Fix hang reading value channel from a redirection

Fixes #600
Fixes #1010

pkg/eval/compile_effect.go

@@ -497,21 +497,31 @@ type invalidFD struct{ fd int }
 
 func (err invalidFD) Error() string { return fmt.Sprintf("invalid fd: %d", err.fd) }
 
+func (op *redirOp) fileChan() chan interface{} {
+	if op.mode == parse.Read {
+		// ClosedChan is used when redirecting from a named file because we
+		// don't support reading "values" (as opposed to bytes) from a file.
+		return ClosedChan
+	}
+	// TODO: Replace BlackholeChan for output with a value sink that will
+	// throw an exception if it sees any values since we don't support writing
+	// values to a file without first converting them to bytes.
+	return BlackholeChan
+}
+
 func (op *redirOp) exec(fm *Frame) error {
 	var dst int
-	if op.dstOp == nil {
-		// use default dst fd
+	if op.dstOp == nil { // the common case: use default destination file-descriptor
 		switch op.mode {
 		case parse.Read:
-			dst = 0
+			dst = 0 // stdin
 		case parse.Write, parse.ReadWrite, parse.Append:
-			dst = 1
+			dst = 1 // stdout
 		default:
 			return fm.errorpf(op, "bad RedirMode; parser bug")
 		}
-	} else {
+	} else { // use the destination file-descriptor requested by the user
 		var err error
-		// dst must be a valid fd
 		dst, err = evalForFd(fm, op.dstOp, false, "redirection destination")
 		if err != nil {
 			return fm.errorp(op, err)
@@ -547,15 +557,9 @@ func (op *redirOp) exec(fm *Frame) error {
 		if err != nil {
 			return fm.errorpf(op, "failed to open file %s: %s", vals.Repr(src, vals.NoPretty), err)
 		}
-		fm.ports[dst] = &Port{
-			File: f, Chan: BlackholeChan,
-			CloseFile: true,
-		}
+		fm.ports[dst] = &Port{File: f, CloseFile: true, Chan: op.fileChan()}
 	case vals.File:
-		fm.ports[dst] = &Port{
-			File: src, Chan: BlackholeChan,
-			CloseFile: false,
-		}
+		fm.ports[dst] = &Port{File: src, CloseFile: false, Chan: op.fileChan()}
 	case vals.Pipe:
 		var f *os.File
 		switch op.mode {
@@ -566,10 +570,7 @@ func (op *redirOp) exec(fm *Frame) error {
 		default:
 			return fm.errorpf(op, "can only use < or > with pipes")
 		}
-		fm.ports[dst] = &Port{
-			File: f, Chan: BlackholeChan,
-			CloseFile: false,
-		}
+		fm.ports[dst] = &Port{File: f, CloseFile: false, Chan: op.fileChan()}
 	default:
 		return fm.errorp(op.srcOp, errs.BadValue{
 			What:  "redirection source",

pkg/eval/compile_effect_test.go

@@ -167,6 +167,12 @@ func TestCompileEffect(t *testing.T) {
 		That(`p = (pipe); echo haha > $p; pwclose $p; slurp < $p; prclose $p`).
 			Puts("haha\n"),
 
+		// We can't read values from a file and shouldn't hang when iterating
+		// over input from a file.
+		// Regression test for https://github.com/elves/elvish/issues/1010
+		That("echo abc > bytes", "each $echo~ < bytes").Prints("abc\n"),
+		That("echo def > bytes", "only-values < bytes | count").Puts("0"),
+
 		// Invalid redirection destination.
 		That("echo []> test").Throws(
 			errs.BadValue{

website/ref/language.md

@@ -1576,6 +1576,14 @@ Redirections may appear anywhere in the command, except at the beginning,
 although this may be restricted in future. It's usually good style to write
 redirections at the end of command forms.
 
+**Important:** Elvish only supports reading and writing bytes from/to the target
+of a redirection. Elvish does not support reading values from or writing values
+to a file or [pipe](builtin.html#pipe) via redirection. Assume a file named
+_data_ contains a single line with a single word such as "hello". This will
+output zero even though there is one word in the file:
+`only-values < data | count`. Whereas `only-bytes < data | count` will report
+one.
+
 # Special Commands
 
 **Special commands** obey the same syntax rules as normal commands, but have