rust: Name the Rayon threads

backend-lib/Cargo.lock

@@ -2391,6 +2391,7 @@ dependencies = [
  "log-panics",
  "orchard",
  "paranoid-android",
+ "rayon",
  "schemer",
  "secp256k1",
  "secrecy",

backend-lib/Cargo.toml

@@ -26,6 +26,9 @@ zcash_primitives = "0.11"
 zcash_proofs = "0.11"
 orchard = { version = "0.4", default-features = false }
 
+# Initialization
+rayon = "1.7"
+
 # Logging
 log-panics = "2.0.0"
 paranoid-android = "0.2"

backend-lib/src/main/rust/lib.rs

@@ -126,7 +126,13 @@ pub unsafe extern "C" fn Java_cash_z_ecc_android_sdk_internal_jni_RustBackend_in
         error!("Failed to reload tracing subscriber with NDK APIs: {}", e);
     }
 
-    debug!("logs have been initialized successfully");
+    // Manually build the Rayon thread pool, so we can name the threads.
+    rayon::ThreadPoolBuilder::new()
+        .thread_name(|i| format!("zc-rayon-{}", i))
+        .build_global()
+        .expect("Only initialized once");
+
+    debug!("Rust backend has been initialized successfully");
     print_debug_state();
 }