fix: ensure the pool counter's always decremented via scopeguard (#1604)

also report machine parallelism @ startup

Issue SYNC-4421

syncserver-common/Cargo.toml

@@ -19,3 +19,5 @@ serde_json.workspace = true
 slog.workspace = true
 slog-scope.workspace = true
 hkdf.workspace = true
+
+scopeguard = "1.2"

syncserver-common/src/lib.rs

@@ -121,16 +121,17 @@ impl BlockingThreadpool {
         E: fmt::Debug + Send + InternalError + 'static,
     {
         self.spawned_tasks.fetch_add(1, Ordering::Relaxed);
+        // Ensure the counter's always decremented (whether the task completed,
+        // was cancelled or panicked)
+        scopeguard::defer! {
+            self.spawned_tasks.fetch_sub(1, Ordering::Relaxed);
+        }
 
-        let result = web::block(f).await.unwrap_or_else(|_| {
+        web::block(f).await.unwrap_or_else(|_| {
             Err(E::internal_error(
                 "Blocking threadpool operation canceled".to_owned(),
             ))
-        });
-
-        self.spawned_tasks.fetch_sub(1, Ordering::Relaxed);
-
-        result
+        })
     }
 
     pub fn active_threads(&self) -> u64 {

syncserver-settings/src/lib.rs

@@ -161,7 +161,16 @@ impl Settings {
         let db = Url::parse(&self.syncstorage.database_url)
             .map(|url| url.scheme().to_owned())
             .unwrap_or_else(|_| "<invalid db>".to_owned());
-        format!("http://{}:{} ({}) {}", self.host, self.port, db, quota)
+        let parallelism = format!(
+            "available_parallelism: {:?} num_cpus: {} num_cpus (phys): {}",
+            std::thread::available_parallelism(),
+            num_cpus::get(),
+            num_cpus::get_physical()
+        );
+        format!(
+            "http://{}:{} ({db}) ({parallelism}) {quota}",
+            self.host, self.port
+        )
     }
 }