namesys: Add recursive resolution

This allows direct access to the earlier protocol-specific Resolve
implementations.  The guts of each protocol-specific resolver are in
the internal resolveOnce method, and we've added a new:

  ResolveN(ctx, name, depth)

method to the public interface.  There's also:

  Resolve(ctx, name)

which wraps ResolveN using DefaultDepthLimit.  The extra API endpoint
is intended to reduce the likelyhood of clients accidentally calling
the more dangerous ResolveN with a nonsensically high or infinite
depth.  On IRC on 2015-05-17, Juan said:

15:34 <jbenet> If 90% of uses is the reduced API with no chance to
  screw it up, that's a huge win.
15:34 <wking> Why would those 90% not just set depth=0 or depth=1,
  depending on which they need?
15:34 <jbenet> Because people will start writing `r.Resolve(ctx, name,
  d)` where d is a variable.
15:35 <wking> And then accidentally set that variable to some huge
  number?
15:35 <jbenet> Grom experience, i've seen this happen _dozens_ of
  times. people screw trivial things up.
15:35 <wking> Why won't those same people be using ResolveN?
15:36 <jbenet> Because almost every example they see will tell them to
  use Resolve(), and they will mostly stay away from ResolveN.

The per-prodocol versions also resolve recursively within their
protocol.  For example:

  DNSResolver.Resolve(ctx, "ipfs.io", 0)

will recursively resolve DNS links until the referenced value is no
longer a DNS link.

I also renamed the multi-protocol ipfs NameSystem (defined in
namesys/namesys.go) to 'mpns' (for Multi-Protocol Name System),
because I wasn't clear on whether IPNS applied to the whole system or
just to to the DHT-based system.  The new name is unambiguously
multi-protocol, which is good.  It would be nice to have a distinct
name for the DHT-based link system.

Now that resolver output is always prefixed with a namespace and
unprefixed mpns resolver input is interpreted as /ipfs/,
core/corehttp/ipns_hostname.go can dispense with it's old manual
/ipfs/ injection.

Now that the Resolver interface handles recursion, we don't need the
resolveRecurse helper in core/pathresolver.go.  The pathresolver
cleanup also called for an adjustment to FromSegments to more easily
get slash-prefixed paths.

Now that recursive resolution with the namesys/namesys.go composite
resolver always gets you to an /ipfs/... path, there's no need for the
/ipns/ special case in fuse/ipns/ipns_unix.go.

Now that DNS links can be things other than /ipfs/ or DHT-link
references (e.g. they could be /ipns/<domain-name> references) I've
also loosened the ParsePath logic to only attempt multihash validation
on IPFS paths.  It checks to ensure that other paths have a
known-protocol prefix, but otherwise leaves them alone.

I also changed some key-stringification from .Pretty() to .String()
following the potential deprecation mentioned in util/key.go.

core/commands/resolve.go

@@ -75,7 +75,7 @@ Resolve te value of another name:
 			name = req.Arguments()[0]
 		}
 
-		output, err := n.Namesys.Resolve(n.Context(), name)
+		output, err := n.Namesys.Resolve(n.Context(), "/ipns/"+name)
 		if err != nil {
 			res.SetError(err, cmds.ErrNormal)
 			return

core/corehttp/gateway_test.go

@@ -22,18 +22,17 @@ import (
 type mockNamesys map[string]path.Path
 
 func (m mockNamesys) Resolve(ctx context.Context, name string) (value path.Path, err error) {
+	return m.ResolveN(ctx, name, namesys.DefaultDepthLimit)
+}
+
+func (m mockNamesys) ResolveN(ctx context.Context, name string, depth int) (value path.Path, err error) {
 	p, ok := m[name]
 	if !ok {
 		return "", namesys.ErrResolveFailed
 	}
 	return p, nil
 }
 
-func (m mockNamesys) CanResolve(name string) bool {
-	_, ok := m[name]
-	return ok
-}
-
 func (m mockNamesys) Publish(ctx context.Context, name ci.PrivKey, value path.Path) error {
 	return errors.New("not implemented for mockNamesys")
 }

core/corehttp/ipns_hostname.go

@@ -20,7 +20,7 @@ func IPNSHostnameOption() ServeOption {
 
 			host := strings.SplitN(r.Host, ":", 2)[0]
 			if p, err := n.Namesys.Resolve(ctx, host); err == nil {
-				r.URL.Path = "/ipfs/" + p.String() + r.URL.Path
+				r.URL.Path = p.String() + r.URL.Path
 			}
 			childMux.ServeHTTP(w, r)
 		})

core/pathresolver.go

@@ -2,7 +2,6 @@ package core
 
 import (
 	"errors"
-	"fmt"
 	"strings"
 
 	context "github.com/ipfs/go-ipfs/Godeps/_workspace/src/golang.org/x/net/context"
@@ -11,64 +10,42 @@ import (
 	path "github.com/ipfs/go-ipfs/path"
 )
 
-const maxLinks = 32
+// ErrNoNamesys is an explicit error for when an IPFS node doesn't
+// (yet) have a name system
+var ErrNoNamesys = errors.New(
+	"core/resolve: no Namesys on IpfsNode - can't resolve ipns entry")
 
-// errors returned by Resolve function
-var (
-	ErrTooManyLinks = errors.New("core/resolve: exceeded maximum number of links in ipns entry")
-	ErrNoNamesys    = errors.New("core/resolve: no Namesys on IpfsNode - can't resolve ipns entry")
-)
-
-// Resolve resolves the given path by parsing out /ipns/ entries and then going
-// through the /ipfs/ entries and returning the final merkledage node.
-// Effectively enables /ipns/ in CLI commands.
+// Resolve resolves the given path by parsing out protocol-specific
+// entries (e.g. /ipns/<node-key>) and then going through the /ipfs/
+// entries and returning the final merkledage node.  Effectively
+// enables /ipns/, /dns/, etc. in commands.
 func Resolve(ctx context.Context, n *IpfsNode, p path.Path) (*merkledag.Node, error) {
-	r := resolver{ctx, n, p}
-	return r.resolveRecurse(0)
-}
-
-type resolver struct {
-	ctx context.Context
-	n   *IpfsNode
-	p   path.Path
-}
-
-func (r *resolver) resolveRecurse(depth int) (*merkledag.Node, error) {
-	if depth >= maxLinks {
-		return nil, ErrTooManyLinks
-	}
-	// for now, we only try to resolve ipns paths if
-	// they begin with "/ipns/". Otherwise, ambiguity
-	// emerges when resolving just a <hash>. Is it meant
-	// to be an ipfs or an ipns resolution?
-
-	if strings.HasPrefix(r.p.String(), "/ipns/") {
+	if strings.HasPrefix(p.String(), "/") {
+		// namespaced path (/ipfs/..., /ipns/..., etc.)
 		// TODO(cryptix): we sould be able to query the local cache for the path
-		if r.n.Namesys == nil {
+		if n.Namesys == nil {
 			return nil, ErrNoNamesys
 		}
-		// if it's an ipns path, try to resolve it.
-		// if we can't, we can give that error back to the user.
-		seg := r.p.Segments()
-		if len(seg) < 2 || seg[1] == "" { // just "/ipns/"
-			return nil, fmt.Errorf("invalid path: %s", string(r.p))
-		}
 
-		ipnsPath := seg[1]
+		seg := p.Segments()
 		extensions := seg[2:]
-		respath, err := r.n.Namesys.Resolve(r.ctx, ipnsPath)
+		resolvable, err := path.FromSegments("/", seg[0], seg[1])
+		if err != nil {
+			return nil, err
+		}
+
+		respath, err := n.Namesys.Resolve(ctx, resolvable.String())
 		if err != nil {
 			return nil, err
 		}
 
 		segments := append(respath.Segments(), extensions...)
-		r.p, err = path.FromSegments(segments...)
+		p, err = path.FromSegments("/", segments...)
 		if err != nil {
 			return nil, err
 		}
-		return r.resolveRecurse(depth + 1)
 	}
 
 	// ok, we have an ipfs path now (or what we'll treat as one)
-	return r.n.Resolver.ResolvePath(r.ctx, r.p)
+	return n.Resolver.ResolvePath(ctx, p)
 }

fuse/ipns/ipns_test.go

@@ -462,7 +462,7 @@ func TestFastRepublish(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	pubkeyHash := u.Key(h).Pretty()
+	pubkeyPath := "/ipns/" + u.Key(h).String()
 
 	// set them back
 	defer func() {
@@ -482,9 +482,9 @@ func TestFastRepublish(t *testing.T) {
 	writeFileData(t, dataA, fname) // random
 	<-time.After(shortRepublishTimeout * 2)
 	log.Debug("resolving first hash")
-	resolvedHash, err := node.Namesys.Resolve(context.Background(), pubkeyHash)
+	resolvedHash, err := node.Namesys.Resolve(context.Background(), pubkeyPath)
 	if err != nil {
-		t.Fatal("resolve err:", pubkeyHash, err)
+		t.Fatal("resolve err:", pubkeyPath, err)
 	}
 
 	// constantly keep writing to the file
@@ -501,7 +501,7 @@ func TestFastRepublish(t *testing.T) {
 	}(shortRepublishTimeout)
 
 	hasPublished := func() bool {
-		res, err := node.Namesys.Resolve(context.Background(), pubkeyHash)
+		res, err := node.Namesys.Resolve(context.Background(), pubkeyPath)
 		if err != nil {
 			t.Fatalf("resolve err: %v", err)
 		}

fuse/ipns/ipns_unix.go

@@ -150,9 +150,6 @@ func (s *Root) Lookup(ctx context.Context, name string) (fs.Node, error) {
 	if segments[0] == "ipfs" {
 		p := strings.Join(resolved.Segments()[1:], "/")
 		return &Link{s.IpfsRoot + "/" + p}, nil
-	} else if segments[0] == "ipns" {
-		p := strings.Join(resolved.Segments()[1:], "/")
-		return &Link{s.IpnsRoot + "/" + p}, nil
 	} else {
 		log.Error("Invalid path.Path: ", resolved)
 		return nil, errors.New("invalid path from ipns record")

ipnsfs/system.go

@@ -141,7 +141,7 @@ func (fs *Filesystem) newKeyRoot(parent context.Context, k ci.PrivKey) (*KeyRoot
 		return nil, err
 	}
 
-	name := u.Key(hash).Pretty()
+	name := "/ipns/" + u.Key(hash).String()
 
 	root := new(KeyRoot)
 	root.key = k

namesys/base.go

@@ -0,0 +1,54 @@
+package namesys
+
+import (
+	"strings"
+
+	context "github.com/ipfs/go-ipfs/Godeps/_workspace/src/golang.org/x/net/context"
+
+	path "github.com/ipfs/go-ipfs/path"
+)
+
+type resolver interface {
+	// resolveOnce looks up a name once (without recursion).
+	resolveOnce(ctx context.Context, name string) (value path.Path, err error)
+}
+
+// resolve is a helper for implementing Resolver.ResolveN using resolveOnce.
+func resolve(ctx context.Context, r resolver, name string, depth int, prefixes ...string) (path.Path, error) {
+	for {
+		p, err := r.resolveOnce(ctx, name)
+		if err != nil {
+			log.Warningf("Could not resolve %s", name)
+			return "", err
+		}
+		log.Debugf("Resolved %s to %s", name, p.String())
+
+		if strings.HasPrefix(p.String(), "/ipfs/") {
+			// we've bottomed out with an IPFS path
+			return p, nil
+		}
+
+		if depth == 1 {
+			return p, ErrResolveRecursion
+		}
+
+		matched := false
+		for _, prefix := range prefixes {
+			if strings.HasPrefix(p.String(), prefix) {
+				matched = true
+				if len(prefixes) == 1 {
+					name = strings.TrimPrefix(p.String(), prefix)
+				}
+				break
+			}
+		}
+
+		if !matched {
+			return p, nil
+		}
+
+		if depth > 1 {
+			depth--
+		}
+	}
+}

namesys/dns.go

@@ -17,16 +17,25 @@ type DNSResolver struct {
 	// cache would need a timeout
 }
 
-// CanResolve implements Resolver
-func (r *DNSResolver) CanResolve(name string) bool {
-	return isd.IsDomain(name)
+// Resolve implements Resolver.
+func (r *DNSResolver) Resolve(ctx context.Context, name string) (path.Path, error) {
+	return r.ResolveN(ctx, name, DefaultDepthLimit)
+}
+
+// ResolveN implements Resolver.
+func (r *DNSResolver) ResolveN(ctx context.Context, name string, depth int) (path.Path, error) {
+	return resolve(ctx, r, name, depth, "/ipns/")
 }
 
-// Resolve implements Resolver
+// resolveOnce implements resolver.
 // TXT records for a given domain name should contain a b58
 // encoded multihash.
-func (r *DNSResolver) Resolve(ctx context.Context, name string) (path.Path, error) {
-	log.Info("DNSResolver resolving %v", name)
+func (r *DNSResolver) resolveOnce(ctx context.Context, name string) (path.Path, error) {
+	if !isd.IsDomain(name) {
+		return "", errors.New("not a valid domain name")
+	}
+
+	log.Infof("DNSResolver resolving %s", name)
 	txt, err := net.LookupTXT(name)
 	if err != nil {
 		return "", err
@@ -43,7 +52,7 @@ func (r *DNSResolver) Resolve(ctx context.Context, name string) (path.Path, erro
 }
 
 func parseEntry(txt string) (path.Path, error) {
-	p, err := path.ParseKeyToPath(txt)
+	p, err := path.ParseKeyToPath(txt) // bare IPFS multihashes
 	if err == nil {
 		return p, nil
 	}

namesys/interface.go

@@ -37,9 +37,24 @@ import (
 	path "github.com/ipfs/go-ipfs/path"
 )
 
+const (
+	// DefaultDepthLimit is the default depth limit used by Resolve.
+	DefaultDepthLimit = 32
+
+	// UnlimitedDepth allows infinite recursion in ResolveN.  You
+	// probably don't want to use this, but it's here if you absolutely
+	// trust resolution to eventually complete and can't put an upper
+	// limit on how many steps it will take.
+	UnlimitedDepth = 0
+)
+
 // ErrResolveFailed signals an error when attempting to resolve.
 var ErrResolveFailed = errors.New("could not resolve name.")
 
+// ErrResolveRecursion signals a recursion-depth limit.
+var ErrResolveRecursion = errors.New(
+	"could not resolve name (recursion limit exceeded).")
+
 // ErrPublishFailed signals an error when attempting to publish.
 var ErrPublishFailed = errors.New("could not publish name.")
 
@@ -58,11 +73,30 @@ type NameSystem interface {
 // Resolver is an object capable of resolving names.
 type Resolver interface {
 
-	// Resolve looks up a name, and returns the value previously published.
+	// Resolve performs a recursive lookup, returning the dereferenced
+	// path.  For example, if ipfs.io has a DNS TXT record pointing to
+	//   /ipns/QmatmE9msSfkKxoffpHwNLNKgwZG8eT9Bud6YoPab52vpy
+	// and there is a DHT IPNS entry for
+	//   QmatmE9msSfkKxoffpHwNLNKgwZG8eT9Bud6YoPab52vpy
+	//   -> /ipfs/Qmcqtw8FfrVSBaRmbWwHxt3AuySBhJLcvmFYi3Lbc4xnwj
+	// then
+	//   Resolve(ctx, "/ipns/ipfs.io")
+	// will resolve both names, returning
+	//   /ipfs/Qmcqtw8FfrVSBaRmbWwHxt3AuySBhJLcvmFYi3Lbc4xnwj
+	//
+	// There is a default depth-limit to avoid infinite recursion.  Most
+	// users will be fine with this default limit, but if you need to
+	// adjust the limit you can use ResolveN.
 	Resolve(ctx context.Context, name string) (value path.Path, err error)
 
-	// CanResolve checks whether this Resolver can resolve a name
-	CanResolve(name string) bool
+	// ResolveN performs a recursive lookup, returning the dereferenced
+	// path.  The only difference from Resolve is that the depth limit
+	// is configurable.  You can use DefaultDepthLimit, UnlimitedDepth,
+	// or a depth limit of your own choosing.
+	//
+	// Most users should use Resolve, since the default limit works well
+	// in most real-world situations.
+	ResolveN(ctx context.Context, name string, depth int) (value path.Path, err error)
 }
 
 // Publisher is an object capable of publishing particular names.