fix: [CODE-1858]: fix reading too long lines in diff hunk headers (#2036)

git/parser/diff_headers.go

@@ -43,13 +43,22 @@ func ParseDiffFileHeader(line string) (DiffFileHeader, bool) {
 // GetHunkHeaders parses git diff output and returns all diff headers for all files.
 // See for documentation: https://git-scm.com/docs/git-diff#generate_patch_text_with_p
 func GetHunkHeaders(r io.Reader) ([]*DiffFileHunkHeaders, error) {
-	scanner := bufio.NewScanner(r)
+	bufrd := bufio.NewReader(r)
 
 	var currentFile *DiffFileHunkHeaders
 	var result []*DiffFileHunkHeaders
 
-	for scanner.Scan() {
-		line := scanner.Text()
+	for {
+		// Consume the line but get only the first 4K of it...
+		// We're interested only in the hunk headers anyway, and they are never longer than this.
+		line, err := readLinePrefix(bufrd, 4096)
+		if err != nil && err != io.EOF { //nolint:errorlint
+			return nil, err
+		}
+
+		if len(line) == 0 {
+			break
+		}
 
 		if h, ok := ParseDiffFileHeader(line); ok {
 			if currentFile != nil {
@@ -79,13 +88,38 @@ func GetHunkHeaders(r io.Reader) ([]*DiffFileHunkHeaders, error) {
 		}
 	}
 
-	if err := scanner.Err(); err != nil {
-		return nil, err
-	}
-
 	if currentFile != nil {
 		result = append(result, currentFile)
 	}
 
 	return result, nil
 }
+
+// readLinePrefix will consume the entire line from the reader,
+// but will return only the first maxLen bytes from it - the rest is discarded.
+// Returns io.EOF when the end of the input has been reached.
+func readLinePrefix(br *bufio.Reader, maxLen int) (line string, err error) {
+	for {
+		var raw []byte
+		var isPrefix bool
+
+		raw, isPrefix, err = br.ReadLine()
+		if err != nil && err != io.EOF { //nolint:errorlint
+			return "", err
+		}
+
+		if needMore := maxLen - len(line); needMore > 0 {
+			if len(raw) > needMore {
+				line += string(raw[:needMore])
+			} else {
+				line += string(raw)
+			}
+		}
+
+		if !isPrefix || len(raw) == 0 {
+			break
+		}
+	}
+
+	return line, err
+}

git/parser/diff_headers_test.go

@@ -15,6 +15,8 @@
 package parser
 
 import (
+	"bufio"
+	"io"
 	"strings"
 	"testing"
 
@@ -106,3 +108,120 @@ index f043b93..0000000
 		t.Errorf(diff)
 	}
 }
+
+func TestReadLinePrefix(t *testing.T) {
+	const maxLen = 256
+	tests := []struct {
+		name    string
+		wf      func(w io.Writer)
+		expLens []int
+	}{
+		{
+			name:    "empty",
+			wf:      func(io.Writer) {},
+			expLens: nil,
+		},
+		{
+			name: "single",
+			wf: func(w io.Writer) {
+				_, _ = w.Write([]byte("aaa"))
+			},
+			expLens: []int{3},
+		},
+		{
+			name: "single-eol",
+			wf: func(w io.Writer) {
+				_, _ = w.Write([]byte("aaa\n"))
+			},
+			expLens: []int{3},
+		},
+		{
+			name: "two-lines",
+			wf: func(w io.Writer) {
+				_, _ = w.Write([]byte("aa\nbb"))
+			},
+			expLens: []int{2, 2},
+		},
+		{
+			name: "two-lines-crlf",
+			wf: func(w io.Writer) {
+				_, _ = w.Write([]byte("aa\r\nbb\r\n"))
+			},
+			expLens: []int{2, 2},
+		},
+		{
+			name: "empty-line",
+			wf: func(w io.Writer) {
+				_, _ = w.Write([]byte("aa\n\ncc"))
+			},
+			expLens: []int{2, 0, 2},
+		},
+		{
+			name: "too-long",
+			wf: func(w io.Writer) {
+				for i := 0; i < maxLen; i++ {
+					_, _ = w.Write([]byte("a"))
+				}
+				_, _ = w.Write([]byte("\n"))
+				for i := 0; i < maxLen*2; i++ {
+					_, _ = w.Write([]byte("b"))
+				}
+				_, _ = w.Write([]byte("\n"))
+				for i := 0; i < maxLen/2; i++ {
+					_, _ = w.Write([]byte("c"))
+				}
+				_, _ = w.Write([]byte("\n"))
+			},
+			expLens: []int{maxLen, maxLen, maxLen / 2},
+		},
+		{
+			name: "overflow-buffer",
+			wf: func(w io.Writer) {
+				for i := 0; i < bufio.MaxScanTokenSize+1; i++ {
+					_, _ = w.Write([]byte("a"))
+				}
+				_, _ = w.Write([]byte("\n"))
+				for i := 0; i < bufio.MaxScanTokenSize*2; i++ {
+					_, _ = w.Write([]byte("b"))
+				}
+				_, _ = w.Write([]byte("\n"))
+				for i := 0; i < bufio.MaxScanTokenSize; i++ {
+					_, _ = w.Write([]byte("c"))
+				}
+			},
+			expLens: []int{maxLen, maxLen, maxLen},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			pr, pw := io.Pipe()
+			defer pr.Close()
+
+			go func() {
+				test.wf(pw)
+				_ = pw.Close()
+			}()
+
+			br := bufio.NewReader(pr)
+
+			for i, expLen := range test.expLens {
+				expLine := strings.Repeat(string(rune('a'+i)), expLen)
+				line, err := readLinePrefix(br, maxLen)
+				if err != nil && err != io.EOF { //nolint:errorlint
+					t.Errorf("got error: %s", err.Error())
+					return
+				}
+				if want, got := expLine, line; want != got {
+					t.Errorf("line %d mismatch want=%s got=%s", i, want, got)
+					return
+				}
+			}
+
+			line, err := readLinePrefix(br, maxLen)
+			if line != "" || err != io.EOF { //nolint:errorlint
+				t.Errorf("expected empty line and EOF but got: line=%s err=%v", line, err)
+			}
+		})
+	}
+}