more generalized/powerful error handling in the walk functions

This commit was moved from ipfs/go-merkledag@27ea6f8

ipld/merkledag/merkledag.go

@@ -286,19 +286,29 @@ func GetLinksWithDAG(ng ipld.NodeGetter) GetLinks {
 // that 'fetchNodes' will start at a time
 const defaultConcurrentFetch = 32
 
-// WalkOptions represent the parameters of a graph walking algorithm
-type WalkOptions struct {
+// walkOptions represent the parameters of a graph walking algorithm
+type walkOptions struct {
 	WithRoot     bool
-	IgnoreErrors bool
 	Concurrency  int
+	ErrorHandler func(c cid.Cid, err error) error
 }
 
-// WalkOption is a setter for WalkOptions
-type WalkOption func(*WalkOptions)
+// WalkOption is a setter for walkOptions
+type WalkOption func(*walkOptions)
+
+func (wo *walkOptions) addHandler(handler func(c cid.Cid, err error) error) {
+	if wo.ErrorHandler != nil {
+		wo.ErrorHandler = func(c cid.Cid, err error) error {
+			return handler(c, wo.ErrorHandler(c, err))
+		}
+	} else {
+		wo.ErrorHandler = handler
+	}
+}
 
 // WithRoot is a WalkOption indicating that the root node should be visited
 func WithRoot() WalkOption {
-	return func(walkOptions *WalkOptions) {
+	return func(walkOptions *walkOptions) {
 		walkOptions.WithRoot = true
 	}
 }
@@ -308,7 +318,7 @@ func WithRoot() WalkOption {
 // NOTE: When using that option, the walk order is *not* guarantee.
 // NOTE: It *does not* make multiple concurrent calls to the passed `visit` function.
 func Concurrent() WalkOption {
-	return func(walkOptions *WalkOptions) {
+	return func(walkOptions *walkOptions) {
 		walkOptions.Concurrency = defaultConcurrentFetch
 	}
 }
@@ -318,16 +328,52 @@ func Concurrent() WalkOption {
 // NOTE: When using that option, the walk order is *not* guarantee.
 // NOTE: It *does not* make multiple concurrent calls to the passed `visit` function.
 func Concurrency(worker int) WalkOption {
-	return func(walkOptions *WalkOptions) {
+	return func(walkOptions *walkOptions) {
 		walkOptions.Concurrency = worker
 	}
 }
 
 // IgnoreErrors is a WalkOption indicating that the walk should attempt to
 // continue even when an error occur.
 func IgnoreErrors() WalkOption {
-	return func(walkOptions *WalkOptions) {
-		walkOptions.IgnoreErrors = true
+	return func(walkOptions *walkOptions) {
+		walkOptions.addHandler(func(c cid.Cid, err error) error {
+			return nil
+		})
+	}
+}
+
+// IgnoreMissing is a WalkOption indicating that the walk should continue when
+// a node is missing.
+func IgnoreMissing() WalkOption {
+	return func(walkOptions *walkOptions) {
+		walkOptions.addHandler(func(c cid.Cid, err error) error {
+			if err == ipld.ErrNotFound {
+				return nil
+			}
+			return err
+		})
+	}
+}
+
+// OnMissing is a WalkOption adding a callback that will be triggered on a missing
+// node.
+func OnMissing(callback func(c cid.Cid)) WalkOption {
+	return func(walkOptions *walkOptions) {
+		walkOptions.addHandler(func(c cid.Cid, err error) error {
+			if err == ipld.ErrNotFound {
+				callback(c)
+			}
+			return err
+		})
+	}
+}
+
+// OnError is a WalkOption adding a custom error handler.
+// If this handler return a nil error, the walk will continue.
+func OnError(handler func(c cid.Cid, err error) error) WalkOption {
+	return func(walkOptions *walkOptions) {
+		walkOptions.addHandler(handler)
 	}
 }
 
@@ -344,7 +390,7 @@ func Walk(ctx context.Context, getLinks GetLinks, c cid.Cid, visit func(cid.Cid)
 // depth to a given visit function. The visit function can be used to limit DAG
 // exploration.
 func WalkDepth(ctx context.Context, getLinks GetLinks, c cid.Cid, visit func(cid.Cid, int) bool, options ...WalkOption) error {
-	opts := &WalkOptions{}
+	opts := &walkOptions{}
 	for _, opt := range options {
 		opt(opts)
 	}
@@ -356,15 +402,18 @@ func WalkDepth(ctx context.Context, getLinks GetLinks, c cid.Cid, visit func(cid
 	}
 }
 
-func sequentialWalkDepth(ctx context.Context, getLinks GetLinks, root cid.Cid, depth int, visit func(cid.Cid, int) bool, options *WalkOptions) error {
+func sequentialWalkDepth(ctx context.Context, getLinks GetLinks, root cid.Cid, depth int, visit func(cid.Cid, int) bool, options *walkOptions) error {
 	if depth != 0 || options.WithRoot {
 		if !visit(root, depth) {
 			return nil
 		}
 	}
 
 	links, err := getLinks(ctx, root)
-	if err != nil && !options.IgnoreErrors {
+	if err != nil && options.ErrorHandler != nil {
+		err = options.ErrorHandler(root, err)
+	}
+	if err != nil {
 		return err
 	}
 
@@ -402,7 +451,7 @@ func (p *ProgressTracker) Value() int {
 	return p.Total
 }
 
-func parallelWalkDepth(ctx context.Context, getLinks GetLinks, root cid.Cid, visit func(cid.Cid, int) bool, options *WalkOptions) error {
+func parallelWalkDepth(ctx context.Context, getLinks GetLinks, root cid.Cid, visit func(cid.Cid, int) bool, options *walkOptions) error {
 	type cidDepth struct {
 		cid   cid.Cid
 		depth int
@@ -445,7 +494,10 @@ func parallelWalkDepth(ctx context.Context, getLinks GetLinks, root cid.Cid, vis
 
 				if shouldVisit {
 					links, err := getLinks(ctx, ci)
-					if err != nil && !options.IgnoreErrors {
+					if err != nil && options.ErrorHandler != nil {
+						err = options.ErrorHandler(root, err)
+					}
+					if err != nil {
 						select {
 						case errChan <- err:
 						case <-fetchersCtx.Done():