Make pcap.Reader's API look like bufio.Scanner's.

The Next/Packet/Err structure makes read loops much
cleaner, and presents a more familiar interface for people
familiar with the stdlib.

pcap/reader.go

@@ -27,6 +27,9 @@ type Reader struct {
 	r     io.Reader
 	order binary.ByteOrder
 	tmult int64
+
+	pkt *Packet
+	err error
 }
 
 // Packet is one raw packet and its metadata.
@@ -94,8 +97,26 @@ func NewReader(r io.Reader) (*Reader, error) {
 	return ret, nil
 }
 
-// Next returns the next packet in r.
-func (r *Reader) Next() (*Packet, error) {
+// Packet returns the packet read by the last call to Next.
+func (r *Reader) Packet() *Packet {
+	return r.pkt
+}
+
+// Err returns the first non-EOF error encountered by the Reader.
+func (r *Reader) Err() error {
+	if r.err == io.EOF {
+		return nil
+	}
+	return r.err
+}
+
+// Next advances the Reader to the next packet in the input, which
+// will then be available through the Packet method. It returns false
+// when the Reader stops, either by reaching the end of the input or
+// an error. After Next returns false, the Err method will return any
+// error that occured while reading, except that if it was io.EOF, Err
+// will return nil.
+func (r *Reader) Next() bool {
 	hdr := struct {
 		Sec     uint32
 		SubSec  uint32
@@ -104,17 +125,20 @@ func (r *Reader) Next() (*Packet, error) {
 	}{}
 
 	if err := binary.Read(r.r, r.order, &hdr); err != nil {
-		return nil, err
+		r.err = err
+		return false
 	}
 
 	bs := make([]byte, hdr.Len)
 	if _, err := io.ReadFull(r.r, bs); err != nil {
-		return nil, err
+		r.err = err
+		return false
 	}
 
-	return &Packet{
+	r.pkt = &Packet{
 		Timestamp: time.Unix(int64(hdr.Sec), r.tmult*int64(hdr.SubSec)),
 		Length:    int(hdr.OrigLen),
 		Bytes:     bs,
-	}, nil
+	}
+	return true
 }

pcap/reader_test.go

@@ -2,7 +2,6 @@ package pcap
 
 import (
 	"fmt"
-	"io"
 	"io/ioutil"
 	"os"
 	"testing"
@@ -23,18 +22,15 @@ func TestFiles(t *testing.T) {
 		if r.LinkType != LinkEthernet {
 			t.Errorf("Expected link type %d, got %d", LinkEthernet, r.LinkType)
 		}
+
 		pkts := []*Packet{}
-	ReadLoop:
-		for {
-			pkt, err := r.Next()
-			if err != nil {
-				if err == io.EOF {
-					break ReadLoop
-				}
-				t.Fatalf("Unexpected error reading packets: %s", err)
-			}
-			pkts = append(pkts, pkt)
+		for r.Next() {
+			pkts = append(pkts, r.Packet())
 		}
+		if r.Err() != nil {
+			t.Fatalf("Reading packets from %s.pcap: %s", fname, r.Err())
+		}
+
 		res := pretty.Sprintf("%# v", pkts)
 		expectedFile := fmt.Sprintf("testdata/%s.parsed", fname)
 		expected, err := ioutil.ReadFile(expectedFile)

pcap/writer_test.go

@@ -3,7 +3,6 @@ package pcap
 import (
 	"bytes"
 	"encoding/binary"
-	"io"
 	"reflect"
 	"testing"
 	"time"
@@ -63,16 +62,11 @@ func TestReadback(t *testing.T) {
 			t.Fatalf("Wrote link type %d, read back %d", LinkEthernet, r.LinkType)
 		}
 
-	ReadLoop:
-		for {
-			pkt, err := r.Next()
-			if err != nil {
-				if err == io.EOF {
-					break ReadLoop
-				}
-				t.Fatalf("Unexpected error reading packets: %s", err)
-			}
-			readBack = append(readBack, pkt)
+		for r.Next() {
+			readBack = append(readBack, r.Packet())
+		}
+		if r.Err() != nil {
+			t.Fatalf("Reading packets back: %s", r.Err())
 		}
 
 		if !reflect.DeepEqual(pkts, readBack) {