问题
I am trying to create a simple function that downloads a remote file to a local filepath using hyper. I need the file write to be asynchronous as well (in my case I am using tokio_fs
for that). Here is the code:
View in the playground
// Parts of the code were omitted, see the playground for full source code
pub fn download_file(
uri: Uri,
file_location: &Path,
) -> Box<Future<Item = (), Error = DownloadFileError>> {
let temp_dir_path = tempfile::tempdir().unwrap().into_path();
let file_name = match file_location.file_name() {
Some(file_name) => file_name,
None => return Box::new(futures::failed(DownloadFileError::IncorrectFilePath)),
};
let temp_filepath = temp_dir_path.join(&file_name);
let connector = HttpsConnector::new(2).unwrap();
let client: Client<_, Body> = Client::builder().build(connector);
let response_future = client
.get(uri)
.map_err(|err| DownloadFileError::GetRequest(err));
let create_file_future =
File::create(temp_filepath).map_err(|err| DownloadFileError::CreateFile(err));
Box::new(
response_future
.join(create_file_future)
.and_then(move |(res, file)| {
res.into_body()
.map_err(|e| DownloadFileError::GetRequest(e))
.for_each(move |chunk| {
io::write_all(file, chunk)
.map(|_| ())
.map_err(|_| DownloadFileError::FileWrite)
})
}),
)
}
However, I get the following error:
error[E0507]: cannot move out of captured outer variable in an `FnMut` closure
--> src/lib.rs:79:39
|
75 | .and_then(move |(res, file)| {
| ---- captured outer variable
...
79 | io::write_all(file, chunk)
| ^^^^ cannot move out of captured outer variable in an `FnMut` closure
Conceptually, I understand what the error means: Since a FnMut
captures variables by mutable reference, I cannot move a captured variable. However, I do not understand how can I work around this problem in the example given, since I need to write the stream to the file returned by the Join
future.
The write_all
method from the Write
trait would work here since it takes the file as a mutable reference, but the problem is that it does the writing on the same thread.
回答1:
You do not want to use for_each
. io::write_all consumes the target and the buffer in exchange for a future that will return the target and the buffer when it is done. You can combine this with Stream::fold to reuse the file:
.fold(file, |file, chunk| {
io::write_all(file, chunk)
.map(|(f, _c)| f)
.map_err(|_| DownloadFileError::FileWrite)
})
.map(drop)
See also:
- How do I write a futures::Stream to disk without storing it entirely in memory first?
- How to save a file downloaded from S3 with Rusoto to my hard drive?
来源:https://stackoverflow.com/questions/53330556/writing-a-chunk-stream-to-a-file-asynchronously-using-hyper