Make from json --objects streaming (#12949)

# Description

Makes the `from json --objects` command produce a stream, and read
lazily from an input stream to produce its output.

Also added a helper, `PipelineData::get_type()`, to make it easier to
construct a wrong type error message when matching on `PipelineData`. I
expect checking `PipelineData` for either a string value or an `Unknown`
or `String` typed `ByteStream` will be very, very common. I would have
liked to have a helper that just returns a readable stream from either,
but that would either be a bespoke enum or a `Box<dyn BufRead>`, which
feels like it wouldn't be so great for performance. So instead, taking
the approach I did here is probably better - having a function that
accepts the `impl BufRead` and matching to use it.

# User-Facing Changes

- `from json --objects` no longer collects its input, and can be used
for large datasets or streams that produce values over time.

# Tests + Formatting
All passing.

# After Submitting
- [ ] release notes

---------

Co-authored-by: Ian Manske <ian.manske@pm.me>

crates/nu-command/src/formats/from/json.rs

@@ -1,4 +1,10 @@
+use std::{
+    io::{BufRead, Cursor},
+    sync::{atomic::AtomicBool, Arc},
+};
+
 use nu_engine::command_prelude::*;
+use nu_protocol::ListStream;
 
 #[derive(Clone)]
 pub struct FromJson;
@@ -45,6 +51,15 @@ impl Command for FromJson {
                     "b" => Value::test_int(2),
                 })),
             },
+            Example {
+                example: r#"'{ "a": 1 }
+{ "b": 2 }' | from json --objects"#,
+                description: "Parse a stream of line-delimited JSON values",
+                result: Some(Value::test_list(vec![
+                    Value::test_record(record! {"a" => Value::test_int(1)}),
+                    Value::test_record(record! {"b" => Value::test_int(2)}),
+                ])),
+            },
         ]
     }
 
@@ -56,40 +71,47 @@ impl Command for FromJson {
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let span = call.head;
+
+        let strict = call.has_flag(engine_state, stack, "strict")?;
+
+        // TODO: turn this into a structured underline of the nu_json error
+        if call.has_flag(engine_state, stack, "objects")? {
+            // Return a stream of JSON values, one for each non-empty line
+            match input {
+                PipelineData::Value(Value::String { val, .. }, metadata) => {
+                    Ok(PipelineData::ListStream(
+                        read_json_lines(Cursor::new(val), span, strict, engine_state.ctrlc.clone()),
+                        metadata,
+                    ))
+                }
+                PipelineData::ByteStream(stream, metadata)
+                    if stream.type_() != ByteStreamType::Binary =>
+                {
+                    if let Some(reader) = stream.reader() {
+                        Ok(PipelineData::ListStream(
+                            read_json_lines(reader, span, strict, None),
+                            metadata,
+                        ))
+                    } else {
+                        Ok(PipelineData::Empty)
+                    }
+                }
+                _ => Err(ShellError::OnlySupportsThisInputType {
+                    exp_input_type: "string".into(),
+                    wrong_type: input.get_type().to_string(),
+                    dst_span: call.head,
+                    src_span: input.span().unwrap_or(call.head),
+                }),
+            }
+        } else {
+            // Return a single JSON value
             let (string_input, span, metadata) = input.collect_string_strict(span)?;
 
             if string_input.is_empty() {
                 return Ok(Value::nothing(span).into_pipeline_data());
             }
 
-        let strict = call.has_flag(engine_state, stack, "strict")?;
-
-        // TODO: turn this into a structured underline of the nu_json error
-        if call.has_flag(engine_state, stack, "objects")? {
-            let lines = string_input.lines().filter(|line| !line.trim().is_empty());
-
-            let converted_lines: Vec<_> = if strict {
-                lines
-                    .map(|line| {
-                        convert_string_to_value_strict(line, span)
-                            .unwrap_or_else(|err| Value::error(err, span))
-                    })
-                    .collect()
-            } else {
-                lines
-                    .map(|line| {
-                        convert_string_to_value(line, span)
-                            .unwrap_or_else(|err| Value::error(err, span))
-                    })
-                    .collect()
-            };
-
-            Ok(converted_lines.into_pipeline_data_with_metadata(
-                span,
-                engine_state.ctrlc.clone(),
-                metadata,
-            ))
-        } else if strict {
+            if strict {
                 Ok(convert_string_to_value_strict(&string_input, span)?
                     .into_pipeline_data_with_metadata(metadata))
             } else {
@@ -97,6 +119,30 @@ impl Command for FromJson {
                     .into_pipeline_data_with_metadata(metadata))
             }
         }
+    }
+}
+
+/// Create a stream of values from a reader that produces line-delimited JSON
+fn read_json_lines(
+    input: impl BufRead + Send + 'static,
+    span: Span,
+    strict: bool,
+    interrupt: Option<Arc<AtomicBool>>,
+) -> ListStream {
+    let iter = input
+        .lines()
+        .filter(|line| line.as_ref().is_ok_and(|line| !line.trim().is_empty()) || line.is_err())
+        .map(move |line| {
+            let line = line.err_span(span)?;
+            if strict {
+                convert_string_to_value_strict(&line, span)
+            } else {
+                convert_string_to_value(&line, span)
+            }
+        })
+        .map(move |result| result.unwrap_or_else(|err| Value::error(err, span)));
+
+    ListStream::new(iter, span, interrupt)
 }
 
 fn convert_nujson_to_value(value: nu_json::Value, span: Span) -> Value {

crates/nu-command/tests/format_conversions/json.rs

@@ -96,6 +96,32 @@ fn from_json_text_recognizing_objects_independently_to_table() {
     })
 }
 
+#[test]
+fn from_json_text_objects_is_stream() {
+    Playground::setup("filter_from_json_test_2_is_stream", |dirs, sandbox| {
+        sandbox.with_files(&[FileWithContentToBeTrimmed(
+            "katz.txt",
+            r#"
+                {"name":   "Yehuda", "rusty_luck": 1}
+                {"name": "JT", "rusty_luck": 1}
+                {"name":   "Andres", "rusty_luck": 1}
+                {"name":"GorbyPuff", "rusty_luck": 3}
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open katz.txt
+                | from json -o
+                | describe -n
+            "#
+        ));
+
+        assert_eq!(actual.out, "stream");
+    })
+}
+
 #[test]
 fn from_json_text_recognizing_objects_independently_to_table_strict() {
     Playground::setup("filter_from_json_test_2_strict", |dirs, sandbox| {

crates/nu-protocol/src/pipeline/pipeline_data.rs

@@ -3,7 +3,7 @@ use crate::{
     engine::{EngineState, Stack},
     process::{ChildPipe, ChildProcess, ExitStatus},
     ByteStream, ByteStreamType, Config, ErrSpan, ListStream, OutDest, PipelineMetadata, Range,
-    ShellError, Span, Value,
+    ShellError, Span, Type, Value,
 };
 use nu_utils::{stderr_write_all_and_flush, stdout_write_all_and_flush};
 use std::{
@@ -99,6 +99,24 @@ impl PipelineData {
         }
     }
 
+    /// Get a type that is representative of the `PipelineData`.
+    ///
+    /// The type returned here makes no effort to collect a stream, so it may be a different type
+    /// than would be returned by [`Value::get_type()`] on the result of [`.into_value()`].
+    ///
+    /// Specifically, a `ListStream` results in [`list stream`](Type::ListStream) rather than
+    /// the fully complete [`list`](Type::List) type (which would require knowing the contents),
+    /// and a `ByteStream` with [unknown](crate::ByteStreamType::Unknown) type results in
+    /// [`any`](Type::Any) rather than [`string`](Type::String) or [`binary`](Type::Binary).
+    pub fn get_type(&self) -> Type {
+        match self {
+            PipelineData::Empty => Type::Nothing,
+            PipelineData::Value(value, _) => value.get_type(),
+            PipelineData::ListStream(_, _) => Type::ListStream,
+            PipelineData::ByteStream(stream, _) => stream.type_().into(),
+        }
+    }
+
     pub fn into_value(self, span: Span) -> Result<Value, ShellError> {
         match self {
             PipelineData::Empty => Ok(Value::nothing(span)),