cranpose_services/

http.rs

#[cfg(not(target_arch = "wasm32"))]
use std::path::Path;
use std::{
    future::Future,
    pin::Pin,
    sync::{
        Arc, Mutex, PoisonError, Weak,
        atomic::{AtomicBool, Ordering},
    },
};

use cranpose_core::{CompositionLocal, compositionLocalOfWithPolicy};
#[cfg(all(target_arch = "wasm32", feature = "web-http"))]
use futures_util::{StreamExt, stream};

/// Why an HTTP transfer failed.
#[derive(thiserror::Error, Debug, Clone)]
pub enum HttpError {
    /// The platform could not create its HTTP client. The text describes the cause.
    #[error("Failed to build HTTP client: {0}")]
    ClientInit(String),
    /// The request could not be sent or got no response.
    #[error("Request failed for {url}: {message}")]
    RequestFailed {
        /// The URL that was requested.
        url: String,
        /// What the platform reported.
        message: String,
    },
    /// The server answered with a status outside the 2xx range.
    #[error("Request failed with status {status} for {url}")]
    HttpStatus {
        /// The URL that was requested.
        url: String,
        /// The status code of the response.
        status: u16,
    },
    /// The response body ended early or could not be read.
    #[error("Failed to read response body for {url}: {message}")]
    BodyReadFailed {
        /// The URL that was requested.
        url: String,
        /// What the platform reported.
        message: String,
    },
    /// The response is not usable, such as a body that is not valid UTF-8 text.
    #[error("Invalid response for {url}: {message}")]
    InvalidResponse {
        /// The URL that was requested.
        url: String,
        /// What is wrong with the response.
        message: String,
    },
    /// The browser gave no window object to send the request through.
    #[error("No window object available")]
    NoWindow,
    /// A worker thread that ran part of the operation panicked.
    #[error("{operation} worker thread panicked")]
    WorkerPanicked {
        /// The operation that the worker thread ran.
        operation: &'static str,
    },
    /// The operation needs a `cranpose-services` feature that is turned off.
    #[error("{operation} requires cranpose-services feature `{feature}`")]
    UnsupportedFeature {
        /// The operation that the build cannot run.
        operation: &'static str,
        /// Name of the `cranpose-services` feature that the operation needs.
        feature: &'static str,
    },
    /// The caller cancelled the transfer through its [`HttpControl`].
    #[error("Download was cancelled")]
    Cancelled,
    /// A downloaded file could not be written to disk.
    #[error("Failed to write downloaded file {path}: {message}")]
    FileWrite {
        /// The path of the file that failed to write.
        path: String,
        /// What the system reported, such as
        /// [`std::io::ErrorKind::StorageFull`] for a full disk.
        kind: std::io::ErrorKind,
        /// The text of the system error.
        message: String,
    },
}

/// How much of a transfer has arrived.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct HttpProgress {
    /// Bytes transferred so far, counting anything a resumed transfer skipped.
    pub transferred: u64,
    /// The whole size, when the server said. A chunked response does not, and
    /// a progress bar that invents a total for one is lying about it.
    pub total: Option<u64>,
}

impl HttpProgress {
    /// How far through the transfer is, in `0..=1`, or `None` when the server
    /// never said how large it is.
    pub fn fraction(&self) -> Option<f32> {
        let total = self.total?;
        if total == 0 {
            return None;
        }
        Some((self.transferred as f32 / total as f32).clamp(0.0, 1.0))
    }

    /// Whether everything the server promised has arrived.
    pub fn is_complete(&self) -> bool {
        self.total.is_some_and(|total| self.transferred >= total)
    }
}

/// What a progress report is handed to.
pub type ProgressHandler = Arc<dyn Fn(HttpProgress) + Send + Sync>;

/// Cancellation and progress shared with an in-flight request.
///
/// One control covers every request rather than only file downloads: a slow
/// response is a slow response whether it is being written to disk or read into
/// memory, and a screen that leaves is a screen that leaves.
#[derive(Clone, Default)]
pub struct HttpControl {
    cancel: Arc<CancelState>,
    progress: Option<ProgressHandler>,
}

#[derive(Default)]
struct CancelState {
    cancelled: AtomicBool,
    waiting: Mutex<Vec<Weak<dyn CancelTransfer>>>,
}

/// A transfer that stops at once when its [`HttpControl`] is cancelled,
/// registered with [`HttpControl::on_cancel`] by the backend running it.
pub trait CancelTransfer: Send + Sync {
    /// Stops the transfer; the caller sees [`HttpError::Cancelled`].
    fn cancel(&self);
}

impl HttpControl {
    /// A live control with no progress reporting.
    pub fn new() -> Self {
        Self::default()
    }

    /// Reports transferred and, where the server says so, total bytes.
    ///
    /// Called from whichever thread is doing the transfer, so the handler must
    /// be prepared for that — the framework's own event stream is the ordinary
    /// way to get it back to composition.
    pub fn with_progress(
        mut self,
        progress: impl Fn(HttpProgress) + Send + Sync + 'static,
    ) -> Self {
        self.progress = Some(Arc::new(progress));
        self
    }

    /// Requests cancellation.
    ///
    /// A native transfer that waits for the response or the next chunk ends
    /// with [`HttpError::Cancelled`] at once; other backends stop at the next
    /// chunk boundary. A partial file left by a cancelled download stays on
    /// disk, which is what makes the next attempt able to resume rather than
    /// start again.
    pub fn cancel(&self) {
        self.cancel.cancelled.store(true, Ordering::Release);
        let waiting = std::mem::take(
            &mut *self
                .cancel
                .waiting
                .lock()
                .unwrap_or_else(PoisonError::into_inner),
        );
        for transfer in waiting.iter().filter_map(Weak::upgrade) {
            transfer.cancel();
        }
    }

    /// A control cancelled with this one that reports no progress.
    #[cfg(not(target_arch = "wasm32"))]
    fn cancel_only(&self) -> Self {
        Self {
            cancel: Arc::clone(&self.cancel),
            progress: None,
        }
    }

    /// Whether cancellation was requested.
    pub fn is_cancelled(&self) -> bool {
        self.cancel.cancelled.load(Ordering::Acquire)
    }

    /// Cancels `transfer` when this control is cancelled, or now if it already
    /// is. The control holds it weakly, so a transfer that ended is dropped
    /// at the next registration; a backend keeps it alive while it runs.
    pub fn on_cancel(&self, transfer: Weak<dyn CancelTransfer>) {
        let mut waiting = self
            .cancel
            .waiting
            .lock()
            .unwrap_or_else(PoisonError::into_inner);
        if self.is_cancelled() {
            drop(waiting);
            if let Some(transfer) = transfer.upgrade() {
                transfer.cancel();
            }
            return;
        }
        waiting.retain(|transfer| transfer.strong_count() > 0);
        waiting.push(transfer);
    }

    /// Hands a progress reading to whatever [`with_progress`](Self::with_progress)
    /// registered, and does nothing where nothing did.
    ///
    /// Called by the backend doing the transfer - including an
    /// [`HttpClient`] an application provides through [`local_http_client`],
    /// which is why this is part of the control's public surface rather than
    /// private to the backends shipped here.
    pub fn report(&self, progress: HttpProgress) {
        if let Some(handler) = &self.progress {
            handler(progress);
        }
    }
}

/// The verbs a request can use.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
pub enum HttpMethod {
    /// Reads the resource.
    #[default]
    Get,
    /// Asks for the headers only, without the body.
    Head,
    /// Sends the request body to the server for processing.
    Post,
    /// Replaces the resource with the request body.
    Put,
    /// Removes the resource.
    Delete,
}

impl HttpMethod {
    /// The name that goes on the wire.
    pub fn name(self) -> &'static str {
        match self {
            HttpMethod::Get => "GET",
            HttpMethod::Head => "HEAD",
            HttpMethod::Post => "POST",
            HttpMethod::Put => "PUT",
            HttpMethod::Delete => "DELETE",
        }
    }
}

/// One request.
#[derive(Clone, Debug, Default, PartialEq, Eq)]
pub struct HttpRequest {
    /// The absolute URL to request.
    pub url: String,
    /// The HTTP method of the request.
    pub method: HttpMethod,
    /// The request headers as name and value pairs, in the order they are sent.
    pub headers: Vec<(String, String)>,
    /// The request body, or `None` for a request without one.
    pub body: Option<Vec<u8>>,
    /// Resume an interrupted transfer from this byte offset.
    ///
    /// Sent as a `Range` header. A server free to ignore it answers `200` with
    /// the whole body instead of `206` with the rest, which
    /// [`HttpResponse::resumed`] reports — a caller that appends to a partial
    /// file must check it, or it writes the start of the file over the middle.
    pub resume_from: Option<u64>,
}

impl HttpRequest {
    /// A `GET` for `url`.
    pub fn get(url: impl Into<String>) -> Self {
        Self {
            url: url.into(),
            ..Self::default()
        }
    }

    /// Sets the HTTP method of the request.
    pub fn method(mut self, method: HttpMethod) -> Self {
        self.method = method;
        self
    }

    /// Adds a header to the request. Calling it again with the same name adds a second header.
    pub fn header(mut self, name: impl Into<String>, value: impl Into<String>) -> Self {
        self.headers.push((name.into(), value.into()));
        self
    }

    /// Sets the request body.
    pub fn body(mut self, body: impl Into<Vec<u8>>) -> Self {
        self.body = Some(body.into());
        self
    }

    /// Resumes an interrupted transfer from `offset`.
    pub fn resume_from(mut self, offset: u64) -> Self {
        self.resume_from = (offset > 0).then_some(offset);
        self
    }
}

/// The future that an HTTP operation returns. It is `Send`, so it can run on any thread.
#[cfg(not(target_arch = "wasm32"))]
pub type HttpFuture<'a, T> = Pin<Box<dyn Future<Output = Result<T, HttpError>> + Send + 'a>>;

/// The future that an HTTP operation returns. It is not `Send` on wasm, because browser objects
/// belong to the one thread of the page.
#[cfg(target_arch = "wasm32")]
pub type HttpFuture<'a, T> = Pin<Box<dyn Future<Output = Result<T, HttpError>> + 'a>>;

/// A response body, read a chunk at a time.
///
/// A body is not a `Vec<u8>`: a model pack, a video, an application package are
/// all larger than the memory a phone will hand over, and the only way to read
/// one is to not hold it. Everything that wants the whole thing anyway asks for
/// it explicitly through [`HttpResponse::read_all`].
pub trait HttpBody {
    /// The next chunk, or `None` at the end of the body.
    fn read_chunk(&self) -> HttpFuture<'_, Option<Vec<u8>>>;
}

/// Shared handle to a response body.
///
/// Bounded exactly as [`HttpFuture`] is: a native transfer runs on a thread of
/// its own and its body crosses to whoever awaits it, while a browser's body is
/// a `ReadableStream` reader that belongs to the one thread a page has and
/// cannot claim otherwise.
#[cfg(not(target_arch = "wasm32"))]
pub type HttpBodyRef = Arc<dyn HttpBody + Send + Sync>;

/// Shared handle to a response body.
///
/// A browser's body is a `ReadableStream` reader that belongs to the one
/// thread a page has and cannot claim otherwise, so it is never `Send + Sync`
/// there; `Rc` avoids paying for synchronisation the target cannot use.
#[cfg(target_arch = "wasm32")]
pub type HttpBodyRef = std::rc::Rc<dyn HttpBody>;

/// Wraps `body` in an [`HttpBodyRef`].
///
/// A trait-object alias cannot expose its own `new`, so this is the one place
/// that picks `Arc` on native and `Rc` on wasm; callers that need an
/// [`HttpBodyRef`] from a concrete body go through this instead of repeating
/// that choice.
#[cfg(not(target_arch = "wasm32"))]
pub fn http_body_ref<B: HttpBody + Send + Sync + 'static>(body: B) -> HttpBodyRef {
    Arc::new(body)
}

/// See the native definition of [`http_body_ref`] for why this is `Rc` on wasm.
#[cfg(target_arch = "wasm32")]
pub fn http_body_ref<B: HttpBody + 'static>(body: B) -> HttpBodyRef {
    std::rc::Rc::new(body)
}

struct EmptyBody;

impl HttpBody for EmptyBody {
    fn read_chunk(&self) -> HttpFuture<'_, Option<Vec<u8>>> {
        Box::pin(async { Ok(None) })
    }
}

/// A body already in memory, handed out in `CHUNK_LEN`-sized pieces.
///
/// For a backend that has the bytes already — a cache, a bundled asset, a test
/// double — so it satisfies the same chunked contract as one still on the wire
/// and its callers cannot tell the difference.
pub struct BytesBody {
    bytes: Vec<u8>,
    offset: std::sync::Mutex<usize>,
    chunk: usize,
}

impl BytesBody {
    /// A body over `bytes`, delivered in one chunk per read.
    pub fn new(bytes: impl Into<Vec<u8>>) -> Self {
        Self::chunked(bytes, usize::MAX)
    }

    /// A body over `bytes`, delivered at most `chunk` bytes at a time.
    pub fn chunked(bytes: impl Into<Vec<u8>>, chunk: usize) -> Self {
        Self {
            bytes: bytes.into(),
            offset: std::sync::Mutex::new(0),
            chunk: chunk.max(1),
        }
    }

    /// How long the body is.
    pub fn len(&self) -> u64 {
        self.bytes.len() as u64
    }

    /// Whether the body carries nothing.
    pub fn is_empty(&self) -> bool {
        self.bytes.is_empty()
    }
}

impl HttpBody for BytesBody {
    fn read_chunk(&self) -> HttpFuture<'_, Option<Vec<u8>>> {
        Box::pin(async move {
            let mut offset = self.offset.lock().unwrap_or_else(PoisonError::into_inner);
            if *offset >= self.bytes.len() {
                return Ok(None);
            }
            let end = offset.saturating_add(self.chunk).min(self.bytes.len());
            let chunk = self.bytes[*offset..end].to_vec();
            *offset = end;
            Ok(Some(chunk))
        })
    }
}

/// What a server answered, with its body still to be read.
pub struct HttpResponse {
    /// The status line's code.
    pub status: u16,
    /// The response headers, with their names lower-cased.
    pub headers: Vec<(String, String)>,
    /// How long the body is, when the server said.
    pub content_length: Option<u64>,
    /// Whether the server honoured [`HttpRequest::resume_from`].
    ///
    /// `false` from a resumed request means the body starts at the beginning
    /// again, and a caller appending to a partial file must truncate it first.
    pub resumed: bool,
    /// The URL the request went to, for the errors reading the body reports.
    pub url: String,
    body: HttpBodyRef,
}

impl HttpResponse {
    /// Builds a response around a body.
    pub fn new(url: impl Into<String>, status: u16, body: HttpBodyRef) -> Self {
        Self {
            status,
            headers: Vec::new(),
            content_length: None,
            resumed: false,
            url: url.into(),
            body,
        }
    }

    /// A response with no body, which is what a `HEAD` answers with.
    pub fn empty(url: impl Into<String>, status: u16) -> Self {
        Self::new(url, status, http_body_ref(EmptyBody))
    }

    /// Sets the response headers as name and value pairs.
    pub fn with_headers(mut self, headers: Vec<(String, String)>) -> Self {
        self.headers = headers;
        self
    }

    /// Sets the length of the body in bytes, when the server said.
    pub fn with_content_length(mut self, content_length: Option<u64>) -> Self {
        self.content_length = content_length;
        self
    }

    /// Sets whether the server honoured [`HttpRequest::resume_from`].
    pub fn with_resumed(mut self, resumed: bool) -> Self {
        self.resumed = resumed;
        self
    }

    /// Whether the status is in the 2xx range.
    pub fn is_success(&self) -> bool {
        (200..300).contains(&self.status)
    }

    /// One header, matched without regard to case.
    pub fn header(&self, name: &str) -> Option<&str> {
        let name = name.to_ascii_lowercase();
        self.headers
            .iter()
            .find(|(key, _)| key.eq_ignore_ascii_case(&name))
            .map(|(_, value)| value.as_str())
    }

    /// Fails with [`HttpError::HttpStatus`] unless the status is a success.
    pub fn error_for_status(self) -> Result<Self, HttpError> {
        if self.is_success() {
            Ok(self)
        } else {
            Err(HttpError::HttpStatus {
                url: self.url.clone(),
                status: self.status,
            })
        }
    }

    /// The next chunk of the body, or `None` at its end.
    pub async fn read_chunk(&self) -> Result<Option<Vec<u8>>, HttpError> {
        self.body.read_chunk().await
    }

    /// The whole body.
    ///
    /// Named for what it does, so a caller reading something that might not fit
    /// in memory can see that it is asking for exactly that.
    pub async fn read_all(&self) -> Result<Vec<u8>, HttpError> {
        let mut out = Vec::with_capacity(self.content_length.unwrap_or(0).min(1 << 20) as usize);
        while let Some(chunk) = self.read_chunk().await? {
            out.extend_from_slice(&chunk);
        }
        Ok(out)
    }

    /// The whole body as text.
    pub async fn read_text(&self) -> Result<String, HttpError> {
        String::from_utf8(self.read_all().await?).map_err(|error| HttpError::InvalidResponse {
            url: self.url.clone(),
            message: error.to_string(),
        })
    }
}

/// A platform's HTTP.
///
/// One method carries the whole contract: everything else — text, bytes, a file
/// on disk — is that method plus what the caller does with the body, so a
/// backend implements the transfer once and cannot make `get_text` behave
/// differently from `download_to`.
pub trait HttpClient: Send + Sync {
    /// Sends `request`, resolving when the response's status line has arrived
    /// and its body is ready to be read.
    fn send<'a>(
        &'a self,
        request: &'a HttpRequest,
        control: HttpControl,
    ) -> HttpFuture<'a, HttpResponse>;

    /// The body of a successful `GET`, as text.
    fn get_text<'a>(&'a self, url: &'a str) -> HttpFuture<'a, String> {
        Box::pin(async move {
            self.send(&HttpRequest::get(url), HttpControl::new())
                .await?
                .error_for_status()?
                .read_text()
                .await
        })
    }

    /// The body of a successful `GET`.
    fn get_bytes<'a>(&'a self, url: &'a str) -> HttpFuture<'a, Vec<u8>> {
        Box::pin(async move {
            self.send(&HttpRequest::get(url), HttpControl::new())
                .await?
                .error_for_status()?
                .read_all()
                .await
        })
    }

    /// Streams a response into `target`, resuming from its current length when
    /// the server supports byte ranges, and returns the file's whole length.
    ///
    /// This is the appropriate call for model packs and other files that should
    /// not be held in memory. A cancelled transfer leaves the partial file
    /// where it is, which is what lets the next attempt continue rather than
    /// start again.
    #[cfg(not(target_arch = "wasm32"))]
    fn download_to<'a>(
        &'a self,
        url: &'a str,
        target: &'a Path,
        control: HttpControl,
    ) -> HttpFuture<'a, u64> {
        Box::pin(async move { download_through(self, url, target, control).await })
    }
}

/// A platform's own transfer for [`HttpClient::download_to`], one that goes
/// on while the application is suspended or closed.
///
/// It keeps the contract of `download_to`: the file at `target` grows from its
/// current length, a cancelled transfer keeps what arrived for the next call,
/// and the result is the file's whole length. A transfer that finished while
/// the application was closed is in `target` when the application starts
/// again, so a call that finds the whole file returns at once.
#[cfg(not(target_arch = "wasm32"))]
pub trait PlatformDownloader: Send + Sync {
    /// Starts or continues the download of `url` into `target`.
    ///
    /// Returns the whole length of the file in bytes.
    fn download<'a>(
        &'a self,
        url: &'a str,
        target: &'a Path,
        control: HttpControl,
    ) -> HttpFuture<'a, u64>;

    /// The bytes of `target` this downloader keeps apart from the file, as
    /// a paused transfer's, added to the file's own; `None` when it keeps
    /// none.
    fn downloaded_length(&self, target: &Path) -> Option<u64>;
}

/// How much of a download into `target` is on the device: the file's
/// length, or more where the platform keeps a paused transfer's bytes apart
/// from the file, as iOS does. An application shows a paused download's
/// progress with it.
#[cfg(not(target_arch = "wasm32"))]
pub fn downloaded_length(target: &Path) -> u64 {
    PLATFORM_DOWNLOADER
        .get()
        .and_then(|downloader| downloader.downloaded_length(target))
        .unwrap_or_else(|| std::fs::metadata(target).map_or(0, |metadata| metadata.len()))
}

#[cfg(not(target_arch = "wasm32"))]
static PLATFORM_DOWNLOADER: crate::registry::ServiceRegistry<dyn PlatformDownloader> =
    crate::registry::ServiceRegistry::new();

/// Installs the platform's downloader; [`default_http_client`] then runs
/// `download_to` through it. iOS installs one on a background `URLSession`.
// forwards on purpose: the registry is private to this crate, and this is the
// one way a platform backend installs into it.
#[cfg(not(target_arch = "wasm32"))]
pub fn set_platform_downloader(downloader: Arc<dyn PlatformDownloader>) {
    PLATFORM_DOWNLOADER.set(downloader);
}

#[cfg(not(target_arch = "wasm32"))]
async fn download_through<C: HttpClient + ?Sized>(
    client: &C,
    url: &str,
    target: &Path,
    control: HttpControl,
) -> Result<u64, HttpError> {
    use std::io::Write;

    if control.is_cancelled() {
        return Err(HttpError::Cancelled);
    }
    let existing = std::fs::metadata(target).map_or(0, |metadata| metadata.len());
    let mut request = HttpRequest::get(url);
    if existing > 0 {
        request = request.resume_from(existing);
    }
    // The request reports nothing itself: its count starts at zero, and this
    // function reports the whole file's.
    let response = client
        .send(&request, control.cancel_only())
        .await?
        .error_for_status()?;

    let base = if response.resumed { existing } else { 0 };
    let total = response.content_length.map(|remaining| base + remaining);
    let mut output = if response.resumed {
        std::fs::OpenOptions::new().append(true).open(target)
    } else {
        std::fs::File::create(target)
    }
    .map_err(|error| file_write_error(target, &error))?;

    let mut transferred = base;
    control.report(HttpProgress { transferred, total });
    while let Some(chunk) = response.read_chunk().await? {
        if control.is_cancelled() {
            return Err(HttpError::Cancelled);
        }
        output
            .write_all(&chunk)
            .map_err(|error| file_write_error(target, &error))?;
        transferred += chunk.len() as u64;
        control.report(HttpProgress { transferred, total });
    }
    output
        .flush()
        .map_err(|error| file_write_error(target, &error))?;
    Ok(transferred)
}

#[cfg(not(target_arch = "wasm32"))]
fn file_write_error(target: &Path, error: &std::io::Error) -> HttpError {
    HttpError::FileWrite {
        path: target.display().to_string(),
        kind: error.kind(),
        message: error.to_string(),
    }
}

/// Shared handle to an [`HttpClient`].
pub type HttpClientRef = Arc<dyn HttpClient>;

/// Runs `task` for every item with at most `concurrency` tasks at a time, each on its own
/// thread. Results keep the order of `items`. A `concurrency` of zero counts as one.
/// Returns [`HttpError::WorkerPanicked`] when a task panics.
#[cfg(not(target_arch = "wasm32"))]
pub async fn map_ordered_concurrent<I, T, F, Fut>(
    items: &[I],
    concurrency: usize,
    task: F,
) -> Result<Vec<T>, HttpError>
where
    I: Clone + Send,
    T: Send,
    F: Fn(I) -> Fut + Send + Sync + 'static,
    Fut: Future<Output = T> + Send,
{
    let task = Arc::new(task);
    let mut results = Vec::with_capacity(items.len());

    for chunk in items.chunks(concurrency.max(1)) {
        let chunk_results = std::thread::scope(|scope| {
            let mut handles = Vec::with_capacity(chunk.len());
            for item in chunk.iter().cloned() {
                let task = Arc::clone(&task);
                handles.push(scope.spawn(move || pollster::block_on(task(item))));
            }

            let mut chunk_results = Vec::with_capacity(handles.len());
            for handle in handles {
                let value = handle.join().map_err(|_| HttpError::WorkerPanicked {
                    operation: "ordered concurrent task",
                })?;
                chunk_results.push(value);
            }
            Ok::<Vec<T>, HttpError>(chunk_results)
        })?;
        results.extend(chunk_results);
    }

    Ok(results)
}

/// Runs `task` for every item with at most `concurrency` tasks in flight on the browser thread.
/// Results keep the order of `items`. A `concurrency` of zero counts as one.
#[cfg(all(target_arch = "wasm32", feature = "web-http"))]
pub async fn map_ordered_concurrent<I, T, F, Fut>(
    items: &[I],
    concurrency: usize,
    task: F,
) -> Result<Vec<T>, HttpError>
where
    I: Clone,
    F: Fn(I) -> Fut + Clone,
    Fut: Future<Output = T>,
{
    let mut results = stream::iter(items.iter().cloned().enumerate().map(|(index, item)| {
        let task = task.clone();
        async move { (index, task(item).await) }
    }))
    .buffer_unordered(concurrency.max(1))
    .collect::<Vec<_>>()
    .await;

    results.sort_by_key(|(index, _)| *index);
    Ok(results.into_iter().map(|(_, value)| value).collect())
}

/// Runs `task` for every item one after another, because this wasm build has no `web-http`
/// feature. `concurrency` is ignored. Results keep the order of `items`.
#[cfg(all(target_arch = "wasm32", not(feature = "web-http")))]
pub async fn map_ordered_concurrent<I, T, F, Fut>(
    items: &[I],
    _concurrency: usize,
    task: F,
) -> Result<Vec<T>, HttpError>
where
    I: Clone,
    F: Fn(I) -> Fut,
    Fut: Future<Output = T>,
{
    Ok(map_ordered_sequential(items, task).await)
}

#[cfg(all(target_arch = "wasm32", not(feature = "web-http")))]
async fn map_ordered_sequential<I, T, F, Fut>(items: &[I], task: F) -> Vec<T>
where
    I: Clone,
    F: Fn(I) -> Fut,
    Fut: Future<Output = T>,
{
    let mut results = Vec::with_capacity(items.len());
    for item in items.iter().cloned() {
        results.push(task(item).await);
    }
    results
}

/// What a [`StubHttpClient`] answers a request with.
pub type StubAnswer = dyn Fn(&HttpRequest) -> Result<HttpResponse, HttpError> + Send + Sync;

/// An HTTP client that answers from a function rather than a network.
///
/// A screen driven from a recorded payload, a robot run that must not depend on
/// a server, a test that wants a specific failure: all of them want one thing,
/// which is to decide what a request answers. Written as a client each time,
/// that is the same dozen lines of body plumbing repeated — and each copy is a
/// chance to answer differently from the real client.
///
/// ```ignore
/// let client: HttpClientRef = Arc::new(StubHttpClient::from_text(move |url| {
///     Ok(fixture_for(url))
/// }));
/// ```
pub struct StubHttpClient {
    answer: Box<StubAnswer>,
}

impl StubHttpClient {
    /// Answers from a function of the whole request.
    pub fn new(
        answer: impl Fn(&HttpRequest) -> Result<HttpResponse, HttpError> + Send + Sync + 'static,
    ) -> Self {
        Self {
            answer: Box::new(answer),
        }
    }

    /// Answers every request with `body` and a `200`.
    pub fn with_body(body: impl Into<Vec<u8>>) -> Self {
        let body = body.into();
        Self::new(move |request| {
            Ok(HttpResponse::new(
                request.url.clone(),
                200,
                http_body_ref(BytesBody::new(body.clone())),
            ))
        })
    }

    /// Answers with text, or an error, from a function of the URL.
    pub fn from_text(
        answer: impl Fn(&str) -> Result<String, HttpError> + Send + Sync + 'static,
    ) -> Self {
        Self::new(move |request| {
            let body = answer(&request.url)?;
            Ok(HttpResponse::new(
                request.url.clone(),
                200,
                http_body_ref(BytesBody::new(body)),
            ))
        })
    }
}

impl HttpClient for StubHttpClient {
    fn send<'a>(
        &'a self,
        request: &'a HttpRequest,
        _control: HttpControl,
    ) -> HttpFuture<'a, HttpResponse> {
        Box::pin(async move { (self.answer)(request) })
    }
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
const CHUNK_LEN: usize = 64 * 1024;

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
const IDLE_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(60);

struct DefaultHttpClient {
    #[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
    native_client: Result<reqwest::blocking::Client, HttpError>,
}

impl DefaultHttpClient {
    fn new() -> Self {
        Self {
            #[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
            native_client: build_native_client(IDLE_TIMEOUT),
        }
    }

    #[cfg(all(test, not(target_arch = "wasm32"), feature = "http-native"))]
    fn with_idle_timeout(idle: std::time::Duration) -> Self {
        Self {
            native_client: build_native_client(idle),
        }
    }
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
struct ResponseHead {
    status: u16,
    headers: Vec<(String, String)>,
    content_length: Option<u64>,
    resumed: bool,
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
struct NativeTransfer {
    head: crate::async_io::Signal<Result<ResponseHead, HttpError>>,
    chunks: crate::async_io::ChunkStream<HttpError>,
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
impl CancelTransfer for NativeTransfer {
    fn cancel(&self) {
        self.head.set(Err(HttpError::Cancelled));
        self.chunks.fail(HttpError::Cancelled);
    }
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
struct ChannelBody {
    transfer: Arc<NativeTransfer>,
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
impl HttpBody for ChannelBody {
    fn read_chunk(&self) -> HttpFuture<'_, Option<Vec<u8>>> {
        Box::pin(self.transfer.chunks.next())
    }
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
async fn send_native(
    client: reqwest::blocking::Client,
    request: HttpRequest,
    control: HttpControl,
) -> Result<HttpResponse, HttpError> {
    use crate::async_io::{ChunkChannel, Signal};

    let head = Signal::new();
    let (chunks, stream) = ChunkChannel::<HttpError>::new();
    let transfer = Arc::new(NativeTransfer {
        head: head.clone(),
        chunks: stream,
    });
    control.on_cancel(Arc::<NativeTransfer>::downgrade(&transfer));
    let url = request.url.clone();

    std::thread::Builder::new()
        .name("cranpose-http".to_string())
        .spawn(move || {
            if let Err(error) = read_native_response(&client, request, &control, &chunks, &head) {
                head.set(Err(error.clone()));
                chunks.fail(error);
            }
        })
        .map_err(|error| HttpError::RequestFailed {
            url: url.clone(),
            message: format!("could not start the transfer: {error}"),
        })?;

    let head = transfer
        .head
        .wait()
        .await
        .ok_or_else(|| HttpError::RequestFailed {
            url: url.clone(),
            message: "the transfer ended before a response arrived".to_string(),
        })??;

    Ok(
        HttpResponse::new(url, head.status, http_body_ref(ChannelBody { transfer }))
            .with_headers(head.headers)
            .with_content_length(head.content_length)
            .with_resumed(head.resumed),
    )
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
fn read_native_response(
    client: &reqwest::blocking::Client,
    request: HttpRequest,
    control: &HttpControl,
    chunks: &crate::async_io::ChunkChannel<HttpError>,
    head_signal: &crate::async_io::Signal<Result<ResponseHead, HttpError>>,
) -> Result<(), HttpError> {
    use std::io::Read;

    if control.is_cancelled() {
        return Err(HttpError::Cancelled);
    }
    let HttpRequest {
        url,
        method,
        headers,
        body,
        resume_from,
    } = request;
    let method = match method {
        HttpMethod::Get => reqwest::Method::GET,
        HttpMethod::Head => reqwest::Method::HEAD,
        HttpMethod::Post => reqwest::Method::POST,
        HttpMethod::Put => reqwest::Method::PUT,
        HttpMethod::Delete => reqwest::Method::DELETE,
    };
    let mut builder = client.request(method, &url);
    for (name, value) in headers {
        builder = builder.header(name, value);
    }
    if let Some(offset) = resume_from {
        builder = builder.header(reqwest::header::RANGE, format!("bytes={offset}-"));
    }
    if let Some(body) = body {
        builder = builder.body(body);
    }

    let mut response = builder.send().map_err(|error| HttpError::RequestFailed {
        url: url.clone(),
        message: error.to_string(),
    })?;

    let status = response.status();
    let resumed = resume_from.is_some() && status == reqwest::StatusCode::PARTIAL_CONTENT;
    let content_length = response.content_length();
    let headers = response
        .headers()
        .iter()
        .map(|(name, value)| {
            (
                name.as_str().to_ascii_lowercase(),
                value.to_str().unwrap_or_default().to_string(),
            )
        })
        .collect();
    head_signal.set(Ok(ResponseHead {
        status: status.as_u16(),
        headers,
        content_length,
        resumed,
    }));

    let mut buffer = vec![0u8; CHUNK_LEN];
    let mut transferred = 0u64;
    loop {
        if control.is_cancelled() {
            return Err(HttpError::Cancelled);
        }
        let count = response
            .read(&mut buffer)
            .map_err(|error| HttpError::BodyReadFailed {
                url: url.clone(),
                message: error.to_string(),
            })?;
        if count == 0 {
            break;
        }
        transferred += count as u64;
        control.report(HttpProgress {
            transferred,
            total: content_length,
        });
        if !chunks.push(buffer[..count].to_vec()) {
            break;
        }
    }
    Ok(())
}

#[cfg(all(not(target_arch = "wasm32"), not(feature = "http-native")))]
async fn send_native(
    _request: HttpRequest,
    _control: HttpControl,
) -> Result<HttpResponse, HttpError> {
    Err(HttpError::UnsupportedFeature {
        operation: "native HTTP requests",
        feature: "http-native",
    })
}

impl HttpClient for DefaultHttpClient {
    fn send<'a>(
        &'a self,
        request: &'a HttpRequest,
        control: HttpControl,
    ) -> HttpFuture<'a, HttpResponse> {
        Box::pin(async move {
            #[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
            {
                let client = self.native_client.as_ref().map_err(Clone::clone)?.clone();
                send_native(client, request.clone(), control).await
            }
            #[cfg(all(not(target_arch = "wasm32"), not(feature = "http-native")))]
            {
                send_native(request.clone(), control).await
            }
            #[cfg(target_arch = "wasm32")]
            {
                send_web(request, control).await
            }
        })
    }

    #[cfg(not(target_arch = "wasm32"))]
    fn download_to<'a>(
        &'a self,
        url: &'a str,
        target: &'a Path,
        control: HttpControl,
    ) -> HttpFuture<'a, u64> {
        Box::pin(async move {
            match PLATFORM_DOWNLOADER.get() {
                Some(downloader) => downloader.download(url, target, control).await,
                None => download_through(self, url, target, control).await,
            }
        })
    }
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
fn build_native_client(idle: std::time::Duration) -> Result<reqwest::blocking::Client, HttpError> {
    configure_native_client_builder(
        reqwest::blocking::Client::builder()
            .connect_timeout(std::time::Duration::from_secs(30))
            .timeout(idle)
            .user_agent(concat!("cranpose/", env!("CARGO_PKG_VERSION"))),
    )?
    .build()
    .map_err(|err| HttpError::ClientInit(err.to_string()))
}

#[cfg(all(not(target_arch = "wasm32"), feature = "http-native"))]
fn configure_native_client_builder(
    builder: reqwest::blocking::ClientBuilder,
) -> Result<reqwest::blocking::ClientBuilder, HttpError> {
    #[cfg(target_os = "android")]
    {
        Ok(builder.tls_certs_only(android_root_certificates()?))
    }

    #[cfg(not(target_os = "android"))]
    {
        Ok(builder)
    }
}

#[cfg(all(target_os = "android", feature = "http-native"))]
fn android_root_certificates() -> Result<Vec<reqwest::Certificate>, HttpError> {
    certificates_from_der_chain(
        webpki_root_certs::TLS_SERVER_ROOT_CERTS
            .iter()
            .map(AsRef::as_ref),
    )
}

#[cfg(any(
    all(test, not(target_arch = "wasm32"), feature = "http-native"),
    all(target_os = "android", feature = "http-native")
))]
fn certificates_from_der_chain<'a, I>(
    certificates: I,
) -> Result<Vec<reqwest::Certificate>, HttpError>
where
    I: IntoIterator<Item = &'a [u8]>,
{
    certificates
        .into_iter()
        .enumerate()
        .map(|(index, der)| {
            reqwest::Certificate::from_der(der).map_err(|err| {
                HttpError::ClientInit(format!(
                    "Failed to load TLS root certificate {index}: {err}"
                ))
            })
        })
        .collect()
}

#[cfg(all(target_arch = "wasm32", feature = "web-http"))]
struct FetchBody {
    url: String,
    reader: web_sys::ReadableStreamDefaultReader,
    control: HttpControl,
    transferred: std::cell::Cell<u64>,
    total: Option<u64>,
}

#[cfg(all(target_arch = "wasm32", feature = "web-http"))]
impl HttpBody for FetchBody {
    fn read_chunk(&self) -> HttpFuture<'_, Option<Vec<u8>>> {
        use wasm_bindgen_futures::JsFuture;

        Box::pin(async move {
            if self.control.is_cancelled() {
                let _ = self.reader.cancel();
                return Err(HttpError::Cancelled);
            }
            let result = JsFuture::from(self.reader.read()).await.map_err(|error| {
                HttpError::BodyReadFailed {
                    url: self.url.clone(),
                    message: format!("{error:?}"),
                }
            })?;
            let done = js_sys::Reflect::get(&result, &wasm_bindgen::JsValue::from_str("done"))
                .ok()
                .and_then(|value| value.as_bool())
                .unwrap_or(true);
            if done {
                return Ok(None);
            }
            let value = js_sys::Reflect::get(&result, &wasm_bindgen::JsValue::from_str("value"))
                .map_err(|error| HttpError::BodyReadFailed {
                    url: self.url.clone(),
                    message: format!("{error:?}"),
                })?;
            let chunk = js_sys::Uint8Array::new(&value).to_vec();
            self.transferred
                .set(self.transferred.get() + chunk.len() as u64);
            self.control.report(HttpProgress {
                transferred: self.transferred.get(),
                total: self.total,
            });
            Ok(Some(chunk))
        })
    }
}

#[cfg(all(target_arch = "wasm32", feature = "web-http"))]
async fn send_web(request: &HttpRequest, control: HttpControl) -> Result<HttpResponse, HttpError> {
    use wasm_bindgen::JsCast;
    use wasm_bindgen_futures::JsFuture;
    use web_sys::{Request, RequestInit, RequestMode, Response};

    if control.is_cancelled() {
        return Err(HttpError::Cancelled);
    }
    let options = RequestInit::new();
    options.set_method(request.method.name());
    options.set_mode(RequestMode::Cors);
    if let Some(body) = &request.body {
        options.set_body(&js_sys::Uint8Array::from(body.as_slice()).into());
    }

    let fetch_request =
        Request::new_with_str_and_init(&request.url, &options).map_err(|error| {
            HttpError::RequestFailed {
                url: request.url.clone(),
                message: format!("{error:?}"),
            }
        })?;
    let headers = fetch_request.headers();
    for (name, value) in &request.headers {
        headers
            .set(name, value)
            .map_err(|error| HttpError::RequestFailed {
                url: request.url.clone(),
                message: format!("{error:?}"),
            })?;
    }
    if let Some(offset) = request.resume_from {
        headers
            .set("Range", &format!("bytes={offset}-"))
            .map_err(|error| HttpError::RequestFailed {
                url: request.url.clone(),
                message: format!("{error:?}"),
            })?;
    }

    let window = web_sys::window().ok_or(HttpError::NoWindow)?;
    let value = JsFuture::from(window.fetch_with_request(&fetch_request))
        .await
        .map_err(|error| HttpError::RequestFailed {
            url: request.url.clone(),
            message: format!("{error:?}"),
        })?;
    let response: Response = value.dyn_into().map_err(|_| HttpError::InvalidResponse {
        url: request.url.clone(),
        message: "the browser answered with something that is not a Response".to_string(),
    })?;

    let status = response.status();
    let mut header_pairs = Vec::new();
    let entries = js_sys::try_iter(&response.headers()).ok().flatten();
    if let Some(entries) = entries {
        for entry in entries.flatten() {
            let pair = js_sys::Array::from(&entry);
            if pair.length() >= 2 {
                let name = pair.get(0).as_string().unwrap_or_default();
                let value = pair.get(1).as_string().unwrap_or_default();
                header_pairs.push((name.to_ascii_lowercase(), value));
            }
        }
    }
    let total = header_pairs
        .iter()
        .find(|(name, _)| name == "content-length")
        .and_then(|(_, value)| value.parse::<u64>().ok());
    let resumed = request.resume_from.is_some() && status == 206;

    let body = response.body().ok_or_else(|| HttpError::InvalidResponse {
        url: request.url.clone(),
        message: "the response carries no body".to_string(),
    })?;
    let reader: web_sys::ReadableStreamDefaultReader =
        body.get_reader()
            .dyn_into()
            .map_err(|_| HttpError::InvalidResponse {
                url: request.url.clone(),
                message: "the response body cannot be read in chunks".to_string(),
            })?;

    Ok(HttpResponse::new(
        request.url.clone(),
        status,
        http_body_ref(FetchBody {
            url: request.url.clone(),
            reader,
            control,
            transferred: std::cell::Cell::new(0),
            total,
        }),
    )
    .with_headers(header_pairs)
    .with_content_length(total)
    .with_resumed(resumed))
}

#[cfg(all(target_arch = "wasm32", not(feature = "web-http")))]
async fn send_web(request: &HttpRequest, _control: HttpControl) -> Result<HttpResponse, HttpError> {
    let _ = request;
    Err(HttpError::UnsupportedFeature {
        operation: "web HTTP requests",
        feature: "web-http",
    })
}

/// The platform's HTTP client.
///
/// A native transfer fails when the response, or the next piece of its body,
/// does not arrive within 60 seconds; a body that keeps arriving runs as long
/// as it needs. A socket left open by a phone that slept or lost its network
/// therefore ends in an error, and [`HttpClient::download_to`] resumes from the
/// bytes already on disk at the next attempt.
pub fn default_http_client() -> HttpClientRef {
    Arc::new(DefaultHttpClient::new())
}

/// The HTTP client a composable reads. It is the platform client unless a composition local
/// provider above it supplies another one.
pub fn local_http_client() -> CompositionLocal<HttpClientRef> {
    crate::composition_locals::cached_local(
        |locals| &locals.http,
        || compositionLocalOfWithPolicy(default_http_client, Arc::ptr_eq),
    )
}

#[cfg(test)]
#[path = "tests/http_tests.rs"]
mod tests;