Go Read 接口的错误处理
背景
io.Reader 和 binary.Read 的错误返回方式不直观,需要理解其特定约定才能正确处理。
Reader 接口约定
看 Reader 的接口定义和注释:
// Reader is the interface that wraps the basic Read method.
//
// Read reads up to len(p) bytes into p. It returns the number of bytes
// read (0 <= n <= len(p)) and any error encountered. Even if Read
// returns n < len(p), it may use all of p as scratch space during the call.
// If some data is available but not len(p) bytes, Read conventionally
// returns what is available instead of waiting for more.
//
// When Read encounters an error or end-of-file condition after
// successfully reading n > 0 bytes, it returns the number of
// bytes read. It may return the (non-nil) error from the same call
// or return the error (and n == 0) from a subsequent call.
// An instance of this general case is that a Reader returning
// a non-zero number of bytes at the end of the input stream may
// return either err == EOF or err == nil. The next Read should
// return 0, EOF.
//
// Callers should always process the n > 0 bytes returned before
// considering the error err. Doing so correctly handles I/O errors
// that happen after reading some bytes and also both of the
// allowed EOF behaviors.
//
// If len(p) == 0, Read should always return n == 0. It may return a
// non-nil error if some error condition is known, such as EOF.
//
// Implementations of Read are discouraged from returning a
// zero byte count with a nil error, except when len(p) == 0.
// Callers should treat a return of 0 and nil as indicating that
// nothing happened; in particular it does not indicate EOF.
//
// Implementations must not retain p.
type Reader interface {
Read(p []byte) (n int, err error)
}
关键点:
- 返回
(n, err),其中n > 0时必须处理返回的数据,即使同时返回了错误 - 错误可能在当前调用返回,或延迟到下一次调用时返回(此时
n = 0) - 不应该返回
n = 0和err = nil的组合(除非len(p) == 0),这表示"什么都没发生"
binary.Read 的特殊约定
// Read reads structured binary data from r into data.
// Data must be a pointer to a fixed-size value or a slice
// of fixed-size values.
// Bytes read from r are decoded using the specified byte order
// and written to successive fields of the data.
// When decoding boolean values, a zero byte is decoded as false, and
// any other non-zero byte is decoded as true.
// When reading into structs, the field data for fields with
// blank (_) field names is skipped; i.e., blank field names
// may be used for padding.
// When reading into a struct, all non-blank fields must be exported
// or Read may panic.
//
// The error is [io.EOF] only if no bytes were read.
// If an [io.EOF] happens after reading some but not all the bytes,
// Read returns [io.ErrUnexpectedEOF].
func Read(r io.Reader, order ByteOrder, data any)
关键点:
io.EOF只在完全没有读到数据时返回- 如果读到了部分数据后遇到 EOF,返回
io.ErrUnexpectedEOF
最后修改于 2025-05-20