I'am trying to implement educational client-server application using Rust and Iron. I've encountered the behaviour that I can't understand. Here is the code:
fn main() {
Iron::new(hello_world).http("localhost:3000").unwrap();
let mut input = String::new();
io::stdin().read_line(&mut input)
.expect("Failed to read line");
println!("You entered: {}", &input)
}
fn hello_world(_: &mut Request) -> IronResult<Response> {
Ok(Response::with((status::Ok, "Hello World!")))
}
When I run it and try to enter something from the keyboard, the line You entered: Some text is not appearing.
But after I changed this line:
Iron::new(hello_world).http("localhost:3000").unwrap();
With this:
let listener = Iron::new(hello_world).http("localhost:3000").unwrap();
I got string You entered: Some text on my console. So it seems to work. But now I have warning about unused variable. This behaviour is confusing.
Can anyone explain why this actually happens?
In the first version of your code, the first line will block waiting for incoming connections. This is because of the following:
Iron::new(hello_world).http("localhost:3000").unwrap() produces an object of type Listening, which will start listening to http requests in a separate thread.
The Listening struct implements the Drop trait, i.e. any objects of type Listening will run a drop function when they fall out of scope. Said drop function will join the listening thread, blocking further execution of your program.
By not assigning the Listening object to a variable, it falls out of scope immediately. This means that the drop function is run right after the object's creation.
Alternative explanation in code
The first version of your program:
fn main() {
Iron::new(hello_world).http("localhost:3000").unwrap();
// The listening thread is joined here, so the program blocks
// The instructions below will never be executed
let mut input = String::new();
io::stdin().read_line(&mut input)
.expect("Failed to read line");
println!("You entered: {}", &input)
}
The results of introducing a variable:
fn main() {
let listener = Iron::new(hello_world).http("localhost:3000").unwrap();
let mut input = String::new();
io::stdin().read_line(&mut input)
.expect("Failed to read line");
println!("You entered: {}", &input)
// The listening thread is joined here, so the program blocks
// As you can see, the program will not exit
}
Related
In the following program I use Tokio's mpsc channels. The Sender is moved to a task named input_message and the Receiver is moved to another task named printer. Both tasks are tokio::spawn()-ed in the main function. The input_message task is to read the user's input and send it through a Channel. The printer task recv() on the channel to get the user's input and simply prints it to stdout:
use std::error::Error;
use tokio::sync::mpsc;
use std::io::{BufRead, Write};
#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
let (tx, mut rx) = mpsc::unbounded_channel::<String>();
let printer = tokio::spawn(async move {
loop {
let res = rx.recv().await; // (11) Comment this ..
// let res = rx.try_recv(); // (12) Uncomment this ,,
if let Some(m) = res { // .. and this
// if let Ok(m) = res { // ,, and this
if m.trim() == "q".to_string() {
break;
}
println!("Received: {}", m.trim());
}
}
println!("Printer exited");
});
let input_message = tokio::spawn(async move {
let stdin = std::io::stdin();
let mut bufr = std::io::BufReader::new(stdin);
let mut buf = String::new();
loop {
// Let the printer thread print the string before asking the user's input.
std::thread::sleep(std::time::Duration::from_millis(1));
print!("Enter input: ");
std::io::stdout().flush().unwrap();
bufr.read_line(&mut buf).unwrap();
if buf.trim() == "q".to_string() {
tx.send(buf).unwrap();
break;
}
tx.send(buf).unwrap();
buf = String::new();
}
println!("InputMessage exited");
});
tokio::join!(input_message, printer);
Ok(())
}
The expected behavior of the program is to:
Ask the user a random input (q to quit)
Print that same input to stdout
Using rx.recv().await as in line 11-13 the program seems to buffer the Strings representing the user's input: the various inputs are not received by the printer task that therefore does not print the strings to stdout. Once the quit message (i.e. q) is sent, the input_message task exits and the messages seems to be flushed out of the channel and the receiver processes them all at once, and so the printer task prints all the inputs at once. Here's an example of wrong output:
Enter input: Hello
Enter input: World
Enter input: q
InputMessage exited
Received: Hello
Received: World
Printer exited
My question here is, how is it possible that the channel buffers the messages and processes them in one go only when the sending thread exits, instead of receiving them as they are sent?
What I tried to do is to use the try_recv() function as in line 12-14 and indeed it fixes the problem. The output is correctly printed, here is an example:
Enter input: Hello
Received: Hello
Enter input: World
Received: World
Enter input: q
InputMessage exited
Printer exited
In light of this, I get confused. I get the difference between the recv().await and the try_recv() functions but I think there's something more in this case that I'm ignoring that makes the latter work and the former not work. Is anybody able to shed some light and elaborate on this? Why does try_recv() work and recv().await not, and why should recv().await not work in this scenario? In terms of efficiency is looping on try_recv() bad or "bad practice" at all?
There are a few things to point out here, but first of all, you are waiting for lines on std::io::stdin() which blocks the thread until a line arrives on that stream. While the thread waiting for input, no other future can be executed on this thread, this blog post is a great resource if you want to dive deeper why you shouldn't do that.
Tokio's io module offers an async handle to stdin(), you can work with this as a quick fix, although the documentation explicitly mentions that you should spin up a dedicated (non-async) thread for interactive user input instead of using the async handle.
Swapping std::io::stdin() for tokio::io::stdin() also entails swapping out the standard library BufReader for tokio's implementation that wraps an R: AsyncRead rather than R: Read.
To prevent interleaved writes between the input task and the output task, you can use a responder channel that signals to the input task when the output has been printed. Instead of sending String over the channel, you could send a Message with these fields:
struct Message {
payload: String,
done_tx: oneshot::Sender<()>,
}
After reading an input line, send the Message over the channel to the printer task. The printer task prints the String and signals through the done_tx that the input task can print the input prompt and wait for a new line.
Putting all that together with some other changes like a while loop to wait for messages, you'd end up with something like this:
use std::error::Error;
use tokio::io::{AsyncBufReadExt, AsyncWriteExt};
use tokio::sync::{mpsc, oneshot};
#[derive(Debug)]
struct Message {
done_tx: oneshot::Sender<()>,
message: String,
}
#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
let (tx, mut rx) = mpsc::unbounded_channel::<Message>();
let printer = tokio::spawn(async move {
while let Some(Message {
message: m,
done_tx,
}) = rx.recv().await
{
if m.trim() == "q".to_string() {
break;
}
println!("Received: {}", m.trim());
done_tx.send(()).unwrap();
}
println!("Printer exited");
});
let input_message = tokio::spawn(async move {
let stdin = tokio::io::stdin();
let mut stdout = tokio::io::stdout();
let mut bufr = tokio::io::BufReader::new(stdin);
let mut buf = String::new();
loop {
// Let the printer thread print the string before asking the user's input.
stdout.write(b"Enter input: ").await.unwrap();
stdout.flush().await.unwrap();
bufr.read_line(&mut buf).await.unwrap();
let end = buf.trim() == "q";
let (done_tx, done) = oneshot::channel();
let message = Message {
message: buf,
done_tx,
};
tx.send(message).unwrap();
if end {
break;
}
done.await.unwrap();
buf = String::new();
}
println!("InputMessage exited");
});
tokio::join!(input_message, printer);
Ok(())
}
I'll try to simplify as much as possible what I'm trying to do accomplish but in a nutshell here is my problem:
I am trying to spawn the node shell as a process in Rust. I would like to pass to the process' stdin javascript code and read the nodejs output from stdout of the process. This would be an interactive usage where the node shell is spawned and keeps receiving JS instructions and executing them.
I do not wish to launch the nodejs app using a file argument.
I have read quite a bit about std::process::Command, tokio and why we can't write and read to a piped input using standard library. One of the solutions that I kept seeing online (in order to not block the main thread while reading/writing) is to use a thread for reading the output. Most solutions did not involve a continuous write/read flow.
What I have done is to spawn 2 threads, one that keeps writing to stdin and one that keeps reading from stdout. That way, I thought, I won't be blocking the main thread. However my issue is that only 1 thread can actively be used. When I have a thread for stdin, stdout does not even receive data.
Here is the code, comments should provide more details
pub struct Runner {
handle: Child,
pub input: Arc<Mutex<String>>,
pub output: Arc<Mutex<String>>,
input_thread: JoinHandle<()>,
output_thread: JoinHandle<()>,
}
impl Runner {
pub fn new() -> Runner {
let mut handle = Command::new("node")
.stdin(Stdio::piped())
.stdout(Stdio::piped())
.spawn()
.expect("Failed to spawn node process!");
// begin stdout thread part
let mut stdout = handle.stdout.take().unwrap();
let output = Arc::new(Mutex::new(String::new()));
let out_clone = Arc::clone(&output);
let output_thread = spawn(move || loop {
// code here never executes...why ?
let mut buf: [u8; 1] = [0];
let mut output = out_clone.lock().unwrap();
let what_i_read = stdout.read(&mut buf);
println!("reading: {:?}", what_i_read);
match what_i_read {
Err(err) => {
println!("{}] Error reading from stream: {}", line!(), err);
break;
}
Ok(bytes_read) => {
if bytes_read != 0 {
let char = String::from_utf8(buf.to_vec()).unwrap();
output.push_str(char.as_str());
} else if output.len() != 0 {
println!("result: {}", output);
out_clone.lock().unwrap().clear();
}
}
}
});
// begin stdin thread block
let mut stdin = handle.stdin.take().unwrap();
let input = Arc::new(Mutex::new(String::new()));
let input_clone = Arc::clone(&input);
let input_thread = spawn(move || loop {
let mut in_text = input_clone.lock().unwrap();
if in_text.len() != 0 {
println!("writing: {}", in_text);
stdin.write_all(in_text.as_bytes()).expect("!write");
stdin.write_all("\n".as_bytes()).expect("!write");
in_text.clear();
}
});
Runner {
handle,
input,
output,
input_thread,
output_thread,
}
}
// this function should receive commands
pub fn execute(&mut self, str: &str) {
let input = Arc::clone(&self.input);
let mut input = input.lock().unwrap();
input.push_str(str);
}
}
In the main thread I'd like use this as
let mut runner = Runner::new();
runner.execute("console.log('foo'");
println!("{:?}", runner.output);
I am still new to Rust but at least I passed the point where the borrow checker makes me bang my head against the wall, I am starting to find it more pleasing now :)
I'm making a small ncurses application in Rust that needs to communicate with a child process. I already have a prototype written in Common Lisp. I'm trying to rewrite it because CL uses a huge amount of memory for such a small tool.
I'm having some trouble figuring out how to interact with the sub-process.
What I'm currently doing is roughly this:
Create the process:
let mut program = match Command::new(command)
.args(arguments)
.stdin(Stdio::piped())
.stdout(Stdio::piped())
.stderr(Stdio::piped())
.spawn()
{
Ok(child) => child,
Err(_) => {
println!("Cannot run program '{}'.", command);
return;
}
};
Pass it to an infinite (until user exits) loop, which reads and handles input and listens for output like this (and writes it to the screen):
fn listen_for_output(program: &mut Child, output_viewer: &TextViewer) {
match program.stdout {
Some(ref mut out) => {
let mut buf_string = String::new();
match out.read_to_string(&mut buf_string) {
Ok(_) => output_viewer.append_string(buf_string),
Err(_) => return,
};
}
None => return,
};
}
The call to read_to_string however blocks the program until the process exits. From what I can see read_to_end and read also seem to block. If I try running something like ls which exits right away, it works, but with something that doesn't exit like python or sbcl it only continues once I kill the subprocess manually.
Based on this answer, I changed the code to use BufReader:
fn listen_for_output(program: &mut Child, output_viewer: &TextViewer) {
match program.stdout.as_mut() {
Some(out) => {
let buf_reader = BufReader::new(out);
for line in buf_reader.lines() {
match line {
Ok(l) => {
output_viewer.append_string(l);
}
Err(_) => return,
};
}
}
None => return,
}
}
However, the problem still remains the same. It will read all lines that are available, and then block. Since the tool is supposed to work with any program, there is no way to guess out when the output will end, before trying to read. There doesn't appear to be a way to set a timeout for BufReader either.
Streams are blocking by default. TCP/IP streams, filesystem streams, pipe streams, they are all blocking. When you tell a stream to give you a chunk of bytes it will stop and wait till it has the given amout of bytes or till something else happens (an interrupt, an end of stream, an error).
The operating systems are eager to return the data to the reading process, so if all you want is to wait for the next line and handle it as soon as it comes in then the method suggested by Shepmaster in Unable to pipe to or from spawned child process more than once (and also in his answer here) works.
Though in theory it doesn't have to work, because an operating system is allowed to make the BufReader wait for more data in read, but in practice the operating systems prefer the early "short reads" to waiting.
This simple BufReader-based approach becomes even more dangerous when you need to handle multiple streams (like the stdout and stderr of a child process) or multiple processes. For example, BufReader-based approach might deadlock when a child process waits for you to drain its stderr pipe while your process is blocked waiting on it's empty stdout.
Similarly, you can't use BufReader when you don't want your program to wait on the child process indefinitely. Maybe you want to display a progress bar or a timer while the child is still working and gives you no output.
You can't use BufReader-based approach if your operating system happens not to be eager in returning the data to the process (prefers "full reads" to "short reads") because in that case a few last lines printed by the child process might end up in a gray zone: the operating system got them, but they're not large enough to fill the BufReader's buffer.
BufReader is limited to what the Read interface allows it to do with the stream, it's no less blocking than the underlying stream is. In order to be efficient it will read the input in chunks, telling the operating system to fill as much of its buffer as it has available.
You might be wondering why reading data in chunks is so important here, why can't the BufReader just read the data byte by byte. The problem is that to read the data from a stream we need the operating system's help. On the other hand, we are not the operating system, we work isolated from it, so as not to mess with it if something goes wrong with our process. So in order to call to the operating system there needs to be a transition to "kernel mode" which might also incur a "context switch". That is why calling the operating system to read every single byte is expensive. We want as few OS calls as possible and so we get the stream data in batches.
To wait on a stream without blocking you'd need a non-blocking stream. MIO promises to have the required non-blocking stream support for pipes, most probably with PipeReader, but I haven't checked it out so far.
The non-blocking nature of a stream should make it possible to read data in chunks regardless of whether the operating system prefers the "short reads" or not. Because non-blocking stream never blocks. If there is no data in the stream it simply tells you so.
In the absense of a non-blocking stream you'll have to resort to spawning threads so that the blocking reads would be performed in a separate thread and thus won't block your primary thread. You might also want to read the stream byte by byte in order to react to the line separator immediately in case the operating system does not prefer the "short reads". Here's a working example: https://gist.github.com/ArtemGr/db40ae04b431a95f2b78.
P.S. Here's an example of a function that allows to monitor the standard output of a program via a shared vector of bytes:
use std::io::Read;
use std::process::{Command, Stdio};
use std::sync::{Arc, Mutex};
use std::thread;
/// Pipe streams are blocking, we need separate threads to monitor them without blocking the primary thread.
fn child_stream_to_vec<R>(mut stream: R) -> Arc<Mutex<Vec<u8>>>
where
R: Read + Send + 'static,
{
let out = Arc::new(Mutex::new(Vec::new()));
let vec = out.clone();
thread::Builder::new()
.name("child_stream_to_vec".into())
.spawn(move || loop {
let mut buf = [0];
match stream.read(&mut buf) {
Err(err) => {
println!("{}] Error reading from stream: {}", line!(), err);
break;
}
Ok(got) => {
if got == 0 {
break;
} else if got == 1 {
vec.lock().expect("!lock").push(buf[0])
} else {
println!("{}] Unexpected number of bytes: {}", line!(), got);
break;
}
}
}
})
.expect("!thread");
out
}
fn main() {
let mut cat = Command::new("cat")
.stdin(Stdio::piped())
.stdout(Stdio::piped())
.stderr(Stdio::piped())
.spawn()
.expect("!cat");
let out = child_stream_to_vec(cat.stdout.take().expect("!stdout"));
let err = child_stream_to_vec(cat.stderr.take().expect("!stderr"));
let mut stdin = match cat.stdin.take() {
Some(stdin) => stdin,
None => panic!("!stdin"),
};
}
With a couple of helpers I'm using it to control an SSH session:
try_s! (stdin.write_all (b"echo hello world\n"));
try_s! (wait_forĖ¢ (&out, 0.1, 9., |s| s == "hello world\n"));
P.S. Note that await on a read call in async-std is blocking as well. It's just instead of blocking a system thread it only blocks a chain of futures (a stack-less green thread essentially). The poll_read is the non-blocking interface. In async-std#499 I've asked the developers whether there's a short read guarantee from these APIs.
P.S. There might be a similar concern in Nom: "we would want to tell the IO side to refill according to the parser's result (Incomplete or not)"
P.S. Might be interesting to see how stream reading is implemented in crossterm. For Windows, in poll.rs, they are using the native WaitForMultipleObjects. In unix.rs they are using mio poll.
Tokio's Command
Here is an example of using tokio 0.2:
use std::process::Stdio;
use futures::StreamExt; // 0.3.1
use tokio::{io::BufReader, prelude::*, process::Command}; // 0.2.4, features = ["full"]
#[tokio::main]
async fn main() {
let mut cmd = Command::new("/tmp/slow.bash")
.stdout(Stdio::piped()) // Can do the same for stderr
.spawn()
.expect("cannot spawn");
let stdout = cmd.stdout().take().expect("no stdout");
// Can do the same for stderr
// To print out each line
// BufReader::new(stdout)
// .lines()
// .for_each(|s| async move { println!("> {:?}", s) })
// .await;
// To print out each line *and* collect it all into a Vec
let result: Vec<_> = BufReader::new(stdout)
.lines()
.inspect(|s| println!("> {:?}", s))
.collect()
.await;
println!("All the lines: {:?}", result);
}
Tokio-Threadpool
Here is an example of using tokio 0.1 and tokio-threadpool. We start the process in a thread using the blocking function. We convert that to a stream with stream::poll_fn
use std::process::{Command, Stdio};
use tokio::{prelude::*, runtime::Runtime}; // 0.1.18
use tokio_threadpool; // 0.1.13
fn stream_command_output(
mut command: Command,
) -> impl Stream<Item = Vec<u8>, Error = tokio_threadpool::BlockingError> {
// Ensure that the output is available to read from and start the process
let mut child = command
.stdout(Stdio::piped())
.spawn()
.expect("cannot spawn");
let mut stdout = child.stdout.take().expect("no stdout");
// Create a stream of data
stream::poll_fn(move || {
// Perform blocking IO
tokio_threadpool::blocking(|| {
// Allocate some space to store anything read
let mut data = vec![0; 128];
// Read 1-128 bytes of data
let n_bytes_read = stdout.read(&mut data).expect("cannot read");
if n_bytes_read == 0 {
// Stdout is done
None
} else {
// Only return as many bytes as we read
data.truncate(n_bytes_read);
Some(data)
}
})
})
}
fn main() {
let output_stream = stream_command_output(Command::new("/tmp/slow.bash"));
let mut runtime = Runtime::new().expect("Unable to start the runtime");
let result = runtime.block_on({
output_stream
.map(|d| String::from_utf8(d).expect("Not UTF-8"))
.fold(Vec::new(), |mut v, s| {
print!("> {}", s);
v.push(s);
Ok(v)
})
});
println!("All the lines: {:?}", result);
}
There's numerous possible tradeoffs that can be made here. For example, always allocating 128 bytes isn't ideal, but it's simple to implement.
Support
For reference, here's slow.bash:
#!/usr/bin/env bash
set -eu
val=0
while [[ $val -lt 10 ]]; do
echo $val
val=$(($val + 1))
sleep 1
done
See also:
How do I synchronously return a value calculated in an asynchronous Future in stable Rust?
If Unix support is sufficient, you can also make the two output streams as non-blocking and poll over them as you would do it on TcpStream with the set_nonblocking function.
The ChildStdout and ChildStderr returned by the Command spawn are Stdio (and contain a file descriptor), you can modify directly the read behavior of these handle to make it non-blocking.
Based on the work of jcreekmore/timeout-readwrite-rs and anowell/nonblock-rs, I use this wrapper to modify the stream handles:
extern crate libc;
use std::io::Read;
use std::os::unix::io::AsRawFd;
use libc::{F_GETFL, F_SETFL, fcntl, O_NONBLOCK};
fn set_nonblocking<H>(handle: &H, nonblocking: bool) -> std::io::Result<()>
where
H: Read + AsRawFd,
{
let fd = handle.as_raw_fd();
let flags = unsafe { fcntl(fd, F_GETFL, 0) };
if flags < 0 {
return Err(std::io::Error::last_os_error());
}
let flags = if nonblocking{
flags | O_NONBLOCK
} else {
flags & !O_NONBLOCK
};
let res = unsafe { fcntl(fd, F_SETFL, flags) };
if res != 0 {
return Err(std::io::Error::last_os_error());
}
Ok(())
}
You can manage the two streams as any other non-blocking stream. The following example is based on the polling crate which makes really easy to handle read event and BufReader for line reading:
use std::process::{Command, Stdio};
use std::path::PathBuf;
use std::io::{BufReader, BufRead};
use std::thread;
extern crate polling;
use polling::{Event, Poller};
fn main() -> Result<(), std::io::Error> {
let path = PathBuf::from("./worker.sh").canonicalize()?;
let mut child = Command::new(path)
.stdin(Stdio::null())
.stdout(Stdio::piped())
.stderr(Stdio::piped())
.spawn()
.expect("Failed to start worker");
let handle = thread::spawn({
let stdout = child.stdout.take().unwrap();
set_nonblocking(&stdout, true)?;
let mut reader_out = BufReader::new(stdout);
let stderr = child.stderr.take().unwrap();
set_nonblocking(&stderr, true)?;
let mut reader_err = BufReader::new(stderr);
move || {
let key_out = 1;
let key_err = 2;
let mut out_closed = false;
let mut err_closed = false;
let poller = Poller::new().unwrap();
poller.add(reader_out.get_ref(), Event::readable(key_out)).unwrap();
poller.add(reader_err.get_ref(), Event::readable(key_err)).unwrap();
let mut line = String::new();
let mut events = Vec::new();
loop {
// Wait for at least one I/O event.
events.clear();
poller.wait(&mut events, None).unwrap();
for ev in &events {
// stdout is ready for reading
if ev.key == key_out {
let len = match reader_out.read_line(&mut line) {
Ok(len) => len,
Err(e) => {
println!("stdout read returned error: {}", e);
0
}
};
if len == 0 {
println!("stdout closed (len is null)");
out_closed = true;
poller.delete(reader_out.get_ref()).unwrap();
} else {
print!("[STDOUT] {}", line);
line.clear();
// reload the poller
poller.modify(reader_out.get_ref(), Event::readable(key_out)).unwrap();
}
}
// stderr is ready for reading
if ev.key == key_err {
let len = match reader_err.read_line(&mut line) {
Ok(len) => len,
Err(e) => {
println!("stderr read returned error: {}", e);
0
}
};
if len == 0 {
println!("stderr closed (len is null)");
err_closed = true;
poller.delete(reader_err.get_ref()).unwrap();
} else {
print!("[STDERR] {}", line);
line.clear();
// reload the poller
poller.modify(reader_err.get_ref(), Event::readable(key_err)).unwrap();
}
}
}
if out_closed && err_closed {
println!("Stream closed, exiting process thread");
break;
}
}
}
});
handle.join().unwrap();
Ok(())
}
Additionally, used with a wrapper over an EventFd, it becomes possible to easily stop the process from another thread without blocking nor active polling and uses and only a single thread.
EDIT: It seems the polling crate sets automatically the polled handles in non-blocking mode following my tests. The set_nonblocking function is still useful in case you want to directly use the nix::poll object.
I have encountered enough use-cases where it was useful to interact with a subprocess over line-delimited text that I wrote a crate for it, interactive_process.
I expect the original problem has long since been solved, but I thought it might be helpful to others.
With a code similar to that of this example, a connection goes on forever because the infinite loop never stops. I modified it a bit in order to allow reading not to be blocking the thread:
use std::net::{TcpListener, TcpStream};
use std::io::{BufRead, BufReader};
use std::thread;
fn main() {
let listener = TcpListener::bind("127.0.0.1:4444").unwrap();
println!("Server is running on 127.0.0.1:4444 ...");
for stream in listener.incoming() {
let stream = stream.unwrap();
stream
.set_nonblocking(true)
.expect("Failed to set stream as nonblocking");
thread::spawn(move || handle_client(stream));
}
}
fn handle_client(stream: TcpStream) {
let mut stream = BufReader::new(stream);
println!("New client logged");
loop {
let mut recv_buffer = String::new();
println!("Before reading");
let recv_data_size = stream
.read_line(&mut recv_buffer)
.expect("Error when reading line");
if recv_data_size > 0 {
println!("{}", recv_buffer);
}
}
}
The client is really simple, connects then send Hello! to the server:
use std::io::prelude::*;
use std::net::TcpStream;
fn main() {
let mut stream = TcpStream::connect("127.0.0.1:4444").unwrap();
let _ = stream.write("Hello!".as_bytes());
}
When client sends its string and disconnects, the server goes into an infinite loop and prints Before reading indefinitely. It never knows when to close connection, even though the client ended it long ago.
Is there a way in Rust to check if distant connection has been closed (if possible without having to set a timeout), then finish properly the thread on which it is running?
From the docs of read_line:
An empty buffer returned indicates that the stream has reached EOF.
So you could do it like this:
fn handle_client(stream: TcpStream) {
let mut stream = BufReader::new(stream);
println!("New client logged");
loop {
let mut recv_buffer = String::new();
println!("Before reading");
let recv_data_size = stream
.read_line(&mut recv_buffer)
.expect("Error when reading line");
if recv_data_size > 0 {
println!("{}", recv_buffer);
} else {
// stream has reached EOF
break;
}
}
}
I'm an absolute Rust beginner trying to build a simple confirmation function (yes or no), but I can't get the user to type anything, the function just keeps looping without waiting for user input:
""
""
""
etc.
is the result of the simplified version below.
use std::process;
use std::io;
pub fn confirm() {
loop {
let mut answer = String::new();
io::stdin().read_line(&mut answer)
.ok()
.expect("Failed to read line");
println!("{:?}", answer);
}
}
I've built my function around the guessing game example, and the rest of my program does nothing much, just reading a file and printing text.
Perhaps is due to the way my program (a git hook) is launched?
Assuming that the problem is that your git commit hook is running in an non-interactive environment, you can follow the advice laid out in that question and directly open /dev/tty. Unlike STDIN, we don't treat it as a magical global variable and instead we pass it into the places we need:
use std::io::{self, BufRead, BufReader};
use std::fs::File;
type Tty = BufReader<File>;
fn open_tty() -> io::Result<Tty> {
let f = try!(File::open("/dev/tty"));
Ok(BufReader::new(f))
}
fn confirm(tty: &mut Tty) -> io::Result<String> {
let mut answer = String::new();
try!(tty.read_line(&mut answer));
Ok(answer)
}
fn inner_main() -> io::Result<()> {
let mut tty = try!(open_tty());
let answer = try!(confirm(&mut tty));
println!("The answer was: {}", answer);
Ok(())
}
fn main() {
inner_main().unwrap()
}
Note that this will not be platform independent. Specifically, this is very unlikely to work on Windows!
I've also gone ahead and allowed the io::Result to propagate throughout the program, only panicking at the outermost shell.
Are you testing the function on the Rust Playground? Running this program in a terminal seems to work fine. That being said, there is no guarantee that stdin will block, but you could change the function to check if the string is empty or not, and only return once it is isn't.
use std::io;
fn main() {
println!("{:?}", confirm());
}
fn confirm() -> String {
loop {
let mut answer = String::new();
io::stdin().read_line(&mut answer)
.ok()
.expect("Failed to read line");
if !answer.is_empty() && answer != "\n" && answer != "\r\n" {
return answer
}
}
}