AP_HAL: fixed bug in vsnprintf()

we need to return the number of bytes that would have been printed if
the buffer was big enough to support use cases such as vasprintf()
which needs to know how many bytes to allocate

libraries/AP_HAL/Util.cpp

@@ -13,16 +13,15 @@
 /* Helper class implements AP_HAL::Print so we can use utility/vprintf */
 class BufferPrinter : public AP_HAL::BetterStream {
 public:
-    BufferPrinter(char* str, size_t size)  : _offs(0), _str(str), _size(size)  {}
+    BufferPrinter(char* str, size_t size)  :
+        _offs(0), _str(str), _size(size)  {}
 
     size_t write(uint8_t c) override {
         if (_offs < _size) {
             _str[_offs] = c;
-            _offs++;
-            return 1;
-        } else {
-            return 0;
         }
+        _offs++;
+        return 1;
     }
     size_t write(const uint8_t *buffer, size_t size) override {
         size_t n = 0;
@@ -52,15 +51,20 @@ int AP_HAL::Util::snprintf(char* str, size_t size, const char *format, ...)
 
 int AP_HAL::Util::vsnprintf(char* str, size_t size, const char *format, va_list ap)
 {
-    if (size == 0) {
-        return 0;
-    }
-    BufferPrinter buf(str, size-1);
+    // note that size==0 must be handled as functions like vasprintf() rely on the return
+    // value being the number of bytes that would be printed if there was enough space.
+    BufferPrinter buf(str, size?size-1:0);
     print_vprintf(&buf, format, ap);
     // null terminate
-    int ret = buf._offs;
-    str[ret] = '\0';
-    return ret;
+    size_t ret = buf._offs;
+    if (ret < size) {
+        // if the string did fit then nul terminate
+        str[ret] = '\0';
+    } else if (size > 0) {
+        // if it didn't fit then terminate using passed in size
+        str[size-1] = 0;
+    }
+    return int(ret);
 }
 
 uint64_t AP_HAL::Util::get_hw_rtc() const