docs: fuzzing with go-fuzz tutorial

This commit adds a tutorial on fuzzing with the go-fuzz library
into the docs folder. It includes an introduction to fuzzing,
setup and installation steps to run go-fuzz with lnd, tips to
generate a valid corpus for use with go-fuzz, and finally it
includes a small explanation of the test harness that was used
to find bugs in lnd.

docs/go-fuzz/wirefuzz.go

@@ -0,0 +1,53 @@
+package wirefuzz
+
+import (
+	"bytes"
+	"fmt"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"reflect"
+)
+
+// Fuzz is used by go-fuzz to fuzz for potentially malicious input
+func Fuzz(data []byte) int {
+	// Because go-fuzz requires this function signature with a []byte parameter,
+	// and we want to emulate the behavior of mainScenario in lnwire_test.go,
+	// we first parse the []byte parameter into a Message type.
+
+	// Parsing []byte into Message
+	r := bytes.NewReader(data)
+	msg, err := lnwire.ReadMessage(r, 0)
+	if err != nil {
+		// Ignore this input - go-fuzz generated []byte that cannot be represented as Message
+		return 0
+	}
+
+	// We will serialize Message into a new bytes buffer
+	var b bytes.Buffer
+	if _, err := lnwire.WriteMessage(&b, msg, 0); err != nil {
+		// Could not serialize Message into bytes buffer, panic
+		panic(err)
+	}
+
+	// Make sure serialized bytes buffer (excluding 2 bytes for message type
+	// is less than max payload size for this specific message,.
+	payloadLen := uint32(b.Len()) - 2
+	if payloadLen > msg.MaxPayloadLength(0) {
+		// Ignore this input - max payload constraint violated
+		return 0
+	}
+
+	// Deserialize the message from the serialized bytes buffer and
+	// assert that the original message is equal to the newly deserialized message.
+	newMsg, err := lnwire.ReadMessage(&b, 0)
+	if err != nil {
+		// Could not deserialize message from bytes buffer, panic
+		panic(err)
+	}
+	if !reflect.DeepEqual(msg, newMsg) {
+		// Deserialized message and original message are not deeply equal
+		panic(fmt.Errorf("Deserialized message and original message " +
+			"are not deeply equal."))
+	}
+
+	return 1
+}