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package iolimit
import (
"errors"
"io"
)
// ErrExpectedLengthExceeded reports that a stream
// produced bytes beyond the expected length.
var ErrExpectedLengthExceeded = errors.New("iolimit: stream exceeded expected length")
// ExpectLengthReader wraps src and enforces an expected byte length.
//
// It returns io.ErrUnexpectedEOF
// if src ends before expected bytes are read.
// It returns ErrExpectedLengthExceeded
// if reads continue beyond the expected boundary
// and src still produces bytes.
//
// This reader does not drain src on close or at the expected boundary.
// As a result,
// overlength streams are detected only
// when a caller reads at or past the boundary.
func ExpectLengthReader(src io.Reader, expected int) io.Reader {
return &expectLengthReader{
src: src,
remaining: expected,
}
}
type expectLengthReader struct {
src io.Reader
remaining int
}
func (reader *expectLengthReader) Read(dst []byte) (int, error) {
if len(dst) == 0 {
return 0, nil
}
if reader.remaining == 0 {
var probe [1]byte
n, err := reader.src.Read(probe[:])
if n > 0 {
return 0, ErrExpectedLengthExceeded
}
if err == nil {
return 0, nil
}
return 0, err
}
if len(dst) > reader.remaining {
dst = dst[:reader.remaining]
}
n, err := reader.src.Read(dst)
if n > 0 {
reader.remaining -= n
}
if errors.Is(err, io.EOF) {
if reader.remaining > 0 {
return n, io.ErrUnexpectedEOF
}
if n > 0 {
return n, nil
}
return 0, io.EOF
}
return n, err
}
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