p2p/dnsdisc: add implementation of EIP-1459 (#20094)

This adds an implementation of node discovery via DNS TXT records to the
go-ethereum library. The implementation doesn't match EIP-1459 exactly,
the main difference being that this implementation uses separate merkle
trees for tree links and ENRs. The EIP will be updated to match p2p/dnsdisc.

To maintain DNS trees, cmd/devp2p provides a frontend for the p2p/dnsdisc
library. The new 'dns' subcommands can be used to create, sign and deploy DNS
discovery trees.

p2p/dnsdisc/error.go

@@ -0,0 +1,63 @@
+// Copyright 2018 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package dnsdisc
+
+import (
+	"errors"
+	"fmt"
+)
+
+// Entry parse errors.
+var (
+	errUnknownEntry = errors.New("unknown entry type")
+	errNoPubkey     = errors.New("missing public key")
+	errBadPubkey    = errors.New("invalid public key")
+	errInvalidENR   = errors.New("invalid node record")
+	errInvalidChild = errors.New("invalid child hash")
+	errInvalidSig   = errors.New("invalid base64 signature")
+	errSyntax       = errors.New("invalid syntax")
+)
+
+// Resolver/sync errors
+var (
+	errNoRoot        = errors.New("no valid root found")
+	errNoEntry       = errors.New("no valid tree entry found")
+	errHashMismatch  = errors.New("hash mismatch")
+	errENRInLinkTree = errors.New("enr entry in link tree")
+	errLinkInENRTree = errors.New("link entry in ENR tree")
+)
+
+type nameError struct {
+	name string
+	err  error
+}
+
+func (err nameError) Error() string {
+	if ee, ok := err.err.(entryError); ok {
+		return fmt.Sprintf("invalid %s entry at %s: %v", ee.typ, err.name, ee.err)
+	}
+	return err.name + ": " + err.err.Error()
+}
+
+type entryError struct {
+	typ string
+	err error
+}
+
+func (err entryError) Error() string {
+	return fmt.Sprintf("invalid %s entry: %v", err.typ, err.err)
+}