Add a jobserver proxy to ensure at least one token is always held

compiler/rustc_data_structures/src/jobserver.rs

@@ -1,7 +1,8 @@
-use std::sync::{LazyLock, OnceLock};
+use std::sync::{Arc, LazyLock, OnceLock};
 
 pub use jobserver_crate::{Acquired, Client, HelperThread};
 use jobserver_crate::{FromEnv, FromEnvErrorKind};
+use parking_lot::{Condvar, Mutex};
 
 // We can only call `from_env_ext` once per process
 
@@ -71,10 +72,93 @@ pub fn client() -> Client {
     GLOBAL_CLIENT_CHECKED.get().expect(ACCESS_ERROR).clone()
 }
 
-pub fn acquire_thread() {
-    GLOBAL_CLIENT_CHECKED.get().expect(ACCESS_ERROR).acquire_raw().ok();
+struct ProxyData {
+    /// The number of tokens assigned to threads.
+    /// If this is 0, a single token is still assigned to this process, but is unused.
+    used: u16,
+
+    /// The number of threads requesting a token
+    pending: u16,
 }
 
-pub fn release_thread() {
-    GLOBAL_CLIENT_CHECKED.get().expect(ACCESS_ERROR).release_raw().ok();
+/// This is a jobserver proxy used to ensure that we hold on to at least one token.
+pub struct Proxy {
+    client: Client,
+    data: Mutex<ProxyData>,
+
+    /// Threads which are waiting on a token will wait on this.
+    wake_pending: Condvar,
+
+    helper: OnceLock<HelperThread>,
+}
+
+impl Proxy {
+    pub fn new() -> Arc<Self> {
+        let proxy = Arc::new(Proxy {
+            client: client(),
+            data: Mutex::new(ProxyData { used: 1, pending: 0 }),
+            wake_pending: Condvar::new(),
+            helper: OnceLock::new(),
+        });
+        let proxy_ = Arc::clone(&proxy);
+        let helper = proxy
+            .client
+            .clone()
+            .into_helper_thread(move |token| {
+                if let Ok(token) = token {
+                    let mut data = proxy_.data.lock();
+                    if data.pending > 0 {
+                        // Give the token to a waiting thread
+                        token.drop_without_releasing();
+                        assert!(data.used > 0);
+                        data.used += 1;
+                        data.pending -= 1;
+                        proxy_.wake_pending.notify_one();
+                    } else {
+                        // The token is no longer needed, drop it.
+                        drop(data);
+                        drop(token);
+                    }
+                }
+            })
+            .expect("failed to create helper thread");
+        proxy.helper.set(helper).unwrap();
+        proxy
+    }
+
+    pub fn acquire_thread(&self) {
+        let mut data = self.data.lock();
+
+        if data.used == 0 {
+            // There was a free token around. This can
+            // happen when all threads release their token.
+            assert_eq!(data.pending, 0);
+            data.used += 1;
+        } else {
+            // Request a token from the helper thread. We can't directly use `acquire_raw`
+            // as we also need to be able to wait for the final token in the process which
+            // does not get a corresponding `release_raw` call.
+            self.helper.get().unwrap().request_token();
+            data.pending += 1;
+            self.wake_pending.wait(&mut data);
+        }
+    }
+
+    pub fn release_thread(&self) {
+        let mut data = self.data.lock();
+
+        if data.pending > 0 {
+            // Give the token to a waiting thread
+            data.pending -= 1;
+            self.wake_pending.notify_one();
+        } else {
+            data.used -= 1;
+
+            // Release the token unless it's the last one in the process
+            if data.used > 0 {
+                drop(data);
+                self.client.release_raw().ok();
+            }
+        }
+    }
 }