/corebluetooth runtime mtu

src/corebluetooth/internal.rs

@@ -42,6 +42,21 @@ use std::{
 use tokio::runtime;
 use uuid::Uuid;
 
+/// ATT Write Command PDUs reserve one byte for the opcode and two bytes for
+/// the attribute handle (Bluetooth Core Specification, Vol 3, Part F, 3.4.5.3).
+const ATT_WRITE_COMMAND_HEADER_LEN: usize = 3;
+
+fn maximum_write_value_length_to_att_mtu(maximum_write_value_length: usize) -> Result<u16, String> {
+    maximum_write_value_length
+        .checked_add(ATT_WRITE_COMMAND_HEADER_LEN)
+        .and_then(|mtu| u16::try_from(mtu).ok())
+        .ok_or_else(|| {
+            format!(
+                "CoreBluetooth maximum write value length {maximum_write_value_length} cannot be represented as a u16 ATT MTU"
+            )
+        })
+}
+
 struct DescriptorInternal {
     pub descriptor: Retained<CBDescriptor>,
     pub uuid: Uuid,
@@ -159,7 +174,7 @@ pub enum CoreBluetoothReply {
     ReadResult(Vec<u8>),
     ReadRssi(i16),
     Connected,
-    ServicesDiscovered(BTreeSet<Service>),
+    ServicesDiscovered(BTreeSet<Service>, u16),
     State(CBPeripheralState),
     Ok,
     Err(String),
@@ -346,12 +361,24 @@ impl PeripheralInternal {
                         .collect(),
                 })
                 .collect();
+            // CoreBluetooth exposes the maximum characteristic value length for
+            // a write, not the ATT MTU. Sample it after discovery, then account
+            // for the ATT Write Command header to infer the full ATT MTU.
+            let maximum_write_value_length = unsafe {
+                self.peripheral.maximumWriteValueLengthForType(
+                    CBCharacteristicWriteType::CBCharacteristicWriteWithoutResponse,
+                )
+            };
+            let reply = match maximum_write_value_length_to_att_mtu(maximum_write_value_length) {
+                Ok(mtu) => CoreBluetoothReply::ServicesDiscovered(services, mtu),
+                Err(error) => CoreBluetoothReply::Err(error),
+            };
             self.services_discovered_future_state
                 .take()
                 .unwrap()
                 .lock()
                 .unwrap()
-                .set_reply(CoreBluetoothReply::ServicesDiscovered(services));
+                .set_reply(reply);
         }
     }
 

src/corebluetooth/peripheral.rs

@@ -288,6 +288,9 @@ impl api::Peripheral for Peripheral {
             .await?;
         match fut.await {
             CoreBluetoothReply::Connected => {
+                self.shared
+                    .mtu
+                    .store(api::DEFAULT_MTU_SIZE, std::sync::atomic::Ordering::Relaxed);
                 self.shared
                     .emit_event(CentralEvent::DeviceConnected(self.shared.uuid.into()));
             }
@@ -330,8 +333,11 @@ impl api::Peripheral for Peripheral {
             })
             .await?;
         match fut.await {
-            CoreBluetoothReply::ServicesDiscovered(services) => {
+            CoreBluetoothReply::ServicesDiscovered(services, mtu) => {
                 *(self.shared.services.lock().map_err(Into::<Error>::into)?) = services;
+                self.shared
+                    .mtu
+                    .store(mtu, std::sync::atomic::Ordering::Relaxed);
                 return Ok(());
             }
             CoreBluetoothReply::Err(msg) => return Err(Error::RuntimeError(msg)),

tests/android/rust/src/lib.rs

@@ -208,8 +208,8 @@ jni_test!(
     test_cases::test_descriptor_discovery
 );
 jni_test!(
-    Java_com_nonpolynomial_btleplug_test_NativeTests_testMtuAfterConnection,
-    test_cases::test_mtu_after_connection
+    Java_com_nonpolynomial_btleplug_test_NativeTests_testMtuAfterServiceDiscovery,
+    test_cases::test_mtu_after_service_discovery
 );
 jni_test!(
     Java_com_nonpolynomial_btleplug_test_NativeTests_testReadRssi,

tests/android/src/androidTest/kotlin/com/nonpolynomial/btleplug/test/BleIntegrationTest.kt

@@ -97,7 +97,7 @@ class BleIntegrationTest {
     @Test fun testDescriptorDiscovery() = NativeTests.testDescriptorDiscovery()
 
     // ── Device Info ─────────────────────────────────────────────────
-    @Test fun testMtuAfterConnection() = NativeTests.testMtuAfterConnection()
+    @Test fun testMtuAfterServiceDiscovery() = NativeTests.testMtuAfterServiceDiscovery()
     @Test fun testReadRssi() = NativeTests.testReadRssi()
     @Test fun testPropertiesContainPeripheralInfo() = NativeTests.testPropertiesContainPeripheralInfo()
     @Test fun testConnectionParameters() = NativeTests.testConnectionParameters()

tests/android/src/main/kotlin/com/nonpolynomial/btleplug/test/NativeTests.kt

@@ -39,7 +39,7 @@ object NativeTests {
     external fun testDescriptorDiscovery()
 
     // Device Info
-    external fun testMtuAfterConnection()
+    external fun testMtuAfterServiceDiscovery()
     external fun testReadRssi()
     external fun testPropertiesContainPeripheralInfo()
     external fun testConnectionParameters()

tests/common/test_cases.rs

@@ -581,9 +581,18 @@ pub async fn test_descriptor_discovery() {
 
 // ── Device Info ─────────────────────────────────────────────────────
 
-pub async fn test_mtu_after_connection() {
+pub async fn test_mtu_after_service_discovery() {
     let peripheral = peripheral_finder::find_and_connect().await;
     let mtu = peripheral.mtu();
+
+    #[cfg(any(target_os = "macos", target_os = "ios"))]
+    assert!(
+        mtu > 23,
+        "CoreBluetooth MTU should be negotiated above the default, got {}",
+        mtu
+    );
+
+    #[cfg(not(any(target_os = "macos", target_os = "ios")))]
     assert!(
         mtu >= 23,
         "MTU should be at least 23 (default), got {}",

tests/test_mtu_after_service_discovery.rs

@@ -0,0 +1,7 @@
+mod common;
+
+#[tokio::test]
+#[ignore = "requires BLE test peripheral"]
+async fn test_mtu_after_service_discovery() {
+    common::test_cases::test_mtu_after_service_discovery().await;
+}