Sending JSON from Python to Node via child_process gets truncated if too long, how to fix?

问题

My Node & Python backend is running just fine, but I now encountered an issue where if a JSON I'm sending from Python back no Node is too long, it gets split into two and my JSON.parse at the Node side fails.

How should I fix this? For example, the first batch clips at

... [1137.6962355826706, -100.78015825640887], [773.3834338399517, -198

and the second one has the remaining few entries

.201506231888], [-87276.575065248, -60597.8827676457], [793.1850250453127, 
-192.1674702207991], [1139.4465453979683, -100.56741252031816], 
[780.498416769341, -196.04064849430705]]}

Do I have to create some logic on the Node side for long JSONs or is this some sort of a buffering issue I'm having on my Python side that I can overcome with proper settings? Here's all I'm doing on the python side:

outPoints, _ = cv2.projectPoints(inPoints, np.asarray(rvec), 
np.asarray(tvec), np.asarray(camera_matrix), np.asarray(dist_coeffs))

# flatten the output to get rid of double brackets per result before JSONifying
flattened = [val for sublist in outPoints for val in sublist]
print(json.dumps({'testdata':np.asarray(flattened).tolist()}))
sys.stdout.flush()

And on the Node side:

// Handle python data from print() function
  pythonProcess.stdout.on('data', function (data){

    try {
      // If JSON handle the data
      console.log(JSON.parse(data.toString()));
    } catch (e) {
      // Otherwise treat as a log entry
      console.log(data.toString());
    }
  });

回答1:

The emitted data is chunked, so if you want to parse a JSON you will need to join all the chunks, and on end perform JSON.parse.

By default, pipes for stdin, stdout, and stderr are established between the parent Node.js process and the spawned child. These pipes have limited (and platform-specific) capacity. If the child process writes to stdout in excess of that limit without the output being captured, the child process will block waiting for the pipe buffer to accept more data.

In linux each chunk is limited to 65536 bytes.

In Linux versions before 2.6.11, the capacity of a pipe was the same as the system page size (e.g., 4096 bytes on i386). Since Linux 2.6.11, the pipe capacity is 65536 bytes.

let result = '';
pythonProcess.stdout.on('data', data => {
    result += data.toString();
    // Or Buffer.concat if you prefer.
});

pythonProcess.stdout.on('end', () => {
    try {
      // If JSON handle the data
      console.log(JSON.parse(result));
    } catch (e) {
      // Otherwise treat as a log entry
      console.log(result);
    }
});

来源：https://stackoverflow.com/questions/50787082/sending-json-from-python-to-node-via-child-process-gets-truncated-if-too-long-h

标签

python

node.js

stdout

child-process