FEAT: don't allow clear on not opened port, allow size? on port

related to: Oldes/Rebol-issues#812

Author: Oldes

src/core/p-file.c

@@ -638,6 +638,7 @@ REBINT Mode_Syms[] = {
 		DECIDE(file->file.index > file->file.size);
 
 	case A_CLEAR:
+		if (!IS_OPEN(file)) Trap1(RE_NOT_OPEN, path);
 		// !! check for write enabled?
 		SET_FLAG(file->modes, RFM_RESEEK);
 		SET_FLAG(file->modes, RFM_TRUNCATE);

src/mezz/base-files.reb

@@ -28,7 +28,7 @@ exists?: func [
 
 size?: func [
 	{Returns the size of a file or vector (bits per value).}
-	target [file! url! vector!]
+	target [file! url! port! vector!]
 ][
 	query/mode target 'size
 ]

src/tests/units/port-test.r3

@@ -353,15 +353,31 @@ if system/platform = 'Windows [
 		]
 		try [delete %file-552]
 
-	--test-- "clear file port"
+	--test-- "CLEAR file port"
 		;@@ https://github.com/Oldes/Rebol-issues/issues/812
-		--assert file? write %file-812 to-binary "Hello World!"
-		--assert port? f: open %file-812
-		--assert "Hello World!" = to-string read f
-		--assert port? clear f
-		--assert 0 = length? f
-		--assert port? close f
-		try [delete %file-812]
+		--assert all [
+			file? write %file-812 "Hello World!"
+			port? f: open %file-812
+			"Hello World!" = read/string f
+			port? clear f ; this actually does not clear the file as we are at the end of the stream
+			0 = length? f
+			"Hello" = read/seek/string/part f 0 5
+			7 = length? f
+			port? clear f ; this should truncate the file
+			0 = length? f
+			port? close f
+			5 = length? f ; because there is still "Hello" left
+			all [
+				error? e: try [clear f]
+				e/id = 'not-open
+			]
+			port? f: open %file-812
+			port? clear f ; this should clear the file completely, because the position is at its head
+			port? close f
+			0 = size? %file-812
+			not error? try [delete %file-812]
+		]
+
 
 	--test-- "RENAME file"
 		;@@ https://github.com/Oldes/Rebol-issues/issues/446