Automattic/node-canvas

Missing async "canvas.toBuffer" for PDF canvases

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#821 opened on Oct 20, 2016

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BugHelp wanted

Description

I need a progress bar for user-feedback because the pdfs i want to produce are huge and i am missing a indication for the time it takes. 600m x 3000mm for example, which means i want to fill 1000 x 1800 = 1.800.000 rectangulars with fillRect.

The problems starts when i want to write the buffer to a file, because toBuffer takes the unpredictable time. My hope was the async usage to get a more reactive way to save the buffer to a file.

Issue

Callback function passed to toBuffer gets never called. Typos dont throw an error no console log.

Steps to Reproduce

var Canvas = require('canvas');
let canvas = new Canvas(600, 600, 'pdf');
let ctx = canvas.getContext('2d');
ctx.fillStyle = 'rgb(0,0,0)';
// this runs in a for loop over 2 dim array with up to 2 milllion entries  
ctx.fillRect(0,0,50,50);
console.log('start buffer');
canvas.toBuffer((err,buf) => {
  // this never runs
  console.log('foobar');
});
console.log('end buffer');

Your Environment

  • Version of node-canvas (^ 1.5.0): 1.6.0 should be installed
  • Environment: node v6.5.0 on Mac OS X 10.11.6

Contributor guide

Tech stack
javascriptnodejscpp
Domain
backendperformance
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Understanding of Node.js async callbacksFamiliarity with C++ addon structure
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the toBuffer implementation in the node canvas library. The issue is that the callback is not being called for PDF canvases. Start by examining the toBuffer method in lib/canvas.js and its C++ counterpart in src/Canvas.cc. Look for how PDF canvases handle the buffer differently from image canvases. Check if there is a missing call to the callback in the PDF case. Also review any existing tests for toBuffer to understand expected behavior.