att_db: allow ATT Read Callback to return custom ATT Error Code

CHANGELOG.md

@@ -22,7 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - GATT Client: Device Information Service Client
 - GATT Client: HID-over-GATT (HOG) Client, Report and Boot Host
 - GATT Client: Scan Parameters Service Client 
-- GATT Server: Scan Parameters Service Server 
+- GATT Server: Scan Parameters Service Server
 - GAP: support scan page configuration with `gap_set_page_scan_activity` and `gap_set_page_scan_type`
 - GAP: support sniff subrating with `gap_sniff_subrating_configure
 - GAP: support QoS setup with `gap_qos_set`
@@ -72,6 +72,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - HFP: provide acl_handle in events to identify connection
 - HCI Transport: extract convenience function declaration for h4, h5, em9304_spi, and usb into separate hci_transport_{type}.h
 - GATT Client: Use ATT_READ_REQUEST for first blob of Read Long Characteristic and Read Long Characteristic Descriptor
+- GATT Server: Allow ATT Read Callback to return custom ATT Error Code
 - HID: Move `src/classic/hid.h` into `src` and prefix with `btstack_` to use it with BLE and avoid name clashes
 
 ## Release v1.3.2

src/ble/att_db.c

@@ -587,6 +587,12 @@ static uint16_t handle_read_by_type_request2(att_connection_t * att_connection,
         }
 #endif
 
+        // allow to return ATT Error Code in ATT Read Callback
+        if (it.value_len > ATT_READ_ERROR_CODE_OFFSET){
+            error_code =  it.value_len - ATT_READ_ERROR_CODE_OFFSET;
+            break;
+        }
+
         // check if value has same len as last one
         uint16_t this_pair_len = 2u + it.value_len;
         if ((offset > 1u) && (pair_len != this_pair_len)) {
@@ -685,6 +691,12 @@ static uint16_t handle_read_request2(att_connection_t * att_connection, uint8_t
     if (it.value_len == ATT_READ_RESPONSE_PENDING) return ATT_READ_RESPONSE_PENDING;
 #endif
 
+    // allow to return ATT Error Code in ATT Read Callback
+    if (it.value_len > ATT_READ_ERROR_CODE_OFFSET){
+        error_code = it.value_len - ATT_READ_ERROR_CODE_OFFSET;
+        return setup_error(response_buffer, request_type, handle, error_code);
+    }
+
     // store
     uint16_t offset   = 1;
     uint16_t bytes_copied = att_copy_value(&it, 0, response_buffer + offset, response_buffer_size - offset, att_connection->con_handle);
@@ -733,6 +745,12 @@ static uint16_t handle_read_blob_request2(att_connection_t * att_connection, uin
     if (it.value_len == ATT_READ_RESPONSE_PENDING) return ATT_READ_RESPONSE_PENDING;
 #endif
 
+    // allow to return ATT Error Code in ATT Read Callback
+    if (it.value_len > ATT_READ_ERROR_CODE_OFFSET){
+        error_code = it.value_len - ATT_READ_ERROR_CODE_OFFSET;
+        return setup_error(response_buffer, request_type, handle, error_code);
+    }
+
     if (value_offset > it.value_len){
         return setup_error_invalid_offset(response_buffer, request_type, handle);
     }
@@ -809,6 +827,12 @@ static uint16_t handle_read_multiple_request2(att_connection_t * att_connection,
         if (read_request_pending) continue;
 #endif
 
+        // allow to return ATT Error Code in ATT Read Callback
+        if (it.value_len > ATT_READ_ERROR_CODE_OFFSET){
+            error_code = it.value_len -ATT_READ_ERROR_CODE_OFFSET;
+            break;
+        }
+
         // store
         uint16_t bytes_copied = att_copy_value(&it, 0, response_buffer + offset, response_buffer_size - offset, att_connection->con_handle);
         offset += bytes_copied;

src/ble/att_db.h

@@ -87,6 +87,9 @@ extern "C" {
 #define ATT_WRITE_COMMAND                0x52
 #define ATT_SIGNED_WRITE_COMMAND         0xD2
 
+// map ATT ERROR CODES on to att_read_callback length
+#define ATT_READ_ERROR_CODE_OFFSET       0xfe00
+
 // custom BTstack ATT Response Pending for att_read_callback
 #define ATT_READ_RESPONSE_PENDING                 0xffff