command substitutions in $PS4

[Stephane Chazelas <stephane.chazelas@gmail.com>]

Hello.

[tested on current git head
f6d4def4e27b13fab174e948b94cd10550d3e10e]

Command substitution doesn't seem to work in $PS4 (used for
xtrace prompt):

$ PS4='$(date +%T)> ' dash -xc :
dash: 1: Syntax error: end of file unexpected (expecting ")")

And with the old syntax:

$ PS4='`date +%T`> ' dash -xc :
> :

And with more than one command:

$ PS4='`date +%T`> ' dash -xc ':;:'
> :

dash doesn't return and seems to go in a forking loop,
presumably because the `date` there triggers another PS4
expansion and so on recursively

If I prevent the recursion with:

--- a/src/eval.c
+++ b/src/eval.c
@@ -776,7 +776,15 @@ evalcommand(union node *cmd, int flags)
 		int sep;
 
 		out = &preverrout;
+
+		/*
+		 * reset xflag temporarily for command substitutions performed
+		 * upon $PS4 expansion
+		 */
+		xflag = 0;
 		outstr(expandstr(ps4val()), out);
+		xflag = 1;
+
 		sep = 0;
 		sep = eprintlist(out, varlist.list, sep);
 		eprintlist(out, arglist.list, sep);

$ PS4='`date +%T`> ' ./src/dash -xc ':;sleep 1; date'
> :
21:43:47> sleep 1
21:43:48> date
Mon Nov 17 21:43:48 GMT 2014

The command substitution still fails upon the first expansion
only. I quickly gave up trying to find out why as I found the
code there hard to follow.

-- 
Stephane