interp: skip cancellable reader test on Windows

And document StdIO and the test a bit better now that I've read up
on the topic a bit.

interp/api.go

@@ -454,10 +454,15 @@ func stdinFile(r io.Reader) (*os.File, error) {
 // standard error. If out or err are nil, they default to a writer that discards
 // the output.
 //
-// Note that providing a non-nil standard input other than [os.File] will require
+// Note that providing a non-nil standard input other than [*os.File] will require
 // an [os.Pipe] and spawning a goroutine to copy into it,
 // as an [os.File] is the only way to share a reader with subprocesses.
+// This may cause the interpreter to consume the entire reader.
 // See [os/exec.Cmd.Stdin].
+//
+// When providing an [*os.File] as standard input, consider using an [os.Pipe]
+// as it has the best chance to support cancellable reads via [os.File.SetReadDeadline],
+// so that cancelling the runner's context can stop a blocked standard input read.
 func StdIO(in io.Reader, out, err io.Writer) RunnerOption {
 	return func(r *Runner) error {
 		stdin, _err := stdinFile(in)

interp/interp_test.go

@@ -4029,14 +4029,21 @@ func TestRunnerContext(t *testing.T) {
 	}
 }
 
-func TestCancelreader(t *testing.T) {
+func TestCancelBlockedStdinRead(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		// TODO: Why is this? The [os.File.SetReadDeadline] docs seem to imply that it should work
+		// across all major platforms, and the file polling  implementation seems to be
+		// for all posix platforms including Windows.
+		// Our previous logic and tests with muesli/cancelreader did not test an os.Pipe
+		// on Windows either, so skipping here is not any worse.
+		t.Skip("os.Pipe on windows appears to not support cancellable reads")
+	}
 	t.Parallel()
 
 	p := syntax.NewParser()
 	file := parse(t, p, "read x")
 	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
-	// Make the linter happy, even though we deliberately wait for the
-	// timeout.
+	// Make the linter happy, even though we deliberately wait for the timeout.
 	defer cancel()
 
 	stdinRead, stdinWrite, err := os.Pipe()