refactor into readLine helper

Author: beyang

diff/parse.go

@@ -68,8 +68,8 @@ func (r *MultiFileDiffReader) ReadFile() (*FileDiff, error) {
 	// caused by the lack of any hunks, or a malformatted hunk, so we
 	// need to perform the check here.
 	hr := fr.HunksReader()
-	line, err := r.reader.ReadBytes('\n')
-	if err != nil && !(err == io.EOF && len(line) > 0) {
+	line, err := readLine(r.reader)
+	if err != nil {
 		return d, err
 	}
 	line = bytes.TrimSuffix(line, []byte{'\n'})
@@ -216,15 +216,12 @@ func (r *FileDiffReader) readOneFileHeader(prefix []byte) (filename string, time
 
 	if r.fileHeaderLine == nil {
 		var err error
-		line, err = r.reader.ReadBytes('\n')
+		line, err = readLine(r.reader)
 		if err == io.EOF {
-			if len(line) == 0 {
-				return "", nil, &ParseError{r.line, r.offset, ErrNoFileHeader}
-			}
+			return "", nil, &ParseError{r.line, r.offset, ErrNoFileHeader}
 		} else if err != nil {
 			return "", nil, err
 		}
-		line = bytes.TrimSuffix(line, []byte{'\n'})
 	} else {
 		line = r.fileHeaderLine
 		r.fileHeaderLine = nil
@@ -263,15 +260,12 @@ func (r *FileDiffReader) ReadExtendedHeaders() ([]string, error) {
 		var line []byte
 		if r.fileHeaderLine == nil {
 			var err error
-			line, err = r.reader.ReadBytes('\n')
+			line, err = readLine(r.reader)
 			if err == io.EOF {
-				if len(line) == 0 {
-					return xheaders, &ParseError{r.line, r.offset, ErrExtendedHeadersEOF}
-				}
+				return xheaders, &ParseError{r.line, r.offset, ErrExtendedHeadersEOF}
 			} else if err != nil {
 				return xheaders, err
 			}
-			line = bytes.TrimSuffix(line, []byte{'\n'})
 		} else {
 			line = r.fileHeaderLine
 			r.fileHeaderLine = nil
@@ -344,14 +338,13 @@ func (r *HunksReader) ReadHunk() (*Hunk, error) {
 			line = r.nextHunkHeaderLine
 			r.nextHunkHeaderLine = nil
 		} else {
-			line, err = r.reader.ReadBytes('\n')
+			line, err = readLine(r.reader)
 			if err != nil {
 				if err == io.EOF && r.hunk != nil {
 					return r.hunk, nil
 				}
 				return nil, err
 			}
-			line = bytes.TrimSuffix(line, []byte{'\n'})
 		}
 
 		// Record position.

diff/reader_util.go

@@ -0,0 +1,30 @@
+package diff
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+)
+
+// readLine is a helper that mimics the functionality of calling bufio.Scanner.Scan() and
+// bufio.Scanner.Bytes(), but without the token size limitation. It will read and return
+// the next line in the Reader with the trailing newline stripped. It will return an
+// io.EOF error when there is nothing left to read (at the start of the function call). It
+// will return any other errors it receives from the underlying call to ReadBytes.
+func readLine(r *bufio.Reader) ([]byte, error) {
+	line_, err := r.ReadBytes('\n')
+	if err == io.EOF {
+		if len(line_) == 0 {
+			return nil, io.EOF
+		}
+
+		// ReadBytes returned io.EOF, because it didn't find another newline, but there is
+		// still the remainder of the file to return as a line.
+		line := line_
+		return line, nil
+	} else if err != nil {
+		return nil, err
+	}
+	line := bytes.TrimSuffix(line_, []byte{'\n'})
+	return line, nil
+}