}
		if n > 0 || s.ioerr != nil {
			s.e += n
			s.buf[s.e] = sentinel
			return
		}
		// n == 0
	}

	s.buf[s.e] = sentinel
	s.ioerr = io.ErrNoProgress
}

// nextSize returns the next bigger size for a buffer of a given size.
func nextSize(size int) int {
	const min = 4 << 10 // 4K: minimum buffer size

		n, err := b.rd.Read(b.buf[b.w:])
		if n < 0 {
			panic(errNegativeRead)
		}
		b.w += n
		if err != nil {
			b.err = err
			return
		}
		if n > 0 {
			return
		}
	}
	b.err = io.ErrNoProgress
}

func (b *Reader) readErr() error {
	err := b.err
	b.err = nil
	return err
}

// Peek returns the next n bytes without advancing the reader. The bytes stop

// middle of reading a fixed-size block or data structure.
var ErrUnexpectedEOF = errors.New("unexpected EOF")

// ErrNoProgress is returned by some clients of a [Reader] when
// many calls to Read have failed to return any data or error,
// usually the sign of a broken [Reader] implementation.
var ErrNoProgress = errors.New("multiple Read calls return no data or error")

// Reader is the interface that wraps the basic Read method.
//

type errReader struct{}

func (errReader) Read(b []byte) (int, error) {
	return 0, io.ErrNoProgress // some error that is not io.EOF
}

func TestIOError(t *testing.T) {
	s := new(Scanner).Init(errReader{})
	errorCalled := false
	s.Error = func(s *Scanner, msg string) {
		if !errorCalled {
			if want := io.ErrNoProgress.Error(); msg != want {
				t.Errorf("msg = %q, want %q", msg, want)
			}
			errorCalled = true

	}

	// Use ReadFrom to read in some data.
	r := &emptyThenNonEmptyReader{r: strings.NewReader("abcd"), n: 100}
	n2, err := w.ReadFrom(r)
	if n2 != 0 || err != io.ErrNoProgress {
		t.Fatalf("buf.Bytes() returned (%v, %v), want (0, io.ErrNoProgress)", n2, err)
	}
}

type readFromWriter struct {
	buf           []byte
	writeBytes    int
	readFromBytes int
}

				d.mergePassInto(img, imagePass, pass)
			}
		}
	}

	// Check for EOF, to verify the zlib checksum.
	n := 0
	for i := 0; n == 0 && err == nil; i++ {
		if i == 100 {
			return nil, io.ErrNoProgress
		}
		n, err = r.Read(d.tmp[:1])
	}
	if err != nil && err != io.EOF {
		return nil, FormatError(err.Error())
	}
	if n != 0 || d.idatLength != 0 {
		return nil, FormatError("too much pixel data")
	}

		{"Closer", Type, 0},
		{"Copy", Func, 0},
		{"CopyBuffer", Func, 5},
		{"CopyN", Func, 0},
		{"Discard", Var, 16},
		{"EOF", Var, 0},
		{"ErrClosedPipe", Var, 0},
		{"ErrNoProgress", Var, 1},
		{"ErrShortBuffer", Var, 0},
		{"ErrShortWrite", Var, 0},
		{"ErrUnexpectedEOF", Var, 0},
		{"LimitReader", Func, 0},
		{"LimitedReader", Type, 0},
		{"LimitedReader.N", Field, 0},

pkg image/draw, const Src = 1
pkg image/jpeg, const DefaultQuality = 75
pkg io, type ByteWriter interface { WriteByte }
pkg io, type ByteWriter interface, WriteByte(uint8) error
pkg io, var ErrNoProgress error
pkg log, const Ldate = 1
pkg log, const Llongfile = 8
pkg log, const Lmicroseconds = 4
pkg log, const Lshortfile = 16
pkg log, const LstdFlags = 3
pkg log, const Ltime = 2