AP_HAL: added update() method for object ringbuffer

to support updating objects for GCS work Tom is doing

libraries/AP_HAL/utility/RingBuffer.cpp

@@ -66,6 +66,28 @@ uint32_t ByteBuffer::write(const uint8_t *data, uint32_t len)
     return len;
 }
 
+/*
+  update bytes at the read pointer. Used to update an object without
+  popping it
+ */
+bool ByteBuffer::update(const uint8_t *data, uint32_t len)
+{
+    if (len > available()) {
+        return false;
+    }
+    // perform as two memcpy calls
+    uint32_t n = size - head;
+    if (n > len) {
+        n = len;
+    }
+    memcpy(&buf[head], data, n);
+    data += n;
+    if (len > n) {
+        memcpy(&buf[0], data, len-n);
+    }
+    return true;
+}
+
 bool ByteBuffer::advance(uint32_t n)
 {
     if (n > available()) {

libraries/AP_HAL/utility/RingBuffer.h

@@ -46,6 +46,12 @@ public:
     // read bytes from ringbuffer. Returns number of bytes read
     uint32_t read(uint8_t *data, uint32_t len);
 
+    /*
+      update bytes at the read pointer. Used to update an object without
+      popping it
+    */
+    bool update(const uint8_t *data, uint32_t len);
+    
     // return size of ringbuffer
     uint32_t get_size(void) const { return size; }
 
@@ -110,7 +116,7 @@ public:
         }
         return buffer->write((uint8_t*)&object, sizeof(T)) == sizeof(T);
     }
-
+   
     /*
       throw away an object
      */
@@ -146,6 +152,12 @@ public:
     bool peek(T &object) {
         return peekbytes(&object, sizeof(T)) == sizeof(T);
     }
+
+    /* update the object at the front of the queue (the one that would
+       be fetched by pop()) */
+    bool update(const T &object) {
+        return buffer->update((uint8_t*)&object, sizeof(T));
+    }
     
 private:
     ByteBuffer *buffer = nullptr;