Be more conservative with stdin.

If no paths are given to ripgrep, only read from stdin if it's a file or
a FIFO. In particular, if something like `rg foo < /dev/null` is used,
then don't try to read from stdin.

Fixes #35, #81

src/args.rs

@@ -279,7 +279,8 @@ impl RawArgs {
             if self.arg_path.is_empty() {
                 if atty::on_stdin()
                     || self.flag_files
-                    || self.flag_type_list {
+                    || self.flag_type_list
+                    || !atty::stdin_is_readable() {
                     vec![Path::new("./").to_path_buf()]
                 } else {
                     vec![Path::new("-").to_path_buf()]

src/atty.rs

@@ -4,6 +4,29 @@ from (or to) a terminal. Windows and Unix do this differently, so implement
 both here.
 */
 
+#[cfg(unix)]
+pub fn stdin_is_readable() -> bool {
+    use std::fs::File;
+    use std::os::unix::fs::FileTypeExt;
+    use std::os::unix::io::{FromRawFd, IntoRawFd};
+    use libc;
+
+    let file = unsafe { File::from_raw_fd(libc::STDIN_FILENO) };
+    let md = file.metadata();
+    let _ = file.into_raw_fd();
+    let ft = match md {
+        Err(_) => return false,
+        Ok(md) => md.file_type(),
+    };
+    ft.is_file() || ft.is_fifo()
+}
+
+#[cfg(windows)]
+pub fn stdin_is_readable() -> bool {
+    // ???
+    true
+}
+
 #[cfg(unix)]
 pub fn on_stdin() -> bool {
     use libc;