FEAT: handle negative part values when reading from a file port

resolves: Oldes/Rebol-issues#2505

Author: Oldes

src/core/p-file.c

@@ -370,7 +370,7 @@ REBINT Mode_Syms[] = {
 
 /***********************************************************************
 **
-*/	static REBCNT Set_Length(const REBVAL *ds, const REBREQ *file, const REBCNT arg)
+*/	static REBCNT Set_Length(const REBVAL *ds, REBREQ *file, const REBCNT arg)
 /*
 **		Computes the length of data based on the argument number
 **		provided for the ARG_*_PART stack value (which, when there,
@@ -395,6 +395,16 @@ REBINT Mode_Syms[] = {
 
 	// Limit size of requested read:
 	cnt = VAL_INT64(D_ARG(arg+1));
+	if (cnt < 0) {
+		cnt = -cnt;
+		if (cnt > file->file.index) {
+			Trap1(RE_OUT_OF_RANGE, D_ARG(arg + 1));
+			//cnt = file->file.index;
+		}
+		file->file.index -= cnt;
+		len += cnt;
+		SET_FLAG(file->modes, RFM_RESEEK);
+	}
 	if (cnt > len) return (REBCNT)len;
 	return (REBCNT)cnt;
 }

src/tests/units/port-test.r3

@@ -331,7 +331,22 @@ if system/platform = 'Windows [
 			port? close p
 			not error? try [delete %file-552]
 		]
-		
+
+	--test-- "read/part"
+		;@@ https://github.com/Oldes/Rebol-issues/issues/2505
+		--assert all [
+			file? write %12345 "12345"
+			port? p: open/read/seek %12345
+			#{31}   == read/part p 1 ; advances!
+			#{3233} == read/part p 2
+			#{3132} == read/part head p 2
+			#{}     == read/part tail p 2
+			#{35}   == read/part tail p -1 ;- no crash!
+			#{3435} == read/part tail p -2
+			all [error? e: try [read/part p -20]  e/id = 'out-of-range]
+			port? close p
+			not error? try [delete %12345]
+		]
 
 	--test-- "CLEAR file port"
 		;@@ https://github.com/Oldes/Rebol-issues/issues/812