Re: Crash fix on Windows/MinGW

[Paul Eggert]

Stepan Kasal <address@hidden> writes:

> What's the difference between:
>
> exec 5>foo
> cat >&5 <<END
> Prologue
> END
>
> and
>
> cat >foo <<END
> Prologue
> END
> exec 5>>foo

It affects whether O_APPEND is set in the file description
corresponding to file descriptor 5.  O_APPEND causes all output to be
appended to the file, regardless of where the current seek position is.

> Can you imagine an Autoconf bug which would be caused by this?

Yes.  In the former case a misbehaving script can lose parts of the log.
The most common error is something like this, executed after the above code:

    echo log1 >>foo
    echo log2 >&5

In the former case, "log1" will be overwritten and lost, which is bad.
Even in the latter case, a misbehaving script can lose parts of the
log by invoking fcntl to clear O_APPEND.  But this is far, far less
likely in practice: I've never seen it, but I have seen problems like
the above in the former case.

> The reason is that I think we are in a ``freeze'' now.

We are not adding features, but we are fixing bugs.  At this point, we
need not limit ourselves to bugs reported by users; we can fix bugs
that we find ourselves.  The old code wasn't consistent about using
">>" to append to logs leading problems like the one mentioned above.
The new code is consistent.

> (A particular concern/superstition is about 5>&- to close a file descriptor;

Fair enough.  I'll remove that.  Though the usage is quite portable in
practice: it was in 7th edition Unix and I've never seen a shell where
it didn't work.

> A patch is attached below.  What do you think about it?

Looks OK.  Thanks.  You did a deeper analysis of the problem than I
did.  I installed it, along with the 5>&- removal, as follows (with a
slight reworking of comments):

2006-04-16  Paul Eggert  <address@hidden>

        * lib/autoconf/general.m4 (_AC_INIT_CONFIG_LOG): Don't
        use ">&-" since we're only 99.999% sure that this is portable,
        and since the MinGW bug is fixed in a different way.
        * lib/autotest/general.m4 (AT_INIT): Likewise.

2006-04-16  Stepan Kasal  <address@hidden>

        * lib/autoconf/status.m4 (_AC_OUTPUT_CONFIG_STATUS): Handle --recheck
        before opening config.log, to avoid hitting a bug on MinGW.

--- lib/autoconf/general.m4     15 Apr 2006 01:13:32 -0000      1.907
+++ lib/autoconf/general.m4     17 Apr 2006 06:01:22 -0000
@@ -1103,7 +1103,6 @@ fi])dnl
 m4_define([_AC_INIT_CONFIG_LOG],
 [m4_divert_text([INIT_PREPARE],
 [m4_define([AS_MESSAGE_LOG_FD], 5)dnl
-exec AS_MESSAGE_LOG_FD>&- # Work around a MinGW bug.
 cat >config.log <<_ACEOF
 This file contains any messages produced by compilers while
 running configure, to aid debugging if configure makes a mistake.
--- lib/autotest/general.m4     15 Apr 2006 01:13:33 -0000      1.210
+++ lib/autotest/general.m4     17 Apr 2006 06:01:22 -0000
@@ -601,7 +601,6 @@ m4_define([AS_MESSAGE_LOG_FD], [5])
 if $at_debug_p; then
   at_suite_log=/dev/null
 else
-  exec AS_MESSAGE_LOG_FD>&- # Work around a MinGW bug.
   : >"$at_suite_log"
 fi
 exec AS_MESSAGE_LOG_FD>>"$at_suite_log"
--- lib/autoconf/status.m4      12 Apr 2006 20:40:21 -0000      1.95
+++ lib/autoconf/status.m4      17 Apr 2006 06:01:22 -0000
@@ -1270,6 +1270,19 @@ if $ac_cs_silent; then
   ac_configure_extra_args="$ac_configure_extra_args --silent"
 fi
 
+_ACEOF
+cat >>$CONFIG_STATUS <<_ACEOF
+dnl Check this before opening the log, to avoid a bug on MinGW,
+dnl which prohibits the recursive instance from truncating an open log.
+if \$ac_cs_recheck; then
+  echo "running CONFIG_SHELL=$SHELL $SHELL $[0] " $ac_configure_args 
\$ac_configure_extra_args " --no-create --no-recursion" >&AS_MESSAGE_FD
+  CONFIG_SHELL=$SHELL
+  export CONFIG_SHELL
+  exec $SHELL "$[0]" $ac_configure_args \$ac_configure_extra_args --no-create 
--no-recursion
+fi
+
+_ACEOF
+cat >>$CONFIG_STATUS <<\_ACEOF
 dnl Open the log:
 m4_rename([_AC_save_AS_MESSAGE_LOG_FD], [AS_MESSAGE_LOG_FD])dnl
 exec AS_MESSAGE_LOG_FD>>config.log
@@ -1281,12 +1294,6 @@ exec AS_MESSAGE_LOG_FD>>config.log
 
 _ACEOF
 cat >>$CONFIG_STATUS <<_ACEOF
-if \$ac_cs_recheck; then
-  echo "running CONFIG_SHELL=$SHELL $SHELL $[0] " $ac_configure_args 
\$ac_configure_extra_args " --no-create --no-recursion" >&AS_MESSAGE_FD
-  CONFIG_SHELL=$SHELL
-  export CONFIG_SHELL
-  exec $SHELL "$[0]" $ac_configure_args \$ac_configure_extra_args --no-create 
--no-recursion
-fi
 m4_ifdef([_AC_OUTPUT_COMMANDS_INIT],
 [#
 # INIT-COMMANDS