InputStream: add ReadFull method

Convenient method that behave differently than Read, and that will be used by
tag scanners.

This method will return in case of error, if the whole data is read or is EOF
is reached.

src/input/InputStream.cxx

@@ -132,10 +132,39 @@ InputStream::LockRead(void *ptr, size_t _size, Error &error)
 	return Read(ptr, _size, error);
 }
 
+bool
+InputStream::ReadFull(void *_ptr, size_t _size, Error &error)
+{
+	uint8_t *ptr = (uint8_t *)_ptr;
+
+	size_t nbytes_total = 0;
+	while (_size > 0) {
+		size_t nbytes = Read(ptr + nbytes_total, _size, error);
+		if (nbytes == 0)
+			return false;
+
+		nbytes_total += nbytes;
+		_size -= nbytes;
+	}
+	return true;
+}
+
+bool
+InputStream::LockReadFull(void *ptr, size_t _size, Error &error)
+{
+#if !CLANG_CHECK_VERSION(3,6)
+	/* disabled on clang due to -Wtautological-pointer-compare */
+	assert(ptr != nullptr);
+#endif
+	assert(_size > 0);
+
+	const ScopeLock protect(mutex);
+	return ReadFull(ptr, _size, error);
+}
+
 bool
 InputStream::LockIsEOF()
 {
 	const ScopeLock protect(mutex);
 	return IsEOF();
 }
-