Re: fewer forks during shell detection

[Eric Blake]

Eric Blake <ebb9 <at> byu.net> writes:

> or must we use (./foo) in order to squelch messages from some shell in the
>  case where ./foo is not executable or not a valid binary image?  I'm
> asking because _AS_RUN is currently using the subshell, after already
> verifying that 'test -f shell' passes (but doesn't do 'test -x shell'),
> and I'd like to remove the subshell if it is safe.

Since older shells tend to fork in order to implement pipes, and the 
redirection occurred outside a compound command (that is, we are worried 
about '{ echo | ($shell); } 2>/dev/null'), it seems that the redirection will 
have occurred prior to the pipe, so I went ahead and removed that subshell.

> Here's my proposed patch.  I've tested that $as_suggested has the desired 
> value, and that both bash and pdksh now pass the LINENO test, while ash fails 
> and proceeds to re-exec with bash.

With a bit more testing, here's what I have pushed:


From: Eric Blake <address@hidden>
Date: Wed, 29 Oct 2008 10:39:53 -0600
Subject: [PATCH] Fix LINENO detection to work around bash and pdksh limitations.

* lib/m4sugar/m4sh.m4 (_AS_LINENO_WORKS): Enhance the test, so
that we can choose which of two tests to trust.
(_AS_RUN): Set flag when alternate shell is running.
(_AS_DETECT_EXPAND): New macro.
(_AS_DETECT_BETTER_SHELL): Use it to massage LINENO tests.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog           |   11 ++++++++++-
 lib/m4sugar/m4sh.m4 |   45 +++++++++++++++++++++++++++++++++++----------
 2 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 682b919..5ee5392 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,15 @@
 2008-10-29  Eric Blake  <address@hidden>
 
-       Mention known zsh bug.
+       Fix LINENO detection to work around bash and pdksh limitations.
+       * lib/m4sugar/m4sh.m4 (_AS_LINENO_WORKS): Enhance the test, so
+       that we can choose which of two tests to trust.
+       (_AS_RUN): Set flag when alternate shell is running.
+       (_AS_DETECT_EXPAND): New macro.
+       (_AS_DETECT_BETTER_SHELL): Use it to massage LINENO tests.
+
+2008-10-29  Eric Blake  <address@hidden>
+
+       Mention proper fix for zsh users.
        * lib/m4sugar/m4sh.m4 (_AS_DETECT_BETTER_SHELL): Recommend zsh
        version known to work.
        Suggested by Paolo Bonzini.
diff --git a/lib/m4sugar/m4sh.m4 b/lib/m4sugar/m4sh.m4
index 341a289..22e7e3e 100644
--- a/lib/m4sugar/m4sh.m4
+++ b/lib/m4sugar/m4sh.m4
@@ -153,6 +153,22 @@ m4_define([AS_COPYRIGHT],
 $1], [^], address@hidden:@ ])])])
 
 
+# _AS_DETECT_EXPAND(VAR, SET)
+# ---------------------------
+# Assign the contents of VAR from the contents of SET, expanded in such
+# a manner that VAR can be passed to _AS_RUN.  In order to make
+# _AS_LINENO_WORKS operate correctly, we must specially handle the
+# first instance of $LINENO within any line being expanded (the first
+# instance is important to tests using the current shell, leaving
+# remaining instances for tests using a candidate shell).  Bash loses
+# track of line numbers if a double quote contains a newline, hence,
+# we must piece-meal the assignment of VAR such that $LINENO expansion
+# occurs in a single line.
+m4_define([_AS_DETECT_EXPAND],
+[$1="m4_bpatsubst(m4_dquote(AS_ESCAPE(m4_expand(m4_set_contents([$2], [
+])))), [\\\$LINENO\(.*\)$], [";$1=$$1$LINENO;$1=$$1"\1])"])
+
+
 # _AS_DETECT_REQUIRED(TEST)
 # -------------------------
 # Refuse to execute under a shell that does not pass the given TEST.
@@ -198,10 +214,8 @@ dnl Remove any tests from suggested that are also required
 [m4_pushdef([AS_EXIT], [exit m4_default([$1], 1)])]dnl
 [if test "x$CONFIG_SHELL" = x; then
   as_bourne_compatible="AS_ESCAPE(m4_expand([_AS_BOURNE_COMPATIBLE]))"
-  as_required="AS_ESCAPE(m4_expand(m4_set_contents([_AS_DETECT_REQUIRED_BODY],
-    m4_newline)))"
-  as_suggested="AS_ESCAPE(m4_expand(m4_set_contents
([_AS_DETECT_SUGGESTED_BODY],
-    m4_newline)))"
+  _AS_DETECT_EXPAND([as_required], [_AS_DETECT_REQUIRED_BODY])
+  _AS_DETECT_EXPAND([as_suggested], [_AS_DETECT_SUGGESTED_BODY])
   AS_IF([_AS_RUN(["$as_required"])],
        [as_have_required=yes],
        [as_have_required=no])
@@ -363,8 +377,9 @@ m4_default_quoted([$4], [M4SH-INIT-FN]))])])
 # ----------------------
 # Run TEST under the current shell (if one parameter is used)
 # or under the given SHELL, protecting it from syntax errors.
+# Set as_run in order to assist _AS_LINENO_WORKS.
 m4_define([_AS_RUN],
-[m4_ifval([$2], [{ $as_echo "$as_bourne_compatible"$1 | ($2); }],
+[m4_ifval([$2], [{ $as_echo "$as_bourne_compatible"$1 | as_run=a $2; }],
                [(eval $1)]) 2>/dev/null])
 
 
@@ -956,12 +971,22 @@ m4_defun([_AS_ME_PREPARE],
 # cause "bash -c '_ASLINENO_WORKS'" to fail (with Bash 2.05, anyway),
 # but that bug is irrelevant to our use of LINENO.  We can't use
 # AS_VAR_ARITH, as this is expanded prior to shell functions.
+#
+# Testing for LINENO support is hard; we use _AS_LINENO_WORKS inside
+# _AS_RUN, which sometimes eval's its argument (pdksh gives false
+# negatives if $LINENO is expanded by eval), and sometimes passes the
+# argument to another shell (if the current shell supports LINENO,
+# then expanding $LINENO prior to the string leads to false
+# positives).  Hence, we perform two tests, and coordinate with
+# _AS_DETECT_EXPAND (which ensures that only the first of two LINENO
+# is expanded in advance) and _AS_RUN (which sets $as_run to 'a' when
+# handing the test to another shell), so that we know which test to
+# trust.
 m4_define([_AS_LINENO_WORKS],
-[
-  as_lineno_1=$LINENO
-  as_lineno_2=$LINENO
-  test "x$as_lineno_1" != "x$as_lineno_2" &&
-  test "x`expr $as_lineno_1 + 1`" = "x$as_lineno_2"])
+[  as_lineno_1=$LINENO as_lineno_1a=$LINENO
+  as_lineno_2=$LINENO as_lineno_2a=$LINENO
+  eval 'test "x$as_lineno_1'$as_run'" != "x$as_lineno_2'$as_run'" &&
+  test "x`expr $as_lineno_1'$as_run' + 1`" = "x$as_lineno_2'$as_run'"'])
 
 
 # _AS_LINENO_PREPARE
-- 
1.6.0.2