Automattic/node-canvas

Assertion failure when drawing after calling canvas.toBuffer() for PDF or SVG canvas

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#1195 opened on Jul 2, 2018

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BugHelp wanted

Description

Steps to Reproduce

const canvas = createCanvas(200,200,"svg")
const ctx = canvas.getContext("2d");
canvas.toBuffer()
ctx.fillRect(10,10,100,100) 
canvas.toBuffer()

results in

File: cairo-surface.c, Line 385

Expression: ! surface->finished

toBuffer has a call to cairo_surface_finish. After calling cairo_surface_finish() the only valid operations on a surface are getting and setting user, referencing and destroying, and flushing and finishing it.

We should at least catch that failure so the process doesn't crash. I don't see a way to actually let the canvas continue to be usable for drawing though. (Thought there might be an easy way to copy the content, but not seeing one.)

Your Environment

  • Version of node-canvas (e.g. 1.4.0): master @ b10d204
  • Environment (e.g. node 4.2.0 on Mac OS X 10.8): node 8.x, Win10 x64

Contributor guide

Tech stack
javascriptnodejs
Domain
backend
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.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic knowledge of Node.js native addonsFamiliarity with Cairo graphics library
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The issue is caused by calling cairo surface finish() in toBuffer(), which marks the surface as finished, preventing further drawing. The fix should add error handling around the Cairo surface finish call in the toBuffer implementation. Check the Node.js binding code for toBuffer in the node canvas repository, likely in src/Canvas.cc or similar. Consider using cairo surface copy page or other methods to avoid finishing the surface prematurely. Also look at the referenced line in cairo surface.c to understand the assertion.