Re: Portability problems in autoconf manual

[Eric Blake]

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[moving to autoconf-patches, replies can drop autoconf]

According to address@hidden on 4/29/2009 3:40 AM:
> Hi.

Hi Jens, and sorry it has taken me so long to respond to your suggestions.

> There it says in section "Limitation of Shell Builtins":
> 
>   Posix says that ‘trap - 1 2 13 15’ resets the traps for the specified 
> signals to
>   their default values, but many common shells (e.g., Solaris /bin/sh) 
> misinterpret
>   this and attempt to execute a “command” named - when the specified 
>   conditions arise. There is no portable workaround, except for ‘trap - 0’, 
> for
>   which ‘trap ’’ 0’ is a portable substitute.
> 
> I do not entirely agree with the latter statement.  It seems that 'trap 1 2 
> 13 15' (without any command) reset the traps in a reasonably portable way, as 
> documented also in the sh man pages. I tested the following on AIX, HP-UX 
> (SPARC & x86-64), HP-UX (PA-RISC & IA64), Tru64, Linux (x86, x86-64, IA64, 
> PPC) (yes, I wanted to show off a bit):

As was pointed out in the thread, POSIX 2008 requires this behavior, but
it is not yet portable (at least dash fails this, so your tests were not
exhaustive).

> 
> On Solaris 9, /bin/sh does not execute a trap on exit if in a parenthesised 
> sub-shell if the last command is not a built-in:
> 
>   $ ( trap "echo foo" 0; :; true )
>   $ ( trap "echo foo" 0; true; : )
>   foo
> 
> Of course, the error diagnosis may not entirely precise. Better it would be 
> to say: "On Solaris 9, /bin/sh may not execute a trap on exit if the trap is 
> defined in a parenthesised sub-shell."
> 
> A workaround may be to always exit explicitly with "exit $?" from the 
> sub-shell.

I added an example of this.

> Another reason to be careful with "set -e" on Solaris 10: A failing command 
> substitution may not be caught by any means if option "-e" is set. The shell 
> bails out immediately:
> 
>   $ ( set -e; foo=`false` || echo "foo"; echo "bar" ); echo "baz"
>   baz

I've mentioned this.

> Older Bashes (mine is 2.05b.0(1)) may forget exit values if there is a trap 
> on exit set and if the trap consists of one succeeding external command only. 
>  Seems to be fixed in latest Bashes.
> 
>   $ /bin/bash -c 'trap "/bin/true" 0; exit 2'; echo $?
>   0
>   $ /bin/bash -c 'trap ":; /bin/true" 0; exit 2'; echo $?
>   2
> 
> I hit that problem when I had a "rm -f " clean-up trap in a canned sequence 
> in a makefile, where the sequence failed but "make" did not take any notice 
> of that fact due to this bug ... not too exotic an use-case, I think.

Also good to mention, especially since it has a workaround.

Here's what I'm applying:

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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From 16a04c38e16cfb451bdfe561d2856068aa28c2a2 Mon Sep 17 00:00:00 2001
From: Eric Blake <address@hidden>
Date: Sat, 6 Jun 2009 22:03:16 -0600
Subject: [PATCH] Improve documentation on trap pitfalls.

* doc/autoconf.texi (Limitations of Builtins) <trap>: Mention new
Posix 2008 requirement on trap, and dash bug in implementing it.
Mention various shell bugs with traps defined inside subshells.
Mention older bash limitation with single-command exit trap.
<set>: Mention another 'set -e' limitation.
Reported by Jens Schmidt.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog         |    8 ++++++++
 doc/autoconf.texi |   41 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 48 insertions(+), 1 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 9a7ccf7..a9bd462 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,13 @@
 2009-06-06  Eric Blake  <address@hidden>

+       Improve documentation on trap pitfalls.
+       * doc/autoconf.texi (Limitations of Builtins) <trap>: Mention new
+       Posix 2008 requirement on trap, and dash bug in implementing it.
+       Mention various shell bugs with traps defined inside subshells.
+       Mention older bash limitation with single-command exit trap.
+       <set>: Mention another 'set -e' limitation.
+       Reported by Jens Schmidt.
+
        Avoid ambiguous fallback in AC_PROG_CXX, AC_PROG_OBJC.
        * lib/autoconf/c.m4 (AC_PROG_CXX, AC_PROG_OBJC): Use 'no', rather
        than an ambiguous compiler name, when no compiler is present.
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 215c864..81bce07 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -16188,6 +16188,16 @@ Limitations of Builtins
 "$foo"; then exit 1; fi} rather than @samp{test -n "$foo" && exit 1}.
 Another possibility is to warn @acronym{BSD} users not to use @samp{sh -e}.

+When @samp{set -e} is in effect, a failed command substitution in
+Solaris @command{/bin/sh} cannot be ignored, even with @samp{||}.
+
address@hidden
+$ @kbd{/bin/sh -c 'set -d; foo=`false` || echo foo; echo bar'}
+$ @kbd{bash -c 'set -d; foo=`false` || echo foo; echo bar'}
+foo
+bar
address@hidden example
+
 Portable scripts should not use @samp{set -e} if @command{trap} is used
 to install an exit handler.  This is because Tru64/OSF 5.1 @command{sh}
 sometimes enters the trap handler with the exit status of the command
@@ -16345,7 +16355,11 @@ Limitations of Builtins
 specified signals to their default values, but many common shells (e.g.,
 Solaris @command{/bin/sh}) misinterpret this and attempt to execute a
 ``command'' named @command{-} when the specified conditions arise.
-There is no portable workaround, except for @samp{trap - 0}, for which
+Posix 2008 also added a requirement to support @samp{trap 1 2 13 15} to
+reset traps, as this is supported by a larger set of shells, but there
+are still shells like @command{dash} that mistakenly try to execute
address@hidden instead of resetting the traps.  Therefore, there is no
+portable workaround, except for @samp{trap - 0}, for which
 @samp{trap '' 0} is a portable substitute.

 Although Posix is not absolutely clear on this point, it is widely
@@ -16390,6 +16404,31 @@ Limitations of Builtins
 @noindent
 Fortunately, this bug only affects @command{trap}.

+Several shells fail to execute an exit trap that is defined inside a
+subshell, when the last command of that subshell is not a builtin.  A
+workaround is to use @samp{exit $?} as the shell builtin.
+
address@hidden
+$ @kbd{bash -c '(trap "echo hi" 0; /bin/true)'}
+hi
+$ @kbd{/bin/sh -c '(trap "echo hi" 0; /bin/true)'}
+$ @kbd{/bin/sh -c '(trap "echo hi" 0; /bin/true; exit $?)'}
+hi
address@hidden example
+
address@hidden
+Likewise, older implementations of @command{bash} failed to preserve
address@hidden across an exit trap consisting of a single cleanup command.
+
address@hidden
+$ @kbd{bash -c 'trap "/bin/true" 0; exit 2'; echo $?}
+2
+$ @kbd{bash-2.05b -c 'trap "/bin/true" 0; exit 2'; echo $?}
+0
+$ @kbd{bash-2.05b -c 'trap ":; /bin/true" 0; exit 2'; echo $?}
+2
address@hidden example
+
 @item @command{true}
 @c -----------------
 @prindex @command{true}
-- 
1.6.3.rc3.2.g4b51