Do not panic when failing to decode lines from external stdout (#1364)

Cargo.toml

@@ -195,6 +195,11 @@ name = "cococo"
 path = "crates/nu-test-support/src/bins/cococo.rs"
 required-features = ["test-bins"]
 
+[[bin]]
+name = "nonu"
+path = "crates/nu-test-support/src/bins/nonu.rs"
+required-features = ["test-bins"]
+
 # Core plugins that ship with `cargo install nu` by default
 # Currently, Cargo limits us to installing only one binary
 # unless we use [[bin]], so we use this as a workaround

crates/nu-test-support/src/bins/nonu.rs

@@ -0,0 +1,3 @@
+fn main() {
+    std::env::args().skip(1).for_each(|arg| print!("{}", arg));
+}

src/commands/classified/external.rs

@@ -285,19 +285,27 @@ fn spawn(
                 };
 
                 let file = futures::io::AllowStdIo::new(stdout);
-                let mut stream = FramedRead::new(file, LinesCodec).map(|line| {
+                let mut stream = FramedRead::new(file, LinesCodec);
+
+                while let Some(line) = stream.next().await {
                     if let Ok(line) = line {
-                        Value {
+                        yield Ok(Value {
                             value: UntaggedValue::Primitive(Primitive::Line(line)),
                             tag: name_tag.clone(),
-                        }
+                        });
                     } else {
-                        panic!("Internal error: could not read lines of text from stdin")
+                        yield Ok(Value {
+                            value: UntaggedValue::Error(
+                                ShellError::labeled_error(
+                                    "Unable to read lines from stdout. This usually happens when the output does not end with a newline.",
+                                    "unable to read from stdout",
+                                    &name_tag,
+                                )
+                            ),
+                            tag: name_tag.clone(),
+                        });
+                        return;
                     }
-                });
-
-                while let Some(item) = stream.next().await {
-                    yield Ok(item)
                 }
             }
 

tests/shell/pipeline/commands/external.rs

@@ -78,6 +78,24 @@ mod it_evaluation {
     }
 }
 
+mod stdin_evaluation {
+    use super::nu_error;
+    use nu_test_support::pipeline;
+
+    #[test]
+    fn does_not_panic_with_no_newline_in_stream() {
+        let stderr = nu_error!(
+            cwd: ".",
+            pipeline(r#"
+                nonu "where's the nuline?"
+                | count
+            "#
+        ));
+
+        assert_eq!(stderr, "");
+    }
+}
+
 mod external_words {
     use super::nu;