make stderr works for failed external command

crates/nu-command/tests/commands/redirection.rs

@@ -171,6 +171,22 @@ fn redirection_keep_exit_codes() {
     });
 }
 
+#[test]
+fn redirection_stderr_with_failed_program() {
+    Playground::setup("redirection stderr with failed program", |dirs, _| {
+        let out = nu!(
+            cwd: dirs.test(),
+            r#"$env.FOO = "bar"; nu --testbin echo_env_stderr_fail FOO e> file.txt; echo 3"#
+        );
+        // firstly echo 3 shouldn't run, because previous command runs to failed.
+        // second `file.txt` should contain "bar".
+        assert!(!out.out.contains('3'));
+        let expected_file = dirs.test().join("file.txt");
+        let actual = file_contents(expected_file);
+        assert_eq!(actual, "bar\n");
+    });
+}
+
 #[test]
 fn redirection_with_non_zero_exit_code_should_stop_from_running() {
     Playground::setup("redirection with non zero exit code", |dirs, _| {

crates/nu-engine/src/eval.rs

@@ -383,7 +383,21 @@ pub fn eval_expression_with_input(
         }
     };
 
-    Ok(might_consume_external_result(input))
+    // Given input is PipelineData::ExternalStream
+    // `might_consume_external_result` will consume `stderr` stream if `stdout` is empty.
+    // it's not intended if user want to redirect stderr message.
+    //
+    // e.g:
+    // 1. cargo check e>| less
+    // 2. cargo check e> result.txt
+    //
+    // In these two cases, stdout will be empty, but nushell shouldn't consume the `stderr`
+    // stream it needs be passed to next command.
+    if !redirect_stderr {
+        Ok(might_consume_external_result(input))
+    } else {
+        Ok((input, false))
+    }
 }
 
 // Try to catch and detect if external command runs to failed.

src/main.rs

@@ -269,6 +269,7 @@ fn main() -> Result<()> {
         match testbin.item.as_str() {
             "echo_env" => test_bins::echo_env(true),
             "echo_env_stderr" => test_bins::echo_env(false),
+            "echo_env_stderr_fail" => test_bins::echo_env_and_fail(false),
             "echo_env_mixed" => test_bins::echo_env_mixed(),
             "cococo" => test_bins::cococo(),
             "meow" => test_bins::meow(),

src/test_bins.rs

@@ -16,6 +16,11 @@ pub fn echo_env(to_stdout: bool) {
     }
 }
 
+pub fn echo_env_and_fail(to_stdout: bool) {
+    echo_env(to_stdout);
+    fail();
+}
+
 fn echo_one_env(arg: &str, to_stdout: bool) {
     if let Ok(v) = std::env::var(arg) {
         if to_stdout {

tests/shell/pipeline/commands/external.rs

@@ -158,6 +158,14 @@ fn basic_outerr_pipe_works() {
     assert_eq!(actual.out, "8");
 }
 
+#[test]
+fn err_pipe_with_failed_external_works() {
+    let actual =
+        nu!(r#"with-env [FOO "bar"] { nu --testbin echo_env_stderr_fail FOO e>| str length }"#);
+    // there is a `newline` output from nu --testbin
+    assert_eq!(actual.out, "4");
+}
+
 mod it_evaluation {
     use super::nu;
     use nu_test_support::fs::Stub::{EmptyFile, FileWithContent, FileWithContentToBeTrimmed};