feat: extract TraversalResumerPathState interface and allow it to be shared across traversals

Author: rvagg

v2/options.go

@@ -8,6 +8,7 @@ import (
 	"github.com/multiformats/go-multicodec"
 
 	"github.com/ipld/go-car/v2/internal/carv1"
+	resumetraversal "github.com/ipld/go-car/v2/traversal"
 )
 
 // DefaultMaxIndexCidSize specifies the maximum size in byptes accepted as a section CID by CARv2 index.
@@ -62,6 +63,7 @@ type Options struct {
 	DataPayloadSize              uint64
 	SkipOffset                   uint64
 	TraversalPrototypeChooser    traversal.LinkTargetNodePrototypeChooser
+	TraversalResumerPathState    resumetraversal.TraversalResumerPathState
 
 	MaxAllowedHeaderSize  uint64
 	MaxAllowedSectionSize uint64
@@ -182,3 +184,17 @@ func MaxAllowedSectionSize(max uint64) Option {
 		o.MaxAllowedSectionSize = max
 	}
 }
+
+// WithTraversalResumerPathState provides a custom TraversalResumerPathState
+// that can be reused between selective CAR creations where traversals may need
+// to be resumed at arbitrary points within the DAG.
+//
+// A TraversalResumerPathState shared across multiple traversals using the same
+// selector and DAG will yield the same state. This allows us to resume at
+// arbitrary points within in the DAG and load the minimal additional blocks
+// required to resume the traversal at that point.
+func WithTraversalResumerPathState(pathState resumetraversal.TraversalResumerPathState) Option {
+	return func(o *Options) {
+		o.TraversalResumerPathState = pathState
+	}
+}

v2/selective.go

@@ -321,7 +321,11 @@ func (tc *traversalCar) setup(ctx context.Context, ls *ipld.LinkSystem, opts Opt
 	}
 
 	ls.TrustedStorage = true
-	resumer, err := resumetraversal.WithTraversingLinksystem(&progress)
+	pathState := opts.TraversalResumerPathState
+	if pathState == nil {
+		pathState = resumetraversal.NewTraversalResumerPathState()
+	}
+	resumer, err := resumetraversal.WithTraversingLinksystem(&progress, pathState)
 	if err != nil {
 		return err
 	}

v2/traversal/resumption.go

@@ -17,12 +17,41 @@ import (
 	"github.com/ipld/go-ipld-prime/traversal"
 )
 
+// TraverseResumer allows resuming a progress from a previously encountered path in the selector.
+type TraverseResumer interface {
+	RewindToPath(from datamodel.Path) error
+	RewindToOffset(offset uint64) error
+	Position() uint64
+}
+
+// TraversalResumerPathState tracks a traversal state for the purpose of
+// building a CAR. For each block in the CAR it tracks the path to that block,
+// the Link of the block and where in the CAR the block is located.
+//
+// A TraversalResumerPathState shared across multiple traversals using the same
+// selector and DAG will yield the same state. This allows us to resume at
+// arbitrary points within in the DAG and load the minimal additional blocks
+// required to resume the traversal at that point.
+type TraversalResumerPathState interface {
+	AddPath(path []datamodel.PathSegment, link datamodel.Link, atOffset uint64)
+	GetLinks(root datamodel.Path) []datamodel.Link
+	GetOffsetAfter(root datamodel.Path) (uint64, error)
+}
+
 type pathNode struct {
 	link     datamodel.Link
 	offset   uint64
 	children map[datamodel.PathSegment]*pathNode
 }
 
+// NewTraversalResumerPathState creates a new TraversalResumerPathState.
+//
+// Note that the TraversalResumerPathState returned by this factory is not
+// thread-safe.
+func NewTraversalResumerPathState() TraversalResumerPathState {
+	return newPath(nil, 0)
+}
+
 func newPath(link datamodel.Link, at uint64) *pathNode {
 	return &pathNode{
 		link:     link,
@@ -31,15 +60,15 @@ func newPath(link datamodel.Link, at uint64) *pathNode {
 	}
 }
 
-func (pn pathNode) addPath(p []datamodel.PathSegment, link datamodel.Link, at uint64) {
+func (pn pathNode) AddPath(p []datamodel.PathSegment, link datamodel.Link, atOffset uint64) {
 	if len(p) == 0 {
 		return
 	}
 	if _, ok := pn.children[p[0]]; !ok {
-		child := newPath(link, at)
+		child := newPath(link, atOffset)
 		pn.children[p[0]] = child
 	}
-	pn.children[p[0]].addPath(p[1:], link, at)
+	pn.children[p[0]].AddPath(p[1:], link, atOffset)
 }
 
 func (pn pathNode) allLinks() []datamodel.Link {
@@ -57,7 +86,7 @@ func (pn pathNode) allLinks() []datamodel.Link {
 }
 
 // getPaths returns reconstructed paths in the tree rooted at 'root'
-func (pn pathNode) getLinks(root datamodel.Path) []datamodel.Link {
+func (pn pathNode) GetLinks(root datamodel.Path) []datamodel.Link {
 	segs := root.Segments()
 	switch len(segs) {
 	case 0:
@@ -80,12 +109,12 @@ func (pn pathNode) getLinks(root datamodel.Path) []datamodel.Link {
 		// base case 2: not registered sub-path.
 		return []datamodel.Link{}
 	}
-	return pn.children[next].getLinks(datamodel.NewPathNocopy(segs[1:]))
+	return pn.children[next].GetLinks(datamodel.NewPathNocopy(segs[1:]))
 }
 
 var errInvalid = fmt.Errorf("invalid path")
 
-func (pn pathNode) offsetAfter(root datamodel.Path) (uint64, error) {
+func (pn pathNode) GetOffsetAfter(root datamodel.Path) (uint64, error) {
 	// we look for offset of next sibling.
 	// if no next sibling recurse up the path segments until we find a next sibling.
 	segs := root.Segments()
@@ -100,7 +129,7 @@ func (pn pathNode) offsetAfter(root datamodel.Path) (uint64, error) {
 	closest := chld.offset
 	// try recursive path
 	if len(segs) > 1 {
-		co, err := chld.offsetAfter(datamodel.NewPathNocopy(segs[1:]))
+		co, err := chld.GetOffsetAfter(datamodel.NewPathNocopy(segs[1:]))
 		if err == nil {
 			return co, err
 		}
@@ -121,35 +150,28 @@ func (pn pathNode) offsetAfter(root datamodel.Path) (uint64, error) {
 	return 0, errInvalid
 }
 
-// TraverseResumer allows resuming a progress from a previously encountered path in the selector.
-type TraverseResumer interface {
-	RewindToPath(from datamodel.Path) error
-	RewindToOffset(offset uint64) error
-	Position() uint64
-}
-
 type traversalState struct {
 	wrappedLinksystem  *linking.LinkSystem
 	lsCounter          *loader.Counter
-	blockNumber        int
-	pathOrder          map[int]datamodel.Path
-	pathTree           *pathNode
+	pathTree           TraversalResumerPathState
 	rewindPathTarget   *datamodel.Path
 	rewindOffsetTarget uint64
 	pendingBlockStart  uint64 // on rewinds, we store where the counter was in order to know the length of the last read block.
 	progress           *traversal.Progress
 }
 
+var _ TraverseResumer = (*traversalState)(nil)
+
 func (ts *traversalState) RewindToPath(from datamodel.Path) error {
 	if ts.progress == nil {
 		return nil
 	}
 	// reset progress and traverse until target.
 	ts.progress.SeenLinks = make(map[datamodel.Link]struct{})
-	ts.blockNumber = 0
 	ts.pendingBlockStart = ts.lsCounter.Size()
 	ts.lsCounter.TotalRead = 0
 	ts.rewindPathTarget = &from
+	ts.rewindOffsetTarget = 0
 	return nil
 }
 
@@ -163,10 +185,10 @@ func (ts *traversalState) RewindToOffset(offset uint64) error {
 	}
 	// reset progress and traverse until target.
 	ts.progress.SeenLinks = make(map[datamodel.Link]struct{})
-	ts.blockNumber = 0
 	ts.pendingBlockStart = ts.lsCounter.Size()
 	ts.lsCounter.TotalRead = 0
 	ts.rewindOffsetTarget = offset
+	ts.rewindPathTarget = nil
 	return nil
 }
 
@@ -177,9 +199,7 @@ func (ts *traversalState) Position() uint64 {
 func (ts *traversalState) traverse(lc linking.LinkContext, l ipld.Link) (io.Reader, error) {
 	// when not in replay mode, we track metadata
 	if ts.rewindPathTarget == nil && ts.rewindOffsetTarget == 0 {
-		ts.pathOrder[ts.blockNumber] = lc.LinkPath
-		ts.pathTree.addPath(lc.LinkPath.Segments(), l, ts.lsCounter.Size())
-		ts.blockNumber++
+		ts.pathTree.AddPath(lc.LinkPath.Segments(), l, ts.lsCounter.Size())
 		return ts.wrappedLinksystem.StorageReadOpener(lc, l)
 	}
 
@@ -205,12 +225,12 @@ func (ts *traversalState) traverse(lc linking.LinkContext, l ipld.Link) (io.Read
 				break
 			}
 			if targetSegments[i].String() != s.String() {
-				links := ts.pathTree.getLinks(datamodel.NewPathNocopy(seg[0 : i+1]))
+				links := ts.pathTree.GetLinks(datamodel.NewPathNocopy(seg[0 : i+1]))
 				for _, l := range links {
 					ts.progress.SeenLinks[l] = struct{}{}
 				}
 				var err error
-				ts.lsCounter.TotalRead, err = ts.pathTree.offsetAfter(datamodel.NewPathNocopy(seg[0 : i+1]))
+				ts.lsCounter.TotalRead, err = ts.pathTree.GetOffsetAfter(datamodel.NewPathNocopy(seg[0 : i+1]))
 				if err == errInvalid {
 					ts.lsCounter.TotalRead = ts.pendingBlockStart
 				} else if err != nil {
@@ -222,12 +242,12 @@ func (ts *traversalState) traverse(lc linking.LinkContext, l ipld.Link) (io.Read
 		}
 	}
 	if ts.rewindOffsetTarget != 0 {
-		links := ts.pathTree.getLinks(lc.LinkPath)
+		links := ts.pathTree.GetLinks(lc.LinkPath)
 		for _, l := range links {
 			ts.progress.SeenLinks[l] = struct{}{}
 		}
 		var err error
-		ts.lsCounter.TotalRead, err = ts.pathTree.offsetAfter(lc.LinkPath)
+		ts.lsCounter.TotalRead, err = ts.pathTree.GetOffsetAfter(lc.LinkPath)
 		if err == errInvalid {
 			ts.lsCounter.TotalRead = ts.pendingBlockStart
 		} else if err != nil {
@@ -243,13 +263,12 @@ func (ts *traversalState) traverse(lc linking.LinkContext, l ipld.Link) (io.Read
 // WithTraversingLinksystem extends a progress for traversal such that it can
 // subsequently resume and perform subsets of the walk efficiently from
 // an arbitrary position within the selector traversal.
-func WithTraversingLinksystem(p *traversal.Progress) (TraverseResumer, error) {
+func WithTraversingLinksystem(p *traversal.Progress, pathState TraversalResumerPathState) (TraverseResumer, error) {
 	wls, ctr := loader.CountingLinkSystem(p.Cfg.LinkSystem)
 	ts := &traversalState{
 		wrappedLinksystem: &wls,
 		lsCounter:         ctr.(*loader.Counter),
-		pathOrder:         make(map[int]datamodel.Path),
-		pathTree:          newPath(nil, 0),
+		pathTree:          pathState,
 		progress:          p,
 	}
 	p.Cfg.LinkSystem.StorageReadOpener = ts.traverse

v2/traversal/resumption_test.go

@@ -89,7 +89,7 @@ func TestWalkResumeByPath(t *testing.T) {
 			LinkTargetNodePrototypeChooser: basicnode.Chooser,
 		},
 	}
-	resumer, err := cartraversal.WithTraversingLinksystem(&p)
+	resumer, err := cartraversal.WithTraversingLinksystem(&p, cartraversal.NewTraversalResumerPathState())
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -154,7 +154,7 @@ func TestWalkResumeByPathPartialWalk(t *testing.T) {
 			LinkTargetNodePrototypeChooser: basicnode.Chooser,
 		},
 	}
-	resumer, err := cartraversal.WithTraversingLinksystem(&p)
+	resumer, err := cartraversal.WithTraversingLinksystem(&p, cartraversal.NewTraversalResumerPathState())
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -195,7 +195,7 @@ func TestWalkResumeByOffset(t *testing.T) {
 			LinkTargetNodePrototypeChooser: basicnode.Chooser,
 		},
 	}
-	resumer, err := cartraversal.WithTraversingLinksystem(&p)
+	resumer, err := cartraversal.WithTraversingLinksystem(&p, cartraversal.NewTraversalResumerPathState())
 	if err != nil {
 		t.Fatal(err)
 	}