AP_HAL: add support for external buffer to be used in ByteBuffer

libraries/AP_HAL/utility/RingBuffer.cpp

@@ -7,11 +7,14 @@ ByteBuffer::ByteBuffer(uint32_t _size)
 {
     buf = (uint8_t*)calloc(1, _size);
     size = buf ? _size : 0;
+    external_buf = false;
 }
 
 ByteBuffer::~ByteBuffer(void)
 {
-    free(buf);
+    if (!external_buf) {
+        free(buf);
+    }
 }
 
 /*
@@ -19,6 +22,10 @@ ByteBuffer::~ByteBuffer(void)
  */
 bool ByteBuffer::set_size(uint32_t _size)
 {
+    if (external_buf) {
+        // resize not supported with external buffer
+        return false;
+    }
     head = tail = 0;
     if (_size != size) {
         free(buf);

libraries/AP_HAL/utility/RingBuffer.h

@@ -12,6 +12,11 @@
 class ByteBuffer {
 public:
     ByteBuffer(uint32_t size);
+    ByteBuffer(uint8_t* _buf, uint32_t _size) :
+    buf(_buf),
+    size(_size),
+    external_buf(true)
+    {}
     ~ByteBuffer(void);
 
     // number of bytes available to be read
@@ -92,6 +97,8 @@ private:
 
     std::atomic<uint32_t> head{0}; // where to read data
     std::atomic<uint32_t> tail{0}; // where to write data
+
+    bool external_buf;
 };
 
 /*
@@ -107,9 +114,17 @@ public:
         // 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)));
+        external_buf = false;
     }
+
+    ObjectBuffer(ByteBuffer *_buffer) :
+    buffer(_buffer),
+    external_buf(true)
+    {}
+
     ~ObjectBuffer(void) {
-        delete buffer;
+        if (!external_buf)
+            delete buffer;
     }
 
     // return size of ringbuffer
@@ -246,6 +261,7 @@ public:
 
 private:
     ByteBuffer *buffer = nullptr;
+    bool external_buf = true;
 };
 
 /*