LogWriterFile: split long header messages that exceed the buffer length

Some message formats are longer than the 300 bytes. We can split the writes
because we have to wait until they are written anyway.

src/modules/logger/log_writer_file.cpp

@@ -293,13 +293,33 @@ int LogWriterFile::write_message(LogType type, void *ptr, size_t size, uint64_t
 	if (_need_reliable_transfer) {
 		int ret;
 
-		while ((ret = write(type, ptr, size, dropout_start)) == -1) {
-			unlock();
-			notify();
-			usleep(3000);
-			lock();
+		// if there's a dropout, write it first (because we might split the message)
+		if (dropout_start) {
+			while ((ret = write(type, ptr, 0, dropout_start)) == -1) {
+				unlock();
+				notify();
+				usleep(3000);
+				lock();
+			}
 		}
 
+		uint8_t *uptr = (uint8_t *)ptr;
+
+		do {
+			// Split into several blocks if the data is longer than the write buffer
+			size_t write_size = math::min(size, _buffers[(int)type].buffer_size());
+
+			while ((ret = write(type, uptr, write_size, 0)) == -1) {
+				unlock();
+				notify();
+				usleep(3000);
+				lock();
+			}
+
+			uptr += write_size;
+			size -= write_size;
+		} while (size > 0);
+
 		return ret;
 	}