fix: propagate application write rejections as ATT Error Responses

src/NimBLECharacteristic.cpp

@@ -434,9 +434,30 @@ void NimBLECharacteristic::readEvent(NimBLEConnInfo& connInfo) {
  */
 void NimBLECharacteristic::writeEvent(const uint8_t* val, uint16_t len, NimBLEConnInfo& connInfo) {
     setValue(val, len);
+    m_writeError = 0;
     m_pCallbacks->onWrite(this, connInfo);
 } // writeEvent
 
+/**
+ * @brief Set the ATT error code to return for the current write operation.
+ * @param [in] attError The ATT error code; 0 accepts the write. Use the
+ * Bluetooth-spec vendor-specific range 0x80-0x9F for application errors.
+ * Call this from your NimBLECharacteristicCallbacks::onWrite() implementation
+ * to reject the write; the stack will then send an ATT Error Response instead
+ * of a success response. Ignored for Write Commands (no response exists).
+ */
+void NimBLECharacteristic::setWriteError(uint8_t attError) {
+    m_writeError = attError;
+} // setWriteError
+
+/**
+ * @brief Get the write error code set by the onWrite callback.
+ * @return The ATT error code for the current write operation, 0 if none was set.
+ */
+int NimBLECharacteristic::getWriteError() const {
+    return m_writeError;
+} // getWriteError
+
 /**
  * @brief Set the callback handlers for this characteristic.
  * @param [in] pCallbacks An instance of a NimBLECharacteristicCallbacks class\n

src/NimBLECharacteristic.h

@@ -76,6 +76,22 @@ class NimBLECharacteristic : public NimBLELocalValueAttribute {
 
     NimBLECharacteristicCallbacks* getCallbacks() const;
 
+    /**
+     * @brief Set the ATT error code to return for the current write operation.
+     * @param [in] attError The ATT error code; 0 accepts the write. Use the
+     * Bluetooth-spec vendor-specific range 0x80-0x9F for application errors.
+     * Call this from your NimBLECharacteristicCallbacks::onWrite() implementation
+     * to reject the write; the stack will then send an ATT Error Response instead
+     * of a success response. Ignored for Write Commands (no response exists).
+     */
+    void setWriteError(uint8_t attError);
+
+    /**
+     * @brief Get the write error code set by the onWrite callback.
+     * @return The ATT error code for the current write operation, 0 if none was set.
+     */
+    int getWriteError() const;
+
     /*********************** Template Functions ************************/
 
 # if __cplusplus < 201703L
@@ -305,6 +321,7 @@ class NimBLECharacteristic : public NimBLELocalValueAttribute {
     NimBLEService*                 m_pService{nullptr};
     std::vector<NimBLEDescriptor*> m_vDescriptors{};
     mutable SubPeerArray           m_subPeers{};
+    int                            m_writeError = 0;
 }; // NimBLECharacteristic
 
 /**

src/NimBLEServer.cpp

@@ -18,6 +18,7 @@
 #include "NimBLEServer.h"
 #if CONFIG_BT_NIMBLE_ENABLED && MYNEWT_VAL(BLE_ROLE_PERIPHERAL)
 
+# include "NimBLECharacteristic.h"
 # include "NimBLEDevice.h"
 # include "NimBLELog.h"
 
@@ -774,7 +775,8 @@ int NimBLEServer::handleGattEvent(uint16_t connHandle, uint16_t attrHandle, ble_
             }
 
             pAtt->writeEvent(buf, len, peerInfo);
-            return 0;
+            // Only characteristics carry the write error slot; descriptor writes keep succeeding.
+            return ctxt->op == BLE_GATT_ACCESS_OP_WRITE_CHR ? static_cast<NimBLECharacteristic*>(pAtt)->getWriteError() : 0;
         }
 
         default: