SBCL

input and output buffers get out of sync in io streams

Bug #1600610 reported by Guillaume LE VAILLANT on 2016-07-10

This bug affects 1 person

Affects		Status	Importance	Assigned to	Milestone
	SBCL	Triaged	High	Unassigned

Bug Description

(with-open-file (stream "/tmp/foo"
                        :direction :io
                        :if-exists :overwrite
                        :if-does-not-exist :create)
  (write-string "abc" stream)
  (file-position stream 0)
  (write-char #\x stream)
  (read-char stream))
=> #\a

See original description

Tags:

Stas Boukarev (stassats) on 2016-07-11

summary:	- read-sequence can move file-position beyond end of file + input and output buffers get out of sync in io streams
description:	updated
Changed in sbcl:
status:	New → Triaged
importance:	Undecided → High
tags:	added: streams

Revision history for this message

Guillaume LE VAILLANT (guillaume-le-vaillant) wrote on 2016-10-06:

Flushing the output buffer by hand before reading seems to work.

(with-open-file (stream "string.txt"
                        :direction :io
                        :if-exists :overwrite
                        :if-does-not-exist :create)
  (write-string "abc" stream)
  (file-position stream 0)
  (write-char #\d stream)
  (finish-output stream)
  (read-char stream))
=> #\b

Maybe the code for flushing the output buffer before reading in the case of a file stream opened in io mode is missing...

Revision history for this message

Guillaume LE VAILLANT (guillaume-le-vaillant) wrote on 2016-10-16:

io-fd-stream.patch Edit (1.3 KiB, text/plain)

The attached patch seems to fix the issue. It adds calls to finish-fd-stream-output in input-wrapper and input-wrapper/variable-width. However, I'm not familiar with the internals of SBCL, so it might not be the best place to put them...

Revision history for this message

Stas Boukarev (stassats) wrote on 2016-10-16:

Flushing on each read will kill performance.