Add DAGService.GetLinks() method and use it in the GC and elsewhere.

This method will use the (also new) LinkService if it is available to
retrieving just the links for a MerkleDAG without necessary having to
retrieve the underlying block.

For now the main benefit is that the pinner will not break when a block
becomes invalid due to a change in the backing file.  This is possible
because the metadata for a block (that includes the Links) is stored
separately and thus always available even if the backing file changes.

License: MIT
Signed-off-by: Kevin Atkinson <k@kevina.org>

gc/gc.go

@@ -24,11 +24,12 @@ var log = logging.Logger("gc")
 //
 // The routine then iterates over every block in the blockstore and
 // deletes any block that is not found in the marked set.
-func GC(ctx context.Context, bs bstore.GCBlockstore, pn pin.Pinner, bestEffortRoots []*cid.Cid) (<-chan key.Key, error) {
+func GC(ctx context.Context, bs bstore.GCBlockstore, ls dag.LinkService, pn pin.Pinner, bestEffortRoots []*cid.Cid) (<-chan key.Key, error) {
 	unlocker := bs.GCLock()
 
 	bsrv := bserv.New(bs, offline.Exchange(bs))
 	ds := dag.NewDAGService(bsrv)
+	ds.LinkService = ls
 
 	gcs, err := ColoredSet(ctx, pn, ds, bestEffortRoots)
 	if err != nil {
@@ -74,13 +75,13 @@ func GC(ctx context.Context, bs bstore.GCBlockstore, pn pin.Pinner, bestEffortRo
 func Descendants(ctx context.Context, ds dag.DAGService, set key.KeySet, roots []*cid.Cid, bestEffort bool) error {
 	for _, c := range roots {
 		set.Add(key.Key(c.Hash()))
-		nd, err := ds.Get(ctx, c)
+		links, err := ds.GetLinks(ctx, c)
 		if err != nil {
 			return err
 		}
 
 		// EnumerateChildren recursively walks the dag and adds the keys to the given set
-		err = dag.EnumerateChildren(ctx, ds, nd, func(c *cid.Cid) bool {
+		err = dag.EnumerateChildren(ctx, ds, links, func(c *cid.Cid) bool {
 			k := key.Key(c.Hash())
 			seen := set.Has(k)
 			if seen {

pin.go

@@ -279,12 +279,12 @@ func (p *pinner) isPinnedWithType(c *cid.Cid, mode PinMode) (string, bool, error
 
 	// Default is Indirect
 	for _, rc := range p.recursePin.Keys() {
-		rnd, err := p.dserv.Get(context.Background(), rc)
+		links, err := p.dserv.GetLinks(context.Background(), rc)
 		if err != nil {
 			return "", false, err
 		}
 
-		has, err := hasChild(p.dserv, rnd, k)
+		has, err := hasChild(p.dserv, links, k)
 		if err != nil {
 			return "", false, err
 		}
@@ -317,11 +317,11 @@ func (p *pinner) CheckIfPinned(cids ...*cid.Cid) ([]Pinned, error) {
 	// Now walk all recursive pins to check for indirect pins
 	var checkChildren func(*cid.Cid, *cid.Cid) error
 	checkChildren = func(rk, parentKey *cid.Cid) error {
-		parent, err := p.dserv.Get(context.Background(), parentKey)
+		links, err := p.dserv.GetLinks(context.Background(), parentKey)
 		if err != nil {
 			return err
 		}
-		for _, lnk := range parent.Links {
+		for _, lnk := range links {
 			c := cid.NewCidV0(lnk.Hash)
 
 			if toCheck.Has(c) {
@@ -521,19 +521,19 @@ func (p *pinner) PinWithMode(c *cid.Cid, mode PinMode) {
 	}
 }
 
-func hasChild(ds mdag.DAGService, root *mdag.Node, child key.Key) (bool, error) {
-	for _, lnk := range root.Links {
+func hasChild(ds mdag.DAGService, links []*mdag.Link, child key.Key) (bool, error) {
+	for _, lnk := range links {
 		c := cid.NewCidV0(lnk.Hash)
 		if key.Key(c.Hash()) == child {
 			return true, nil
 		}
 
-		nd, err := ds.Get(context.Background(), c)
+		children, err := ds.GetLinks(context.Background(), c)
 		if err != nil {
 			return false, err
 		}
 
-		has, err := hasChild(ds, nd, child)
+		has, err := hasChild(ds, children, child)
 		if err != nil {
 			return false, err
 		}