From cdfba7a1acc2435cf33d921d90ef84c98f285320 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Tue, 18 Nov 2014 22:08:08 -0600
Subject: [PATCH] Expose SyntaxError reason with a proper error

---
 bencode/api.go    | 22 +++++++++++-----------
 bencode/decode.go | 45 ++++++++++++++++++++++++---------------------
 2 files changed, 35 insertions(+), 32 deletions(-)

diff --git a/bencode/api.go b/bencode/api.go
index 47f24642..b4ddcec9 100644
--- a/bencode/api.go
+++ b/bencode/api.go
@@ -1,10 +1,12 @@
 package bencode
 
-import "bytes"
-import "bufio"
-import "reflect"
-import "strconv"
-import "io"
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"reflect"
+)
 
 //----------------------------------------------------------------------------
 // Errors
@@ -61,14 +63,12 @@ func (e *UnmarshalFieldError) Error() string {
 
 // Malformed bencode input, unmarshaler failed to parse it.
 type SyntaxError struct {
-	Offset int64  // location of the error
-	what   string // error description
+	Offset int64 // location of the error
+	What   error // error description
 }
 
 func (e *SyntaxError) Error() string {
-	return "bencode: syntax error (offset: " +
-		strconv.FormatInt(e.Offset, 10) +
-		"): " + e.what
+	return fmt.Sprintf("bencode: syntax error (offset: %d): %s", e.Offset, e.What)
 }
 
 // A non-nil error was returned after calling MarshalBencode on a type which
@@ -86,7 +86,7 @@ func (e *MarshalerError) Error() string {
 // implements the Unmarshaler interface.
 type UnmarshalerError struct {
 	Type reflect.Type
-	Err error
+	Err  error
 }
 
 func (e *UnmarshalerError) Error() string {
diff --git a/bencode/decode.go b/bencode/decode.go
index 82cad32b..7db899ae 100644
--- a/bencode/decode.go
+++ b/bencode/decode.go
@@ -1,12 +1,15 @@
 package bencode
 
-import "reflect"
-import "runtime"
-import "bufio"
-import "bytes"
-import "strconv"
-import "strings"
-import "io"
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+	"reflect"
+	"runtime"
+	"strconv"
+	"strings"
+)
 
 type decoder struct {
 	*bufio.Reader
@@ -38,7 +41,7 @@ func check_for_unexpected_eof(err error, offset int64) {
 	if err == io.EOF {
 		panic(&SyntaxError{
 			Offset: offset,
-			what:   "unexpected EOF",
+			What:   io.ErrUnexpectedEOF,
 		})
 	}
 }
@@ -70,7 +73,7 @@ func check_for_int_parse_error(err error, offset int64) {
 	if err != nil {
 		panic(&SyntaxError{
 			Offset: offset,
-			what:   err.Error(),
+			What:   err,
 		})
 	}
 }
@@ -82,7 +85,7 @@ func (d *decoder) parse_int(v reflect.Value) {
 	if d.buf.Len() == 0 {
 		panic(&SyntaxError{
 			Offset: start,
-			what:   "empty integer value",
+			What:   errors.New("empty integer value"),
 		})
 	}
 
@@ -138,7 +141,7 @@ func (d *decoder) parse_string(v reflect.Value) {
 		check_for_unexpected_eof(err, d.offset)
 		panic(&SyntaxError{
 			Offset: d.offset,
-			what:   "unexpected I/O error: " + err.Error(),
+			What:   errors.New("unexpected I/O error: " + err.Error()),
 		})
 	}
 
@@ -254,7 +257,7 @@ func (d *decoder) parse_dict(v reflect.Value) {
 				if !ok {
 					panic(&SyntaxError{
 						Offset: d.offset,
-						what:   "unexpected end of dict, no matching value for a given key",
+						What:   errors.New("unexpected end of dict, no matching value for a given key"),
 					})
 				}
 				continue
@@ -265,7 +268,7 @@ func (d *decoder) parse_dict(v reflect.Value) {
 		if !d.parse_value(valuev) {
 			panic(&SyntaxError{
 				Offset: d.offset,
-				what:   "unexpected end of dict, no matching value for a given key",
+				What:   errors.New("unexpected end of dict, no matching value for a given key"),
 			})
 		}
 
@@ -359,7 +362,7 @@ func (d *decoder) read_one_value() bool {
 				check_for_unexpected_eof(err, d.offset)
 				panic(&SyntaxError{
 					Offset: d.offset,
-					what:   "unexpected I/O error: " + err.Error(),
+					What:   errors.New("unexpected I/O error: " + err.Error()),
 				})
 			}
 			break
@@ -368,7 +371,7 @@ func (d *decoder) read_one_value() bool {
 		// unknown value
 		panic(&SyntaxError{
 			Offset: d.offset - 1,
-			what:   "unknown value type (invalid bencode?)",
+			What:   errors.New("unknown value type (invalid bencode?)"),
 		})
 	}
 
@@ -453,7 +456,7 @@ func (d *decoder) parse_value(v reflect.Value) bool {
 		// unknown value
 		panic(&SyntaxError{
 			Offset: d.offset - 1,
-			what:   "unknown value type (invalid bencode?)",
+			What:   errors.New("unknown value type (invalid bencode?)"),
 		})
 	}
 
@@ -487,7 +490,7 @@ func (d *decoder) parse_value_interface() (interface{}, bool) {
 		// unknown value
 		panic(&SyntaxError{
 			Offset: d.offset - 1,
-			what:   "unknown value type (invalid bencode?)",
+			What:   errors.New("unknown value type (invalid bencode?)"),
 		})
 	}
 	panic("unreachable")
@@ -499,7 +502,7 @@ func (d *decoder) parse_int_interface() interface{} {
 	if d.buf.Len() == 0 {
 		panic(&SyntaxError{
 			Offset: start,
-			what:   "empty integer value",
+			What:   errors.New("empty integer value"),
 		})
 	}
 
@@ -524,7 +527,7 @@ func (d *decoder) parse_string_interface() interface{} {
 		check_for_unexpected_eof(err, d.offset)
 		panic(&SyntaxError{
 			Offset: d.offset,
-			what:   "unexpected I/O error: " + err.Error(),
+			What:   errors.New("unexpected I/O error: " + err.Error()),
 		})
 	}
 
@@ -545,7 +548,7 @@ func (d *decoder) parse_dict_interface() interface{} {
 		if !ok {
 			panic(&SyntaxError{
 				Offset: d.offset,
-				what:   "non-string key in a dict",
+				What:   errors.New("non-string key in a dict"),
 			})
 		}
 
@@ -553,7 +556,7 @@ func (d *decoder) parse_dict_interface() interface{} {
 		if !ok {
 			panic(&SyntaxError{
 				Offset: d.offset,
-				what:   "unexpected end of dict, no matching value for a given key",
+				What:   errors.New("unexpected end of dict, no matching value for a given key"),
 			})
 		}
 
-- 
2.48.1