FIX: correctly read POSIX virtual files in chunks using an open port

related to: Oldes/Rebol-issues#2303

Author: Oldes

src/core/p-file.c

@@ -391,14 +391,16 @@ REBINT Mode_Syms[] = {
 **
 ***********************************************************************/
 {
-	REBI64 len;  // maximum size
+	REBI64 len = 0;  // maximum size
 	REBI64 cnt;
 //	int what_if_it_changed;
 
 	// Compute and bound bytes remaining:
-	len = file->file.size - file->file.index; // already read
-	if (len < 0) return 0;
-	len &= MAX_READ_MASK; // limit the size
+	if (file->file.size > 0) {
+		len = file->file.size - file->file.index; // already read
+		if (len < 0) return 0;
+		len &= MAX_READ_MASK; // limit the size
+	}
 
 	// Return requested length:
 	if (!D_REF(arg)) return (REBCNT)len;

src/tests/units/port-test.r3

@@ -598,6 +598,15 @@ if exists? %/proc/cpuinfo [
 			not error? info: try [read/part %/proc/cpuinfo 10]
 			10 == length? info
 		]
+		;; read a POSIX virtual file in chunks using an open port
+		--assert all [
+			port? port: try [open/read %/proc/cpuinfo]
+			bin: make binary! 16000
+			while [not empty? tmp: read/part port 1024][append bin tmp]
+			equal? bin try [read %/proc/cpuinfo]
+			port? try [close port]
+			not open? port
+		]
 ]
 	--test-- "Reading an empty file"
 		--assert all [