Return errors for empty text fragments (#29)

This fixes two issues:

  1. Fragments with only context lines were accepted
  2. Fragments with only a "no newline" marker caused a panic

The panic was found by go-fuzz. While fixing that issue, I
discovered the first problem with other types of empty fragments while
comparing behavior against 'git apply'.

Also extract some helper functions and modify the loop conditions to
clean things up while making changes in this area.

gitdiff/text.go

@@ -79,14 +79,8 @@ func (p *parser) ParseTextChunk(frag *TextFragment) error {
 		return p.Errorf(0, "no content following fragment header")
 	}
 
-	isNoNewlineLine := func(s string) bool {
-		// test for "\ No newline at end of file" by prefix because the text
-		// changes by locale (git claims all versions are at least 12 chars)
-		return len(s) >= 12 && s[:2] == "\\ "
-	}
-
 	oldLines, newLines := frag.OldLines, frag.NewLines
-	for {
+	for oldLines > 0 || newLines > 0 {
 		line := p.Line(0)
 		op, data := line[0], line[1:]
 
@@ -113,13 +107,14 @@ func (p *parser) ParseTextChunk(frag *TextFragment) error {
 			frag.LinesAdded++
 			frag.TrailingContext = 0
 			frag.Lines = append(frag.Lines, Line{OpAdd, data})
-		default:
+		case '\\':
 			// this may appear in middle of fragment if it's for a deleted line
-			if isNoNewlineLine(line) {
-				last := &frag.Lines[len(frag.Lines)-1]
-				last.Line = strings.TrimSuffix(last.Line, "\n")
+			if isNoNewlineMarker(line) {
+				removeLastNewline(frag)
 				break
 			}
+			fallthrough
+		default:
 			// TODO(bkeyes): if this is because we hit the next header, it
 			// would be helpful to return the miscounts line error. We could
 			// either test for the common headers ("@@ -", "diff --git") or
@@ -128,11 +123,6 @@ func (p *parser) ParseTextChunk(frag *TextFragment) error {
 			return p.Errorf(0, "invalid line operation: %q", op)
 		}
 
-		next := p.Line(1)
-		if oldLines <= 0 && newLines <= 0 && !isNoNewlineLine(next) {
-			break
-		}
-
 		if err := p.Next(); err != nil {
 			if err == io.EOF {
 				break
@@ -145,13 +135,35 @@ func (p *parser) ParseTextChunk(frag *TextFragment) error {
 		hdr := max(frag.OldLines-oldLines, frag.NewLines-newLines) + 1
 		return p.Errorf(-hdr, "fragment header miscounts lines: %+d old, %+d new", -oldLines, -newLines)
 	}
+	if frag.LinesAdded == 0 && frag.LinesDeleted == 0 {
+		return p.Errorf(0, "fragment contains no changes")
+	}
 
-	if err := p.Next(); err != nil && err != io.EOF {
-		return err
+	// check for a final "no newline" marker since it is not included in the
+	// counters used to stop the loop above
+	if isNoNewlineMarker(p.Line(0)) {
+		removeLastNewline(frag)
+		if err := p.Next(); err != nil && err != io.EOF {
+			return err
+		}
 	}
+
 	return nil
 }
 
+func isNoNewlineMarker(s string) bool {
+	// test for "\ No newline at end of file" by prefix because the text
+	// changes by locale (git claims all versions are at least 12 chars)
+	return len(s) >= 12 && s[:2] == "\\ "
+}
+
+func removeLastNewline(frag *TextFragment) {
+	if len(frag.Lines) > 0 {
+		last := &frag.Lines[len(frag.Lines)-1]
+		last.Line = strings.TrimSuffix(last.Line, "\n")
+	}
+}
+
 func parseRange(s string) (start int64, end int64, err error) {
 	parts := strings.SplitN(s, ",", 2)
 

gitdiff/text_test.go

@@ -317,6 +317,24 @@ func TestParseTextChunk(t *testing.T) {
 			},
 			Err: true,
 		},
+		"onlyContext": {
+			Input: ` context line
+ context line
+`,
+			Fragment: TextFragment{
+				OldLines: 2,
+				NewLines: 2,
+			},
+			Err: true,
+		},
+		"unexpectedNoNewlineMarker": {
+			Input: `\ No newline at end of file`,
+			Fragment: TextFragment{
+				OldLines: 1,
+				NewLines: 1,
+			},
+			Err: true,
+		},
 	}
 
 	for name, test := range tests {