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Should libedit be preferred to libreadline?

From: Seneca Cunningham <scunning(at)ca(dot)afilias(dot)info>
To: pgsql-hackers(at)postgresql(dot)org
Cc: pgsql-ports(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Should libedit be preferred to libreadline?
Date: 2005-11-21 22:38:17
Message-ID: 43824C59.5060503@ca.afilias.info (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patchespgsql-ports
It would certainly seem so on AIX.

In tracking down why postgres 8.x would segfault on AIX 5.3, it became
apparent to me that libreadline.a is a problematic library to link
against and that libedit.a is preferable (and for reasons other than
that readline is GPL while postgres is BSD-licensed).

With AIX 5, the easiest way to get a shared object is to pass "-bexpall"
to the linker.  This results in all symbols being exported.  The problem
with this is that the linker will export all of libreadline's and
libhistory's symbols.  In the case of libreadline.so.4 (and .5) on AIX 5
this includes symbols like strncpy and memmove, but on .4, not memcpy.
This is likely because libc.a does not export them.

What results from this is that when postgres is linked against readline
on AIX, it gets these memory functions through readline instead of its
own code.  When readline 4.3 is used (what IBM provides in their "AIX
Toolbox for Linux"), postgres is known to crash.  These segfaults (if
postgres was compiled with gcc) have occurred on AIX 5.3ML3, AIX 5.3ML1,
and AIX 5.2ML7.  With readline 5.0, postgres merely gets these functions
through the shared library memory segments instead of the user memory
segments[6].

While it is possible to build libreadline in a manner that doesn't
export strncpy, neither of the prebuilt readlines for AIX 5 that I
checked were both shared and did not export strncpy.  IBM's readline[5]
exports strncpy, UCLA's readline[4] is static.  Building a shared
readline that doesn't export strncpy requires creating export files for
libreadline and libhistory that only list the symbols that they are
supposed to export and editing the shared library Makefile to add the
exports flags to the appropriate linker calls.

Whatever strategy we might take, using readline on AIX requires
considerable trickery and hacking around with the build environments.
Simply put, it's ghastly.

On the other hand, the port of NetBSD's editline that I tried[1] works
without build-hackery to the library and has reasonable exports.  The
only changes to postgres that I needed to make were confined to telling
the configure script to check for libedit before libreadline and adding
a test for histedit.h.  The attached patch contains my modifications.

It is also possible to use a wrapper like rlwrap[2] instead of linking
postgres against libreadline or libedit.

[1] port of NetBSD's editline
    http://www.thrysoee.dk/editline/
[2] rlwrap
    http://utopia.knoware.nl/~hlub/uck/software/
[3] IBM Redbook "AIX 5L Porting Guide", section 9.2
    http://www.redbooks.ibm.com/abstracts/sg246034.html?Open
    http://www.redbooks.ibm.com/redbooks/pdfs/sg246034.pdf
[4] UCLA's readline package
    http://aixpdslib.seas.ucla.edu/packages/readline.html
[5] IBM's readline package
    http://www-03.ibm.com/servers/aix/products/aixos/linux/download.html
[6] IBM Redbook "Developing and Porting C and C++ Applications on AIX",
      page 110
    http://www.redbooks.ibm.com/abstracts/sg245674.html?Open
    http://www.redbooks.ibm.com/redbooks/pdfs/sg245674.pdf

-- 
Seneca Cunningham
scunning(at)ca(dot)afilias(dot)info


Attachment: libedit-pgsql810.patch
Description: text/x-patch (7.1 KB)

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