkLoader,
 	)
 	if err != nil {
+		if _, ok := err.(SkipMe); ok {
+			return nil, err
+		}
 		return nil, fmt.Errorf("error traversing node at %q: could not load link %q: %s", prog.Path, lnk, err)
 	}
 	return nb.Build(), nil