FIX: disallowed loop variable context access

resolves: Oldes/Rebol-issues#2531

Author: Oldes

src/core/n-data.c

@@ -3,7 +3,7 @@
 **  REBOL [R3] Language Interpreter and Run-time Environment
 **
 **  Copyright 2012 REBOL Technologies
-**  Copyright 2012-2022 Rebol Open Source Developers
+**  Copyright 2012-2023 Rebol Open Source Developers
 **  REBOL is a trademark of REBOL Technologies
 **
 **  Licensed under the Apache License, Version 2.0 (the "License");
@@ -352,6 +352,9 @@ static int Check_Char_Range(REBVAL *val, REBINT limit)
 		// case like: ` f: func[a][context? 'a]  f 1 `
 		*D_RET = frame[3]; 
 	} else {
+		if (IS_INT_SERIES(VAL_WORD_FRAME(word)))
+			// in case like: ` foreach x [1] [context? 'x] ` 
+			return R_NONE;
 		SET_OBJECT(D_RET, VAL_WORD_FRAME(word));
 	}
 	return R_RET;

src/core/n-loop.c

@@ -64,6 +64,10 @@ enum loop_each_mode {
 	SET_SELFLESS(frame);
 	SERIES_TAIL(frame) = len+1;
 	SERIES_TAIL(FRM_WORD_SERIES(frame)) = len+1;
+	
+	// Mark the frame as internal series so it is not accessible!
+	// See: https://github.com/Oldes/Rebol-issues/issues/2531
+	INT_SERIES(frame);
 
 	// Setup for loop:
 	word = FRM_WORD(frame, 1); // skip SELF

src/include/sys-value.h

@@ -476,6 +476,7 @@ enum {
 	SER_BARE = 1<<5,	// Series has no links to GC-able values
 	SER_PROT = 1<<6,	// Series is protected from modification
 	SER_MON  = 1<<7,	// Monitoring
+	SER_INT  = 1<<8,	// Series data is internal (loop frames) and should not be accessed by users
 };
 
 #define SERIES_SET_FLAG(s, f) (SERIES_FLAGS(s) |= ((f) << 8))
@@ -489,6 +490,8 @@ enum {
 #define KEEP_SERIES(s,l)  do {SERIES_SET_FLAG(s, SER_KEEP); LABEL_SERIES(s,l);} while(0)
 #define EXT_SERIES(s)     SERIES_SET_FLAG(s, SER_EXT)
 #define IS_EXT_SERIES(s)  SERIES_GET_FLAG(s, SER_EXT)
+#define INT_SERIES(s)     SERIES_SET_FLAG(s, SER_INT)
+#define IS_INT_SERIES(s)  SERIES_GET_FLAG(s, SER_INT)
 #define LOCK_SERIES(s)    SERIES_SET_FLAG(s, SER_LOCK)
 #define IS_LOCK_SERIES(s) SERIES_GET_FLAG(s, SER_LOCK)
 #define BARE_SERIES(s)    SERIES_SET_FLAG(s, SER_BARE)

src/tests/units/object-test.r3

@@ -380,6 +380,10 @@ Rebol [
 		--assert object? o: context? use [x] ['x]
 		--assert object? append o 'self
 
+	--test-- "issue-2531"
+	;@@ https://github.com/Oldes/Rebol-issues/issues/2531
+		--assert none? foreach x [1] [context? 'x]
+
 ===end-group===
 
 ===start-group=== "PROTECT object!"