builtin: implement a double free detection for v strings

vlib/builtin/string.v

@@ -46,10 +46,18 @@ pub struct string {
 pub:
 	str byteptr // points to a C style 0 terminated string of bytes.
 	len int // the length of the .str field, excluding the ending 0 byte. It is always equal to strlen(.str).
-	is_lit bool
+mut:    
+	is_lit int
 }
 // mut:
 // hash_cache int
+//
+// NB string.is_lit is an enumeration of the following:
+// .is_lit == 0 => a fresh string, should be freed by autofree
+// .is_lit == 1 => a literal string from .rodata, should NOT be freed
+// .is_lit == -98761234 => already freed string, protects against double frees.
+//            ^^^^^^^^^ calling free on these is a bug.
+// Any other value means that the string has been corrupted.
 
 pub struct ustring {
 pub mut:
@@ -109,7 +117,7 @@ pub fn tos_lit(s charptr) string {
 	return string{
 		str: byteptr(s)
 		len: C.strlen(s)
-		is_lit:true
+		is_lit: 1
 	}
 }
 
@@ -1186,10 +1194,15 @@ pub fn (c byte) is_letter() bool {
 }
 
 pub fn (s &string) free() {
-	if s.is_lit || s.len == 0 {
+	if s.is_lit == -98761234 {
+		C.printf('double string.free() detected\n')
+		return
+	}
+	if s.is_lit == 1 || s.len == 0 {
 		return
 	}
 	free(s.str)
+	s.is_lit = -98761234
 }
 
 // all_before('23:34:45.234', '.') == '23:34:45'

vlib/v/gen/str.v

@@ -108,7 +108,7 @@ string _STR_TMP(const char *fmt, ...) {
 	//puts(g_str_buf);
 #endif
 	string res = tos(g_str_buf,  len);
-	res.is_lit = true;
+	res.is_lit = 1;
 	return res;
 
 } // endof _STR_TMP