Bitmaps larger than 495 × 495 × 4 leak in WASM

Describe the bug FPDF.Bitmap_Destroy() leaks above a certain size in multiple browsers.

To Reproduce Steps to reproduce the behavior:

1. Go to https://pdfviewer.github.io/

2. Open developer tools

3. Add 2 live expressions to watch: wasmMemory and wasmMemory.buffer.byteLength (only in Chrome-like browsers)

4. You can also open the browser task manager with: <kbd>Shift</kbd> + <kbd>Esc</kbd>, locate the PDF Viewer tab there and watch that

5. Evaluate the following one block at a time in the developer console (reload tab in between each block for test hygiene)

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(_PDFium_Init() is repeated for your copy+paste convenience and repeating/not-repeating it does not change the behavior.)

1. s = 1000; _free(_malloc(()) – evaluate in console ✔️ (no leak)

wasmMemory
wasmMemory.buffer.byteLength

_PDFium_Init(); var s = 1000; for (let i = 0; i < 100; i++) _free(_malloc(s * s * 4)); // ✔️

wasmMemory.buffer.byteLength
wasmMemory

2. s = 100; Destroy(CreateEx(s, s, 4) ✔️

_PDFium_Init(); var s = 100; for (let i = 0; i < 10; i++) FPDF.Bitmap_Destroy(FPDF.Bitmap_CreateEx(s, s, 4)); // ✔️

3. s = 495 ✔️

_PDFium_Init(); var s = 495; for (let i = 0; i < 10; i++) FPDF.Bitmap_Destroy(FPDF.Bitmap_CreateEx(s, s, 4)); // ✔️

4. s >= 496 ❌

_PDFium_Init(); var s = 496; for (let i = 0; i < 10; i++) FPDF.Bitmap_Destroy(FPDF.Bitmap_CreateEx(s, s, 4)); // ❌

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Bindings to confirm bitmap behavior.

_PDFium_Init();
FPDF.Bitmap_GetWidth  = Module.cwrap('FPDFBitmap_GetWidth',  'number', ['number']);
FPDF.Bitmap_GetHeight = Module.cwrap('FPDFBitmap_GetHeight', 'number', ['number']);
FPDF.Bitmap_GetBuffer = Module.cwrap('FPDFBitmap_GetBuffer', 'number', ['number']);
FPDF.Bitmap_Create    = Module.cwrap('FPDFBitmap_Create',    'number', ['number', 'number', 'number']); // ⚠️ same issue

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Expected behavior There should be no leaks at any size.

System (please complete the following information):

• OS: Windows 10 Version 20H2 (OS Build 19042.1055)
• Browsers: Chrome 91.0.4472.77; Firefox 89.0

[quincarter]

We do that already. See: https://github.com/paulocoutinhox/pdfium-lib/issues/71#issuecomment-1136067094 – function H.

i recognize this now. It’s making sense to me.

[quincarter]

This is all thread blocking btw. If i can get my solution working, i will contribute part of it to this repo as it removes a lot (if not all) of the thread blocking that happens in this index.html build. I have refactored a lot but its been difficult as the DOM manipulation has to be extracted to separate areas.