From: ts <decoux@...>
Date: 2008-01-31T22:14:15+09:00
Subject: Re: Core team, I need your help

>>>>> "J" == Jonas Pfenniger <zimbatm@oree.ch> writes:

J> You are right, I have joined the patch to this e-mail.

 Well, with this patch you reverse  [ruby-cvs:17330]

 ruby_scope->local_vars[-1] is where ruby has stored ruby_scope in
 top_local_setup() when it don't have local variable.

 I'll try to explain it in another way

 Reverse (mentally) [ruby-cvs:17330] and look what do top_local_setup()

  * when it has no local variable

 		if (ruby_scope->local_vars) {
		    *vars++ = ruby_scope->local_vars[-1];
		    MEMCPY(vars, ruby_scope->local_vars, VALUE, i);
		    rb_mem_clear(vars+i, len-i);
		}
		else {
		    *vars++ = 0;
		    rb_mem_clear(vars, len);
		}
		ruby_scope->local_vars = vars;
		ruby_scope->flags |= SCOPE_MALLOC;

 it will store 0 in *vars and put (vars + 1) in local_vars

 * now when the GC call obj_free() there is

      case T_SCOPE:
	if (RANY(obj)->as.scope.local_vars &&
            RANY(obj)->as.scope.flags != SCOPE_ALLOCA) {
	    VALUE *vars = RANY(obj)->as.scope.local_vars-1;
	    if (!(RANY(obj)->as.scope.flags & SCOPE_CLONE) && vars[0] == 0)
		RUBY_CRITICAL(free(RANY(obj)->as.scope.local_tbl));
	    if (RANY(obj)->as.scope.flags & SCOPE_MALLOC)
		RUBY_CRITICAL(free(vars));
	}
	break;

 the location vars[0] is the same than *vars in top_local_setup() and the
 GC will free RANY(obj)->as.scope.local_tbl

 Now apply the patch [ruby-cvs:17330]

		if (ruby_scope->local_vars) {
		    *vars++ = ruby_scope->local_vars[-1];
		    MEMCPY(vars, ruby_scope->local_vars, VALUE, i);
		    rb_mem_clear(vars+i, len-i);
		}
		else {
		    *vars++ = (VALUE)ruby_scope;
		    rb_mem_clear(vars, len);
		}
		ruby_scope->local_vars = vars;
		ruby_scope->flags |= SCOPE_MALLOC;

 this time it will store ruby_scope in *vars and it still put (vars + 1) in
 local_vars but the test in obj_free() is not modified and when the GC run
 it will not free scope.local_tbl because in obj_free() you have
 vars[0] == obj != 0

 This is why you see a memory leak



Guy Decoux