internal/runtime/cgroup: lineReader fuzz test

The original unit test is converted to a fuzz test, to be more confident
on future refactors. All sub-tests are converted to seed corpus.

Change-Id: Id0c167ed47729a00ea0614d17746ddcc284697d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/723581
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/runtime/cgroup/line_reader_test.go

@@ -11,21 +11,21 @@ import (
 	"testing"
 )
 
-func TestLineReader(t *testing.T) {
 type nextLine struct {
 	line       string
 	incomplete bool // next call before this line should return incomplete
 }
-	complete := func(s string) nextLine {
+
+func complete(s string) nextLine {
 	return nextLine{line: s}
 }
-	incomplete := func(s string) nextLine {
+func incomplete(s string) nextLine {
 	return nextLine{line: s, incomplete: true}
 }
 
 const scratchSize = 8
 
-	tests := []struct {
+var readerTests = []struct {
 	name     string
 	contents string
 	want     []nextLine
@@ -122,10 +122,9 @@ func TestLineReader(t *testing.T) {
 	},
 }
 
-	for _, tc := range tests {
+func readString(contents string) func(fd int, b []byte) (int, uintptr) {
-		t.Run(tc.name, func(t *testing.T) {
+	r := strings.NewReader(contents)
-			r := strings.NewReader(tc.contents)
+	return func(fd int, b []byte) (int, uintptr) {
-			read := func(fd int, b []byte) (int, uintptr) {
 		n, err := r.Read(b)
 		if err != nil && err != io.EOF {
 			const dummyErrno = 42
@@ -133,9 +132,13 @@ func TestLineReader(t *testing.T) {
 		}
 		return n, 0
 	}
+}
 
+func TestLineReader(t *testing.T) {
+	for _, tc := range readerTests {
+		t.Run(tc.name, func(t *testing.T) {
 			var scratch [scratchSize]byte
-			l := cgroup.NewLineReader(0, scratch[:], read)
+			l := cgroup.NewLineReader(0, scratch[:], readString(tc.contents))
 
 			var got []nextLine
 			for {
@@ -168,3 +171,39 @@ func TestLineReader(t *testing.T) {
 		})
 	}
 }
+
+func FuzzLineReader(f *testing.F) {
+	for _, tc := range readerTests {
+		f.Add(tc.contents)
+	}
+	f.Fuzz(func(t *testing.T, input string) {
+		scratch := make([]byte, scratchSize)
+		reader := cgroup.NewLineReader(0, scratch, readString(input))
+		for expected := range strings.Lines(input) {
+			err := reader.Next()
+			line := reader.Line()
+
+			var expectedErr error
+			if len(expected) > scratchSize {
+				expected = expected[:scratchSize]
+				expectedErr = cgroup.ErrIncompleteLine
+			} else if expected[len(expected)-1] == '\n' {
+				expected = expected[:len(expected)-1]
+			} else {
+				expectedErr = cgroup.ErrIncompleteLine
+			}
+
+			if err != expectedErr {
+				t.Fatalf("got err %v, want %v", err, expectedErr)
+			}
+
+			if string(line) != expected {
+				t.Fatalf("got %q, want %q", string(line), expected)
+			}
+		}
+		err := reader.Next()
+		if err != cgroup.ErrEOF {
+			t.Fatalf("got %v, want EOF", err)
+		}
+	})
+}