fix: use single-threaded tokio runtime in sccache client

The sccache client was using Runtime::new() which creates a multi-threaded
tokio runtime with worker threads equal to the number of CPU cores. On
high-core-count servers running many parallel builds, this created an
excessive number of threads.

For example:
- 96 vCPU server
- 10 concurrent make invocations
- Each make using -j16
- Result: 96 × 10 × 16 = 15,360 threads created by sccache wrappers

While these threads are short-lived, with continuous build pipelines this
constant thread creation/destruction causes unnecessary overhead. The sccache
client only performs simple I/O operations (connecting to server, sending
requests, receiving responses) and doesn't need worker threads.

This change replaces all client-side Runtime::new() calls with
Builder::new_current_thread().enable_all().build() to use a
single-threaded runtime, reducing thread count from num_cpus to 1 per
client invocation.

Author: venkyt-arista

src/commands.rs

@@ -92,7 +92,9 @@ fn run_server_process(startup_timeout: Option<Duration>) -> Result<ServerStartup
     trace!("run_server_process");
     let tempdir = tempfile::Builder::new().prefix("sccache").tempdir()?;
     let socket_path = tempdir.path().join("sock");
-    let runtime = Runtime::new()?;
+    let runtime = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()?;
     let exe_path = env::current_exe()?;
     let workdir = exe_path.parent().expect("executable path has no parent?!");
 
@@ -190,7 +192,9 @@ fn run_server_process(startup_timeout: Option<Duration>) -> Result<ServerStartup
     trace!("run_server_process");
 
     // Create a mini event loop and register our named pipe server
-    let runtime = Runtime::new()?;
+    let runtime = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()?;
     let pipe_name = &format!(r"\\.\pipe\{}", Uuid::new_v4().as_simple());
 
     // Spawn a server which should come back and connect to us
@@ -624,7 +628,9 @@ pub fn run_command(cmd: Command) -> Result<i32> {
                 // If there is no server, spawning a new server would start with zero stats
                 // anyways, so we can just return (mostly) empty stats directly.
                 Err(_) => {
-                    let runtime = Runtime::new()?;
+                    let runtime = tokio::runtime::Builder::new_current_thread()
+                        .enable_all()
+                        .build()?;
                     let storage = storage_from_config(config, runtime.handle()).ok();
                     runtime.block_on(ServerInfo::new(ServerStats::default(), storage.as_deref()))?
                 }
@@ -760,7 +766,9 @@ pub fn run_command(cmd: Command) -> Result<i32> {
             use crate::compiler;
 
             trace!("Command::PackageToolchain({})", executable.display());
-            let runtime = Runtime::new()?;
+            let runtime = tokio::runtime::Builder::new_current_thread()
+                .enable_all()
+                .build()?;
             let jobserver = Client::new();
             let creator = ProcessCommandCreator::new(&jobserver);
             let args: Vec<_> = env::args_os().collect();
@@ -804,7 +812,9 @@ pub fn run_command(cmd: Command) -> Result<i32> {
 
             let jobserver = Client::new();
             let conn = connect_or_start_server(&get_addr(), startup_timeout)?;
-            let mut runtime = Runtime::new()?;
+            let mut runtime = tokio::runtime::Builder::new_current_thread()
+                .enable_all()
+                .build()?;
             let res = do_compile(
                 ProcessCommandCreator::new(&jobserver),
                 &mut runtime,