Fix unexpected flattening of data by par-each (Issue #8497) (#9007)

# Description
Previously, `par-each` acted like a `flatmap`: first mapping the data,
then applying a `flatten`. This is unlike `each`, which just maps the
data. Now `par-each` works like `each` in this regard, leaving nested
data unflattened.

Fixes #8497

# User-Facing Changes
Previously:
`[1 2 3] | par-each {|e| [$e, $e] }` --> `[1,1,2,2,3,3]` 
Now:
`[1 2 3] | par-each {|e| [$e, $e] }` --> `[[1,1],[2,2],[3,3]]`

# Tests
This adds one test that verifies the lack of flattening for `par-each`.

crates/nu-command/src/filters/par_each.rs

@@ -114,8 +114,10 @@ impl Command for ParEach {
         let max_threads = threads.unwrap_or(0);
         let metadata = input.metadata();
         let ctrlc = engine_state.ctrlc.clone();
+        let outer_ctrlc = engine_state.ctrlc.clone();
         let block_id = capture_block.block_id;
         let mut stack = stack.captures_to_stack(&capture_block.captures);
+        let span = call.head;
         let redirect_stdout = call.redirect_stdout;
         let redirect_stderr = call.redirect_stderr;
 
@@ -146,16 +148,15 @@ impl Command for ParEach {
                                 redirect_stdout,
                                 redirect_stderr,
                             ) {
-                                Ok(v) => v,
+                                Ok(v) => v.into_value(span),
+
                                 Err(error) => Value::Error {
                                     error: Box::new(chain_error_with_input(error, val_span)),
-                                }
-                                .into_pipeline_data(),
+                                },
                             }
                         })
                         .collect::<Vec<_>>()
                         .into_iter()
-                        .flatten()
                         .into_pipeline_data(ctrlc)
                 })),
             PipelineData::Value(Value::List { vals: val, .. }, ..) => Ok(create_pool(max_threads)?
@@ -181,16 +182,14 @@ impl Command for ParEach {
                                 redirect_stdout,
                                 redirect_stderr,
                             ) {
-                                Ok(v) => v,
+                                Ok(v) => v.into_value(span),
                                 Err(error) => Value::Error {
                                     error: Box::new(chain_error_with_input(error, val_span)),
-                                }
-                                .into_pipeline_data(),
+                                },
                             }
                         })
                         .collect::<Vec<_>>()
                         .into_iter()
-                        .flatten()
                         .into_pipeline_data(ctrlc)
                 })),
             PipelineData::ListStream(stream, ..) => Ok(create_pool(max_threads)?.install(|| {
@@ -216,16 +215,14 @@ impl Command for ParEach {
                             redirect_stdout,
                             redirect_stderr,
                         ) {
-                            Ok(v) => v,
+                            Ok(v) => v.into_value(span),
                             Err(error) => Value::Error {
                                 error: Box::new(chain_error_with_input(error, val_span)),
-                            }
-                            .into_pipeline_data(),
+                            },
                         }
                     })
                     .collect::<Vec<_>>()
                     .into_iter()
-                    .flatten()
                     .into_pipeline_data(ctrlc)
             })),
             PipelineData::ExternalStream { stdout: None, .. } => Ok(PipelineData::empty()),
@@ -242,7 +239,6 @@ impl Command for ParEach {
                                 return Value::Error {
                                     error: Box::new(err),
                                 }
-                                .into_pipeline_data()
                             }
                         };
 
@@ -264,16 +260,14 @@ impl Command for ParEach {
                             redirect_stdout,
                             redirect_stderr,
                         ) {
-                            Ok(v) => v,
+                            Ok(v) => v.into_value(span),
                             Err(error) => Value::Error {
                                 error: Box::new(error),
-                            }
-                            .into_pipeline_data(),
+                            },
                         }
                     })
                     .collect::<Vec<_>>()
                     .into_iter()
-                    .flatten()
                     .into_pipeline_data(ctrlc)
             })),
             // This match allows non-iterables to be accepted,
@@ -297,6 +291,7 @@ impl Command for ParEach {
                 )
             }
         }
+        .and_then(|x| x.filter(|v| !v.is_nothing(), outer_ctrlc))
         .map(|res| res.set_metadata(metadata))
     }
 }

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

@@ -59,6 +59,7 @@ mod network;
 mod nu_check;
 mod open;
 mod p;
+mod par_each;
 mod parse;
 mod path;
 mod platform;

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

@@ -0,0 +1,12 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn par_each_does_not_flatten_nested_structures() {
+    // This is a regression test for issue #8497
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"[1 2 3] | par-each { |it| [$it, $it] } | sort | to json --raw"#
+    ));
+
+    assert_eq!(actual.out, "[[1,1],[2,2],[3,3]]");
+}