Refactor: Extract new mod jobserver_client.rs

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

src/helpers/jobserver_client.rs

@@ -0,0 +1,20 @@
+use std::num::NonZeroUsize;
+use std::thread::available_parallelism;
+
+use crate::BinstallError;
+
+pub fn create_jobserver_client() -> Result<jobserver::Client, BinstallError> {
+    use jobserver::Client;
+
+    // Safety:
+    //
+    // Client::from_env is unsafe because from_raw_fd is unsafe.
+    // It doesn't do anything that is actually unsafe, like
+    // dereferencing pointer.
+    if let Some(client) = unsafe { Client::from_env() } {
+        Ok(client)
+    } else {
+        let ncore = available_parallelism().map(NonZeroUsize::get).unwrap_or(1);
+        Ok(Client::new(ncore)?)
+    }
+}