Avoid lockup when running without arguments and stdin is connected to tty (#1112)

Up until commit 55287a8 "Run in diff mode iff two positional arguments
are supplied" diff mode was only supported when stdin is connected to a
tty, which meant that, for example, running `delta a b` in a git hook
would do the wrong thing.

As a fix, the tty check was removed, so now `delta a b` always works,
but the only error condition is now to pass only a single file. When
passing no files at all, we try parse to stdin even when connected to a
tty. Usually that wouldn't be that bad, because the user could press
Ctrl-C or Ctrl-D (EOF) to quit, unfortunately something in the pager
setup breaks this, and the only way to exit `delta` is to kill it.

Running `delta` with stdin connected to a tty would mean that the user
has to manually enter a diff (or paste it), which is not really an
expected scenario, as demonstrated by the error message that's currently
only emitted when running `delta` with only a single file argument. So
we can avoid the lockup by also emitting the error message when running
on a tty and there are no files to be diffed.

Fixes #1039, fixes #1090

1 files changed, 10 insertions, 12 deletions

diff --git a/src/main.rs b/src/main.rs
index 1d2b7633..625b5131 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -136,21 +136,19 @@ fn run_app() -> std::io::Result<i32> {
         OutputType::from_mode(&env, config.paging_mode, config.pager.clone(), &config).unwrap();
     let mut writer = output_type.handle().unwrap();
 
-    match (config.minus_file.as_ref(), config.plus_file.as_ref()) {
-        (None, None) => {}
-        (Some(minus_file), Some(plus_file)) => {
-            let exit_code = subcommands::diff::diff(minus_file, plus_file, &config, &mut writer);
-            return Ok(exit_code);
-        }
-        _ => {
-            eprintln!(
-                "\
+    if let (Some(minus_file), Some(plus_file)) = (&config.minus_file, &config.plus_file) {
+        let exit_code = subcommands::diff::diff(minus_file, plus_file, &config, &mut writer);
+        return Ok(exit_code);
+    }
+
+    if atty::is(atty::Stream::Stdin) {
+        eprintln!(
+            "\
     The main way to use delta is to configure it as the pager for git: \
     see https://github.com/dandavison/delta#configuration. \
     You can also use delta to diff two files: `delta file_A file_B`."
-            );
-            return Ok(config.error_exit_code);
-        }
+        );
+        return Ok(config.error_exit_code);
     }
 
     if let Err(error) = delta(io::stdin().lock().byte_lines(), &mut writer, &config) {