[SHELL] Optimize dash -c "command" to avoid a fork

On Sun, Apr 10, 2011 at 07:36:49AM +0000, Jonathan Nieder wrote:
> From: Jilles Tjoelker <jilles@stack.nl>
> Date: Sat, 13 Jun 2009 16:17:45 -0500
> 
> This change only affects strings passed to -c, when the -s option is
> not used.
> 
> Use the EV_EXIT flag to inform the eval machinery that the string
> being passed is the entirety of input.  This way, a fork may be
> omitted in many special cases.
> 
> If there are empty lines after the last command, the evalcmd will not
> see the end early enough and forks will not be omitted. The same thing
> seems to happen in bash.
> 
> Example:
>   sh -c 'ps lT'
> No longer shows a shell process waiting for ps to finish.
> 
> [jn: ported from FreeBSD SVN r194128.  Bugs are mine.]
> 
> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

Instead of detecting EOF using the input layer, I'm going to
use the parser instead.  In either case, we always have to read
ahead in order to complete the parsing of the previous node.
Therefore we always know whether there is more to come, except
in the case where we see a newline/semicolon or similar.

For the purposes of sh -c, this should be sufficient.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/eval.h b/src/eval.h
index ac394e8..4e4de30 100644
--- a/src/eval.h
+++ b/src/eval.h
@@ -46,6 +46,10 @@ struct backcmd {		/* result of evalbackcmd */
 	struct job *jp;		/* job structure for command */
 };
 
+/* flags in argument to evaltree */
+#define EV_EXIT 01		/* exit after evaluating tree */
+#define EV_TESTED 02		/* exit status is checked; ignore -e flag */
+
 int evalstring(char *, int);
 union node;	/* BLETCH for ansi C */
 void evaltree(union node *, int);