Remove need for tokio runtime for supporting serve (rerun-io#6043)

### What

Rust users now no longer need to ensure to have tokio runtime set up
when using `serve`.

* Fixes rerun-io#5907
* Direct follow-up / merges into rerun-io#6042

Last PR in a series of PRs for removal of the tokio runtime:
* removes need for tokio in re_sdk_comms
*similar to the refactor in re_ws_comms only in spirit - a lot simpler:
no fancy broadcast, no other libraries involved. Just a bunch of threads
hammering on blocking sockets).
* remove remaining usages of async in this context
* removes tokio need from all documentation

-----

Dependency count `0.15.1` -> last friday `d90ed2f7e`-> `this pr`

* `cargo build -p rerun --no-default-features` 225 -> 274 -> 275
dependencies
* `cargo build -p rerun -F default` 361 -> 364 -> 364 dependencies
* `cargo build -p rerun -F web_viewer` 409 -> 412 -> 374 dependencies

Notes:
* 0.15.1 already regressed quite a bit compared back to January:
rerun-io#4824
* we currently have a few temporary crates while migrating to re_query2
which weren't present in 0.15

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6043?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6043?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/6043)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

Cargo.toml

@@ -219,7 +219,6 @@ tiny_http = { version = "0.12", default-features = false }
 tinystl = { version = "0.0.3", default-features = false }
 tinyvec = { version = "1.6", features = ["alloc", "rustc_1_55"] }
 tobj = "4.0"
-tokio = { version = "1.24", default-features = false }
 toml = { version = "0.8.10", default-features = false }
 tracing = { version = "0.1", default-features = false }
 tungstenite = { version = "0.20", default-features = false }

crates/re_sdk/src/recording_stream.rs

@@ -461,9 +461,6 @@ impl RecordingStreamBuilder {
     /// Creates a new [`RecordingStream`] that is pre-configured to stream the data through to a
     /// web-based Rerun viewer via WebSockets.
     ///
-    /// This method needs to be called in a context where a Tokio runtime is already running (see
-    /// example below).
-    ///
     /// If the `open_browser` argument is `true`, your default browser will be opened with a
     /// connected web-viewer.
     ///
@@ -481,16 +478,6 @@ impl RecordingStreamBuilder {
     /// ## Example
     ///
     /// ```ignore
-    /// // Ensure we have a running tokio runtime.
-    /// let mut tokio_runtime = None;
-    /// let tokio_runtime_handle = if let Ok(handle) = tokio::runtime::Handle::try_current() {
-    ///     handle
-    /// } else {
-    ///     let rt = tokio::runtime::Runtime::new().expect("Failed to create tokio runtime");
-    ///     tokio_runtime.get_or_insert(rt).handle().clone()
-    /// };
-    /// let _tokio_runtime_guard = tokio_runtime_handle.enter();
-    ///
     /// let rec = re_sdk::RecordingStreamBuilder::new("rerun_example_app")
     ///     .serve("0.0.0.0",
     ///            Default::default(),

crates/re_sdk/src/web_viewer.rs

@@ -122,8 +122,6 @@ impl crate::sink::LogSink for WebViewerSink {
 /// NOTE: you can not connect one `Session` to another.
 ///
 /// This function returns immediately.
-///
-/// The caller needs to ensure that there is a `tokio` runtime running.
 #[must_use = "the sink must be kept around to keep the servers running"]
 pub fn new_sink(
     open_browser: bool,

crates/re_sdk_comms/Cargo.toml

@@ -24,7 +24,7 @@ all-features = true
 client = ["re_log_encoding/encoder"]
 
 ## Enable the server.
-server = ["rand", "tokio", "re_log_encoding/decoder"]
+server = ["rand", "re_log_encoding/decoder"]
 
 
 [dependencies]
@@ -46,9 +46,3 @@ rand = { workspace = true, optional = true, features = [
   "std_rng",
   "small_rng",
 ] }
-
-tokio = { workspace = true, optional = true, features = [
-  "io-util",
-  "net",
-  "rt",
-] }

crates/re_sdk_comms/src/server.rs

@@ -1,7 +1,10 @@
-use std::{io::ErrorKind, time::Instant};
+use std::{
+    io::{ErrorKind, Read as _},
+    net::{TcpListener, TcpStream},
+    time::Instant,
+};
 
 use rand::{Rng as _, SeedableRng};
-use tokio::net::{TcpListener, TcpStream};
 
 use re_log_types::{LogMsg, TimePoint, TimeType, TimelineName};
 use re_smart_channel::{Receiver, Sender};
@@ -13,6 +16,9 @@ pub enum ServerError {
         bind_addr: String,
         err: std::io::Error,
     },
+
+    #[error(transparent)]
+    FailedToSpawnThread(#[from] std::io::Error),
 }
 
 #[derive(thiserror::Error, Debug)]
@@ -68,12 +74,15 @@ impl Default for ServerOptions {
 ///
 /// ``` no_run
 /// # use re_sdk_comms::{serve, ServerOptions};
-/// #[tokio::main]
-/// async fn main() {
-///     let log_msg_rx = serve("0.0.0.0", re_sdk_comms::DEFAULT_SERVER_PORT, ServerOptions::default()).await.unwrap();
+/// fn main() {
+///     let log_msg_rx = serve("0.0.0.0", re_sdk_comms::DEFAULT_SERVER_PORT, ServerOptions::default()).unwrap();
 /// }
 /// ```
-pub async fn serve(
+///
+/// Internally spawns a thread that listens for incoming TCP connections on the given `bind_ip` and `port`
+/// and one thread per connected client.
+// TODO(andreas): Reconsider if we should use `smol` tasks instead of threads both here and in re_ws_comms.
+pub fn serve(
     bind_ip: &str,
     port: u16,
     options: ServerOptions,
@@ -85,13 +94,16 @@ pub async fn serve(
     );
 
     let bind_addr = format!("{bind_ip}:{port}");
-    let listener =
-        TcpListener::bind(&bind_addr)
-            .await
-            .map_err(|err| ServerError::TcpBindError {
-                bind_addr: bind_addr.clone(),
-                err,
-            })?;
+    let listener = TcpListener::bind(&bind_addr).map_err(|err| ServerError::TcpBindError {
+        bind_addr: bind_addr.clone(),
+        err,
+    })?;
+
+    std::thread::Builder::new()
+        .name("rerun_sdk_comms: listener".to_owned())
+        .spawn(move || {
+            listen_for_new_clients(&listener, options, &tx);
+        })?;
 
     if options.quiet {
         re_log::debug!(
@@ -103,19 +115,24 @@ pub async fn serve(
         );
     }
 
-    tokio::spawn(listen_for_new_clients(listener, options, tx));
-
     Ok(rx)
 }
 
-async fn listen_for_new_clients(listener: TcpListener, options: ServerOptions, tx: Sender<LogMsg>) {
-    #[allow(clippy::infinite_loop)] // TODO(emilk): some way of aborting this loop
+fn listen_for_new_clients(listener: &TcpListener, options: ServerOptions, tx: &Sender<LogMsg>) {
+    // TODO(emilk): some way of aborting this loop
+    #[allow(clippy::infinite_loop)]
     loop {
-        match listener.accept().await {
+        match listener.accept() {
             Ok((stream, _)) => {
                 let addr = stream.peer_addr().ok();
                 let tx = tx.clone_as(re_smart_channel::SmartMessageSource::TcpClient { addr });
-                spawn_client(stream, tx, options, addr);
+
+                std::thread::Builder::new()
+                    .name("rerun_sdk_comms: client".to_owned())
+                    .spawn(move || {
+                        spawn_client(stream, &tx, options, addr);
+                    })
+                    .ok();
             }
             Err(err) => {
                 re_log::warn!("Failed to accept incoming SDK client: {err}");
@@ -126,46 +143,41 @@ async fn listen_for_new_clients(listener: TcpListener, options: ServerOptions, t
 
 fn spawn_client(
     stream: TcpStream,
-    tx: Sender<LogMsg>,
+    tx: &Sender<LogMsg>,
     options: ServerOptions,
     peer_addr: Option<std::net::SocketAddr>,
 ) {
-    tokio::spawn(async move {
-        let addr_string =
-            peer_addr.map_or_else(|| "(unknown ip)".to_owned(), |addr| addr.to_string());
+    let addr_string = peer_addr.map_or_else(|| "(unknown ip)".to_owned(), |addr| addr.to_string());
 
-        if options.quiet {
-            re_log::debug!("New SDK client connected: {addr_string}");
-        } else {
-            re_log::info!("New SDK client connected: {addr_string}");
-        }
+    if options.quiet {
+        re_log::debug!("New SDK client connected: {addr_string}");
+    } else {
+        re_log::info!("New SDK client connected: {addr_string}");
+    }
 
-        if let Err(err) = run_client(stream, &tx, options).await {
-            if let ConnectionError::SendError(err) = &err {
-                if err.kind() == ErrorKind::UnexpectedEof {
-                    // Client gracefully severed the connection.
-                    tx.quit(None).ok(); // best-effort at this point
-                    return;
-                }
+    if let Err(err) = run_client(stream, tx, options) {
+        if let ConnectionError::SendError(err) = &err {
+            if err.kind() == ErrorKind::UnexpectedEof {
+                // Client gracefully severed the connection.
+                tx.quit(None).ok(); // best-effort at this point
+                return;
             }
-            re_log::warn_once!("Closing connection to client at {addr_string}: {err}");
-            let err: Box<dyn std::error::Error + Send + Sync + 'static> = err.to_string().into();
-            tx.quit(Some(err)).ok(); // best-effort at this point
         }
-    });
+        re_log::warn_once!("Closing connection to client at {addr_string}: {err}");
+        let err: Box<dyn std::error::Error + Send + Sync + 'static> = err.to_string().into();
+        tx.quit(Some(err)).ok(); // best-effort at this point
+    }
 }
 
-async fn run_client(
+fn run_client(
     mut stream: TcpStream,
     tx: &Sender<LogMsg>,
     options: ServerOptions,
 ) -> Result<(), ConnectionError> {
     #![allow(clippy::read_zero_byte_vec)] // false positive: https://github.com/rust-lang/rust-clippy/issues/9274
 
-    use tokio::io::AsyncReadExt as _;
-
     let mut client_version = [0_u8; 2];
-    stream.read_exact(&mut client_version).await?;
+    stream.read_exact(&mut client_version)?;
     let client_version = u16::from_le_bytes(client_version);
 
     match client_version.cmp(&crate::PROTOCOL_VERSION) {
@@ -190,11 +202,11 @@ async fn run_client(
 
     loop {
         let mut packet_size = [0_u8; 4];
-        stream.read_exact(&mut packet_size).await?;
+        stream.read_exact(&mut packet_size)?;
         let packet_size = u32::from_le_bytes(packet_size);
 
         packet.resize(packet_size as usize, 0_u8);
-        stream.read_exact(&mut packet).await?;
+        stream.read_exact(&mut packet)?;
 
         re_log::trace!("Received packet of size {packet_size}.");
 

crates/re_web_viewer_server/src/lib.rs

@@ -37,10 +37,6 @@ pub enum WebViewerServerError {
 
     #[error("Failed to create server at address {0}: {1}")]
     CreateServerFailed(String, Box<dyn std::error::Error + Send + Sync + 'static>),
-
-    #[cfg(feature = "sync")]
-    #[error("Failed to spawn web viewer thread: {0}")]
-    ThreadSpawnFailed(#[from] std::io::Error),
 }
 
 // ----------------------------------------------------------------------------

crates/rerun-cli/Cargo.toml

@@ -67,7 +67,6 @@ rerun = { workspace = true, features = [
 
 document-features.workspace = true
 mimalloc.workspace = true
-tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }
 
 
 [build-dependencies]

crates/rerun-cli/src/bin/rerun.rs

@@ -16,13 +16,12 @@ use re_memory::AccountingAllocator;
 static GLOBAL: AccountingAllocator<mimalloc::MiMalloc> =
     AccountingAllocator::new(mimalloc::MiMalloc);
 
-#[tokio::main]
-async fn main() -> std::process::ExitCode {
+fn main() -> std::process::ExitCode {
     re_log::setup_logging();
 
     let build_info = re_build_info::build_info!();
 
-    let result = rerun::run(build_info, rerun::CallSource::Cli, std::env::args()).await;
+    let result = rerun::run(build_info, rerun::CallSource::Cli, std::env::args());
 
     match result {
         Ok(exit_code) => std::process::ExitCode::from(exit_code),

crates/rerun/Cargo.toml

@@ -43,7 +43,7 @@ analytics = [
 ]
 
 ## Integration with `clap`.
-clap = ["dep:clap", "dep:tokio"]
+clap = ["dep:clap"]
 
 ## Support for using Rerun's data-loaders directly from the SDK.
 ##
@@ -84,7 +84,6 @@ run = [
   "dep:re_log_encoding",
   "dep:re_sdk_comms",
   "dep:re_ws_comms",
-  "dep:tokio",
 ]
 
 ## Support for running a TCP server that listens to incoming log messages from a Rerun SDK.
@@ -143,12 +142,6 @@ rayon.workspace = true
 
 # Native, optional:
 clap = { workspace = true, optional = true, features = ["derive"] }
-tokio = { workspace = true, optional = true, features = [
-  "macros",
-  "rt-multi-thread",
-  "time",
-] }
-
 
 [build-dependencies]
 re_build_tools.workspace = true