Memory issue with creating a doc with a lot of images

[rsshilli]

I'm creating a document that has 750+ pages where each page has a different screenshot. The total doc size is 48 MB and it takes ~4 mins to create, which seems fine. What's not fine is that it take ~6 GB of memory to create this 48 MB file.

At first I thought I had the same problem as #728 but when I tried @blikblum's branch which includes both the new es6 conversion and #728 I found that it didn't help at all.

I've figured out a fix for it. The problem is in the PNG zlib decrypting. The doc.image() method is returning right away, and if you create a lot of pages and add a lot of images, the image processing piles up on itself until it's literally trying to decode hundreds of images at once. Unfortunately it seems the image decoding takes a few x more memory and when it happens all at once it gets out of hand.

With my fix, the 6 GB of memory went down to about 100 MB of memory. The downside though is that now doc.image() returns a promise instead of the doc, which means they can't be chained together anymore.

[rsshilli]

For those looking for a work-around for version 0.8.x, try this:

    async function writeImage(someImage) {
      doc.image(someImage);
      // Sleep for a bit because of https://github.com/foliojs/pdfkit/issues/861
      await new Promise(resolve => setTimeout(() => resolve(), 100));
    }

[blikblum]

Probably fixed by #889 . The issue still can occur because png-js decodePixels calls zlib inflate async, but odds are much smaller.

Please test and if still occurs reopen with a sample code