svn commit: r358152 - head/bin/sh

[Antoine Brodin]

On Thu, Feb 20, 2020 at 4:01 AM Hiroki Sato <hrs at freebsd.org> wrote:
>
> Author: hrs
> Date: Thu Feb 20 03:01:27 2020
> New Revision: 358152
> URL: https://svnweb.freebsd.org/changeset/base/358152
>
> Log:
>   Improve performance of "read" built-in command when using a seekable
>   fd.
>
>   The read built-in command calls read(2) with a 1-byte buffer because
>   newline characters need to be detected even on a byte stream which
>   comes from a non-seekable file descriptor.  Because of this, the
>   following script calls >6,000 read(2) to show a 6KiB file:
>
>    while read IN; do echo "$IN"; done < /COPYRIGHT
>
>   When the input byte stream is seekable, it is possible to read a data
>   block and then reposition the file pointer to where a newline
>   character found.  This change adds a small buffer to do this and
>   reduces the number of read(2) calls.
>
>   Theoretically, multiple built-in commands reading the same seekable
>   byte stream in a single pipe chain can share the buffer.  However,
>   this change just makes a single invocation of the read built-in
>   allocate a buffer and deallocate it every time for simplicity.
>   Although this causes read(2) to read the same regions multiple times,
>   the performance penalty should be small compared to the reduction of
>   read(2) calls.
>
>   Reviewed by:          jilles
>   MFC after:            1 week
>   Differential Revision:        https://reviews.freebsd.org/D23747

This seems to be broken on at least i386.
Please either fix or revert.

Antoine (with hat: portmgr)