EXPERIMENTAL optimisation for “sh -c 'foo'” to equal “sh -c 'exit foo'” iff

several conditions are met as outlined below; for more background, refer to
http://www.FreeBSD.org/cgi/query-pr.cgi?pr=113860

We don’t yet optimise 「% sh -c '{ echo a; sleep 10;}&'; sleep 1; ps T」 so
the FreeBSD® sh approach cannot work for us, but scanning the “sh -c” argu‐
ment for disallowed characters and, if not, setting a flag that enables the
shell to exec the tree when parsed as TCOM *and not c_trap()* was possible.

Disallowed characters are currently C_QUOTE except space, that is:
Tab Newline " # $ & ' ( ) * ; < = > ? [ \ ] ` |

This should catch all cases of magic, variables, subshells, pipelines, etc.

exec.c

@@ -23,7 +23,7 @@
 
 #include "sh.h"
 
-__RCSID("$MirOS: src/bin/mksh/exec.c,v 1.101 2012/10/03 17:24:18 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/exec.c,v 1.102 2012/10/21 21:26:39 tg Exp $");
 
 #ifndef MKSH_DEFAULT_EXECSHELL
 #define MKSH_DEFAULT_EXECSHELL	"/bin/sh"
@@ -604,11 +604,20 @@ comexec(struct op *t, struct tbl * volatile tp, const char **ap,
 			else
 				/* go on, use the builtin */
 				break;
+#endif
+#ifndef MKSH_SMALL
+		} else if (tp->val.f == c_trap) {
+			t->u.evalflags &= ~DOTCOMEXEC;
+			break;
 #endif
 		} else
 			break;
 		tp = findcom(ap[0], fcflags & (FC_BI|FC_FUNC));
 	}
+#ifndef MKSH_SMALL
+	if (t->u.evalflags & DOTCOMEXEC)
+		flags |= XEXEC;
+#endif
 	l_expand = e->loc;
 	if (keepasn_ok && (!ap[0] || (tp && (tp->flag & KEEPASN))))
 		type_flags = 0;

main.c

@@ -34,7 +34,7 @@
 #include <locale.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/main.c,v 1.229 2012/10/21 17:16:45 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/main.c,v 1.230 2012/10/21 21:26:40 tg Exp $");
 
 extern char **environ;
 
@@ -464,6 +464,16 @@ main_init(int argc, const char *argv[], Source **sp, struct block **lp)
 		s = pushs(SSTRINGCMDLINE, ATEMP);
 		if (!(s->start = s->str = argv[argi++]))
 			errorf("%s %s", "-c", "requires an argument");
+#ifndef MKSH_SMALL
+		while (*s->str) {
+			if (*s->str != ' ' && ctype(*s->str, C_QUOTE))
+				break;
+			s->str++;
+		}
+		if (!*s->str)
+			s->flags |= SF_MAYEXEC;
+		s->str = s->start;
+#endif
 #ifdef MKSH_MIDNIGHTBSD01ASH_COMPAT
 		/* compatibility to MidnightBSD 0.1 /bin/sh (kludge) */
 		if (Flag(FSH) && argv[argi] && !strcmp(argv[argi], "--"))
@@ -818,7 +828,9 @@ shell(Source * volatile s, volatile bool toplevel)
 		}
 		t = compile(s, sfirst);
 		sfirst = false;
-		if (t != NULL && t->type == TEOF) {
+		if (!t)
+			goto source_no_tree;
+		if (t->type == TEOF) {
 			if (wastty && Flag(FIGNOREEOF) && --attempts > 0) {
 				shellf("Use 'exit' to leave mksh\n");
 				s->type = SSTDIN;
@@ -838,12 +850,17 @@ shell(Source * volatile s, volatile bool toplevel)
 				break;
 			}
 		}
-		if (t && (!Flag(FNOEXEC) || (s->flags & SF_TTY)))
+#ifndef MKSH_SMALL
+		  else if ((s->flags & SF_MAYEXEC) && t->type == TCOM)
+			t->u.evalflags |= DOTCOMEXEC;
+#endif
+		if (!Flag(FNOEXEC) || (s->flags & SF_TTY))
 			exstat = execute(t, 0, NULL);
 
-		if (t != NULL && t->type != TEOF && interactive && really_exit)
+		if (t->type != TEOF && interactive && really_exit)
 			really_exit = false;
 
+ source_no_tree:
 		reclaim();
 	}
 	quitenv(NULL);

sh.h

@@ -157,7 +157,7 @@
 #endif
 
 #ifdef EXTERN
-__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.592 2012/10/03 17:24:23 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.593 2012/10/21 21:26:41 tg Exp $");
 #endif
 #define MKSH_VERSION "R40 2012/10/03"
 
@@ -1343,6 +1343,9 @@ struct ioword {
 #define DOTEMP	BIT(8)		/* dito: in word part of ${..[%#=?]..} */
 #define DOVACHECK BIT(9)	/* var assign check (for typeset, set, etc) */
 #define DOMARKDIRS BIT(10)	/* force markdirs behaviour */
+#ifndef MKSH_SMALL
+#define DOTCOMEXEC BIT(11)	/* not an eval flag, used by sh -c hack */
+#endif
 
 /*
  * The arguments of [[ .. ]] expressions are kept in t->args[] and flags
@@ -1486,6 +1489,9 @@ struct source {
 #define SF_ALIASEND	BIT(2)	/* faking space at end of alias */
 #define SF_TTY		BIT(3)	/* type == SSTDIN & it is a tty */
 #define SF_HASALIAS	BIT(4)	/* u.tblp valid (SALIAS, SEOF) */
+#ifndef MKSH_SMALL
+#define SF_MAYEXEC	BIT(5)	/* special sh -c optimisation hack */
+#endif
 
 typedef union {
 	int i;