[Python][Rosbag] Written rosbag has same start and end dates - All topics are published at the end

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Hello everyone,

I've been running around code APIs, forums and other wells of knowledge but I can't find the solution to my problem. Here's what I want to do:

• open a .mat file (Done)
• read the structure and fetch the data (Done)
• create a rosbag with a topic for each message in the mat file (Fail)

I can write to the bag file with

for t in range(MESSAGE_SIZE):
     ros_msg.data = MESSAGE[t]    
     t_msg.secs = t_vec.sec
     t_msg.nsecs = t_vec.nsec 
     bag.write("/topic", ros_msg, t_msg)

Where t_vec is an array, filled with timestamps of said messages. It's under the same format as the ROS time. Execution of the code is done smoothly. However, when I try to replay the bag to check what has been written down in it, I got these issues:

• rosbag info tells me that bag duration is 0.0s
• rosbag info tells me that the start and end date are the same (final value of the time vector used in the bag creation)
• when I play the bag, the duration displayed in the terminal is 17s, whereas the time vector is 89 seconds long.
• all messages of all topics are published only when the bag reaches its end.
• the /clock topic is well published and its time is correct.

I honestly don't know where to look now...

Any help would be greatly appreciated. Thanks in advance!