1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
use std::{error::Error as StdError, fmt, io::Write as _};

use actix_web::{
    body::{BodyStream, MessageBody},
    HttpResponse, Responder,
};
use bytes::{Bytes, BytesMut};
use futures_core::Stream;
use futures_util::TryStreamExt as _;
use pin_project_lite::pin_project;

use crate::util::{InfallibleStream, MutWriter};

pin_project! {
    /// A buffered line formatting body stream.
    ///
    /// Each item yielded by the stream will be written to the response body using its
    /// `Display` implementation.
    ///
    /// This has significant memory efficiency advantages over returning an array of lines when the
    /// data set is very large because it avoids buffering the entire response.
    ///
    /// # Examples
    /// ```
    /// # use actix_web::Responder;
    /// # use actix_web_lab::respond::DisplayStream;
    /// # use futures_core::Stream;
    /// fn streaming_data_source() -> impl Stream<Item = u32> {
    ///     // get item stream from source
    ///     # futures_util::stream::empty()
    /// }
    ///
    /// async fn handler() -> impl Responder {
    ///     let data_stream = streaming_data_source();
    ///
    ///     DisplayStream::new_infallible(data_stream)
    ///         .into_responder()
    /// }
    /// ```
    pub struct DisplayStream<S> {
        // The wrapped item stream.
        #[pin]
        stream: S,
    }
}

impl<S> DisplayStream<S> {
    /// Constructs a new `DisplayStream` from a stream of lines.
    pub fn new(stream: S) -> Self {
        Self { stream }
    }
}

impl<S> DisplayStream<S> {
    /// Constructs a new `DisplayStream` from an infallible stream of lines.
    pub fn new_infallible(stream: S) -> DisplayStream<InfallibleStream<S>> {
        DisplayStream::new(InfallibleStream::new(stream))
    }
}

impl<S, T, E> DisplayStream<S>
where
    S: Stream<Item = Result<T, E>>,
    T: fmt::Display,
    E: Into<Box<dyn StdError>> + 'static,
{
    /// Creates a chunked body stream that serializes as CSV on-the-fly.
    pub fn into_body_stream(self) -> impl MessageBody {
        BodyStream::new(self.into_chunk_stream())
    }

    /// Creates a `Responder` type with a line-by-line serializing stream and `text/plain`
    /// content-type header.
    pub fn into_responder(self) -> impl Responder
    where
        S: 'static,
        T: 'static,
        E: 'static,
    {
        HttpResponse::Ok()
            .content_type(mime::TEXT_PLAIN_UTF_8)
            .message_body(self.into_body_stream())
            .unwrap()
    }

    /// Creates a stream of serialized chunks.
    pub fn into_chunk_stream(self) -> impl Stream<Item = Result<Bytes, E>> {
        self.stream.map_ok(write_display)
    }
}

fn write_display(item: impl fmt::Display) -> Bytes {
    let mut buf = BytesMut::new();
    let mut wrt = MutWriter(&mut buf);

    writeln!(wrt, "{item}").unwrap();

    buf.freeze()
}

#[cfg(test)]
mod tests {
    use std::error::Error as StdError;

    use actix_web::body;
    use futures_util::stream;

    use super::*;

    #[actix_web::test]
    async fn serializes_into_body() {
        let ndjson_body = DisplayStream::new_infallible(stream::iter([123, 789, 345, 901, 456]))
            .into_body_stream();

        let body_bytes = body::to_bytes(ndjson_body)
            .await
            .map_err(Into::<Box<dyn StdError>>::into)
            .unwrap();

        const EXP_BYTES: &str = "123\n\
        789\n\
        345\n\
        901\n\
        456\n";

        assert_eq!(body_bytes, EXP_BYTES);
    }
}