Hide the jobserver Client from the public view

Future commits will be changing the jobserver integration to have custom
logic to enable it to appropriately talk to Cargo, so we can't have
other code that directly talks to the jobserver.

Author: Mark-Simulacrum

src/librustc_codegen_ssa/back/write.rs

@@ -978,10 +978,9 @@ fn start_executing_work<B: ExtraBackendMethods>(
     // get tokens on `coordinator_receive` which will
     // get managed in the main loop below.
     let coordinator_send2 = coordinator_send.clone();
-    let helper = rustc_jobserver::client().into_helper_thread(move |token| {
-        let token = token.expect("acquired token successfully");
+    let helper = rustc_jobserver::helper_thread(move |token| {
         drop(coordinator_send2.send(Box::new(Message::Token::<B>(token))));
-    }).expect("failed to spawn helper thread");
+    });
 
     let mut each_linked_rlib_for_lto = Vec::new();
     drop(link::each_linked_rlib(crate_info, &mut |cnum, path| {

src/librustc_jobserver/lib.rs

@@ -1,4 +1,5 @@
-pub use jobserver_crate::{Acquired, Client};
+pub use jobserver_crate::Acquired;
+use jobserver_crate::{HelperThread, Client};
 use lazy_static::lazy_static;
 
 lazy_static! {
@@ -33,8 +34,12 @@ pub fn initialize() {
     lazy_static::initialize(&GLOBAL_CLIENT)
 }
 
-pub fn client() -> Client {
-    GLOBAL_CLIENT.clone()
+pub fn helper_thread<F>(mut cb: F) -> HelperThread
+    where F: FnMut(Acquired) + Send + 'static,
+{
+    GLOBAL_CLIENT.clone().into_helper_thread(move |token| {
+        cb(token.expect("acquire token"))
+    }).expect("failed to spawn helper thread")
 }
 
 pub fn acquire_thread() {