On Sat, Oct 27, 2018 at 9:29 PM, <bhup...@anvaya.net> wrote:
> Code in question:
>
> // Flush writes any buffered data to the underlying io.Writer.
> func (b *Writer) Flush() error {
> if b.err != nil {
> return b.err
> }
> if b.n == 0 {
> return nil
> }
> n, err := b.wr.Write(b.buf[0:b.n])
> if n < b.n && err == nil {
> err = io.ErrShortWrite
> }
> if err != nil {
> if n > 0 && n < b.n {
> copy(b.buf[0:b.n-n], b.buf[n:b.n])
> }
> b.n -= n
> b.err = err
> return err
> }
> b.n = 0
> return nil
> }
>
> Consider that underlying io.Writer is a TCP socket. Under system load,
> partial write can occur and write() system call does not consider partial
> write as an error (see man 2 write). In other words, if n < b.n, only some
> data got written to the underlying io.Writer and this in itself is not an
> erro. bufio is expected to handle this by keep the bytes not yet written in
> the buffer so that those bytes will get written in future (code above does
> this: copy(b.buf[0:b.n-n], b.buf[n:b.n]). However, the Flush() code is
> buggy (unless I am missing something). It sets b.err to io.ErrShortWrite if
> n < b.n and any further Flush() call by client to continue to write to the
> underlying io.Writer bails out as the first line of this function checks for
> b.err. b.err never gets cleared, and as a result no further writes can
> occur.
>
> I don't see anything else in bufio that would clear b.err. How is this
> supposed to work when underlying io.Writer only write partial data.
You are confusing the write system call with the Write method. The
Write method used to implement io.Writer, as documented at
https://golang.org/pkg/io/#Writer, should either write all the data or
return a non-nil error. Returning io.ErrShortWrite is a check that
the Write method is implemented correctly. When using a TCP socket,
the relevant Write method calls the write system call multiple times,
as needed, to implement this requirement
(https://golang.org/src/internal/poll/fd_unix.go#L253).
Ian
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