log: fix prepFile fail on files without any newline(maybe empty)

I just fixed this bug.
But actually, I think it should be cancelled to cut off invalid part from the end.
Just append new logs to the file would be more simple.

log/handler.go

@@ -102,29 +102,20 @@ func prepFile(path string) (*countingWriter, error) {
 	if err != nil {
 		return nil, err
 	}
-	_, err = f.Seek(-1, io.SeekEnd)
+	fi, err := f.Stat()
 	if err != nil {
 		return nil, err
 	}
 	buf := make([]byte, 1)
-	var cut int64
-	for {
-		if _, err := f.Read(buf); err != nil {
+	var ns int64
+	for ns = fi.Size(); ns > 0; ns-- {
+		if _, err := f.ReadAt(buf, ns-1); err != nil {
 			return nil, err
 		}
 		if buf[0] == '\n' {
 			break
 		}
-		if _, err = f.Seek(-2, io.SeekCurrent); err != nil {
-			return nil, err
 	}
-		cut++
-	}
-	fi, err := f.Stat()
-	if err != nil {
-		return nil, err
-	}
-	ns := fi.Size() - cut
 	if err = f.Truncate(ns); err != nil {
 		return nil, err
 	}

log/handler_test.go

@@ -0,0 +1,86 @@
+package log
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func newTempFileWithData(data []byte) string {
+	for {
+		f, err := ioutil.TempFile("", "ethereum.log")
+		if err != nil {
+			continue
+		}
+
+		err = f.Truncate(0)
+		if err != nil {
+			continue
+		}
+
+		if len(data) != 0 {
+			_, err = f.Write(data)
+			if err != nil {
+				continue
+			}
+		}
+
+		name := f.Name()
+		_ = f.Close()
+
+		return name
+	}
+}
+
+func TestPrepFileWithEmptyFile(t *testing.T) {
+	tmpfile := newTempFileWithData(nil)
+
+	w, err := prepFile(tmpfile)
+	if err != nil {
+		t.Errorf("Expect nil, got %v", err)
+	}
+
+	w.Close()
+
+	defer os.Remove(tmpfile)
+}
+
+func TestPrepFileWithoutNewLine(t *testing.T) {
+	dataWithoutNewLine := make([]byte, 100)
+	for i := range dataWithoutNewLine {
+		dataWithoutNewLine[i] = 'A'
+	}
+
+	tmpfile := newTempFileWithData(dataWithoutNewLine)
+
+	w, err := prepFile(tmpfile)
+	if err != nil {
+		t.Errorf("Expect nil, got %v", err)
+	}
+
+	w.Close()
+
+	defer os.Remove(tmpfile)
+}
+
+func TestPrepFileWithNewLine(t *testing.T) {
+	dataWithoutNewLine := make([]byte, 100)
+	for i := range dataWithoutNewLine {
+		dataWithoutNewLine[i] = 'A'
+	}
+	dataWithoutNewLine[0] = '\n'
+
+	tmpfile := newTempFileWithData(dataWithoutNewLine)
+
+	w, err := prepFile(tmpfile)
+	if err != nil {
+		t.Errorf("Expect nil, got %v", err)
+	}
+	if w.count != 1 {
+		t.Errorf("Expect 1, got %v", w.count)
+	}
+
+	w.Close()
+
+	defer os.Remove(tmpfile)
+}