Properly report DNSLink errors (#12)

* Properly report DNSLink errors

Only report that there is no DNSLink for a name when there are no DNSLink TXT records available for that name.  For all other errors, such as being offline, report the more general "cannot resolve name" error.

* Document that we give precedence to good results from looking up DNSLinks in TXT records from `"_dnslink."+fqdn` over results from `fqdn`.

base.go

@@ -2,7 +2,6 @@ package namesys
 
 import (
 	"context"
-	"fmt"
 	"strings"
 	"time"
 
@@ -37,14 +36,6 @@ func resolve(ctx context.Context, r resolver, name string, options opts.ResolveO
 		}
 	}
 
-	if err == ErrResolveFailed {
-		i := len(name) - 1
-		for i >= 0 && name[i] != '/' {
-			i--
-		}
-		// Wrap error so that it can be tested if it is a ErrResolveFailed
-		err = fmt.Errorf("%w: %q is missing a DNSLink record (https://docs.ipfs.io/concepts/dnslink/)", ErrResolveFailed, name[i+1:])
-	}
 	return p, err
 }
 

dns.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	gpath "path"
 	"strings"
 
 	path "github.com/ipfs/go-path"
@@ -88,6 +89,7 @@ func (r *DNSResolver) resolveOnceAsync(ctx context.Context, name string, options
 
 	go func() {
 		defer close(out)
+		var rootResErr, subResErr error
 		for {
 			select {
 			case subRes, ok := <-subChan:
@@ -98,8 +100,11 @@ func (r *DNSResolver) resolveOnceAsync(ctx context.Context, name string, options
 				if subRes.error == nil {
 					p, err := appendPath(subRes.path)
 					emitOnceResult(ctx, out, onceResult{value: p, err: err})
+					// Return without waiting for rootRes, since this result
+					// (for "_dnslink."+fqdn) takes precedence
 					return
 				}
+				subResErr = subRes.error
 			case rootRes, ok := <-rootChan:
 				if !ok {
 					rootChan = nil
@@ -108,11 +113,24 @@ func (r *DNSResolver) resolveOnceAsync(ctx context.Context, name string, options
 				if rootRes.error == nil {
 					p, err := appendPath(rootRes.path)
 					emitOnceResult(ctx, out, onceResult{value: p, err: err})
+					// Do not return here.  Wait for subRes so that it is
+					// output last if good, thereby giving subRes precedence.
+				} else {
+					rootResErr = rootRes.error
 				}
 			case <-ctx.Done():
 				return
 			}
 			if subChan == nil && rootChan == nil {
+				// If here, then both lookups are done
+				//
+				// If both lookups failed due to no TXT records with a
+				// dnslink, then output a more specific error message
+				if rootResErr == ErrResolveFailed && subResErr == ErrResolveFailed {
+					// Wrap error so that it can be tested if it is a ErrResolveFailed
+					err := fmt.Errorf("%w: %q is missing a DNSLink record (https://docs.ipfs.io/concepts/dnslink/)", ErrResolveFailed, gpath.Base(name))
+					emitOnceResult(ctx, out, onceResult{err: err})
+				}
 				return
 			}
 		}
@@ -126,7 +144,14 @@ func workDomain(r *DNSResolver, name string, res chan lookupRes) {
 
 	txt, err := r.lookupTXT(name)
 	if err != nil {
-		// Error is != nil
+		if dnsErr, ok := err.(*net.DNSError); ok {
+			// If no TXT records found, return same error as when no text
+			// records contain dnslink. Otherwise, return the actual error.
+			if dnsErr.IsNotFound {
+				err = ErrResolveFailed
+			}
+		}
+		// Could not look up any text records for name
 		res <- lookupRes{"", err}
 		return
 	}
@@ -138,6 +163,8 @@ func workDomain(r *DNSResolver, name string, res chan lookupRes) {
 			return
 		}
 	}
+
+	// There were no TXT records with a dnslink
 	res <- lookupRes{"", ErrResolveFailed}
 }