acid: fix memory corruption due to gc

we cannot call gc() in execute() because it will gc
anonyous intermediate results which happens when we
construct a list and the elements are calculated by
calling a function thru ocall() which calls execute().

also, the _thiscmd symbol; which is used to keep
a reference to a statement so it wont get garbage
collected; does not work as yyparse() is recursive
(include statements).

we add execrec() function which *only* gets called from
yyparse() when evaluating a statement. it will
keep a stack on the _thiscmd symbol handling the yyparse()
recursion.

we also only call gc() in execrec() before calling
execute(). so execute() will never gc() while evaluating
a statement which prevents the intermediate results
from getting collected.

sys/src/cmd/acid/acid.h

@@ -190,6 +190,7 @@ void	dostop(int);
 Lsym*	enter(char*, int);
 void	error(char*, ...);
 void	execute(Node*);
+void	execrec(Node*);
 void	fatal(char*, ...);
 void	flatten(Node**, Node*);
 void	gc(void);

sys/src/cmd/acid/dbg.y

@@ -50,11 +50,7 @@ prog		:
 
 bigstmnt	: stmnt
 		{
-			/* make stmnt a root so it isn't collected! */
-			mkvar("_thiscmd")->proc = $1;
-			execute($1);
-			mkvar("_thiscmd")->proc = nil;
-			gc();
+			execrec($1);
 			if(interactive)
 				Bprint(bout, "acid: ");
 		}

sys/src/cmd/acid/exec.c

@@ -56,6 +56,32 @@ unwind(void)
 	}
 }
 
+void
+execrec(Node *n)
+{
+	Value *v;
+	Lsym *s;
+
+	/* make node a root so it isn't collected! */
+	s = mkvar("_thiscmd");
+
+	v = gmalloc(sizeof(Value));
+	memset(v, 0, sizeof(Value));
+	v->type = TCODE;
+	v->cc = n;
+	v->pop = s->v;
+
+	s->v = v;
+	s->proc = n;
+
+	gc();
+	execute(n);
+
+	s->proc = s->v->cc;
+	s->v = v->pop;
+	free(v);
+}
+
 void
 execute(Node *n)
 {
@@ -66,7 +92,6 @@ execute(Node *n)
 	Node res, xx;
 	static int stmnt;
 
-	gc();
 	if(gotint)
 		error("interrupted");
 

sys/src/cmd/acid/lex.c

@@ -569,11 +569,11 @@ enter(char *name, int t)
 	s->lexval = t;
 
 	v = gmalloc(sizeof(Value));
-	s->v = v;
-
+	memset(v, 0, sizeof(Value));
 	v->fmt = 'X';
 	v->type = TINT;
-	memset(v, 0, sizeof(Value));
+
+	s->v = v;
 
 	return s;
 }