AP_HAL: added readptr() and advance() to ObjectBuffer

this gives a more efficient way of accessing an ObjectBuffer, which
reduces interrupt latency in SoftSigReader

libraries/AP_HAL/utility/RingBuffer.h

@@ -98,7 +98,11 @@ template <class T>
 class ObjectBuffer {
 public:
     ObjectBuffer(uint32_t _size) {
-        buffer = new ByteBuffer((_size * sizeof(T))+1);
+        // we set size to 1 more than requested as the byte buffer
+        // gives one less byte than requested. We round up to a full
+        // multiple of the object size so that we always get aligned
+        // elements, which makes the readptr() method possible
+        buffer = new ByteBuffer(((_size+1) * sizeof(T)));
     }
     ~ObjectBuffer(void) {
         delete buffer;
@@ -188,6 +192,25 @@ public:
         return buffer->peekbytes((uint8_t*)&object, sizeof(T)) == sizeof(T);
     }
 
+    /*
+      return a pointer to first contiguous array of available
+      objects. Return nullptr if none available
+     */
+    const T *readptr(uint32_t &n) {
+        uint32_t avail_bytes = 0;
+        const T *ret = (const T *)buffer->readptr(avail_bytes);
+        if (!ret || avail_bytes < sizeof(T)) {
+            return nullptr;
+        }
+        n = avail_bytes / sizeof(T);
+        return ret;
+    }
+
+    // advance the read pointer (discarding objects)
+    bool advance(uint32_t n) {
+        return buffer->advance(n * sizeof(T));
+    }
+    
     /* update the object at the front of the queue (the one that would
        be fetched by pop()) */
     bool update(const T &object) {