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wasm/js/emscripten: Fix page loading race

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Using a stunt webserver which artificially introduces a 3s delay just
before the last line of the HTML output, I have reproduced a
uwer-reported loading/startup race bug:

Previously the wasm loading was started by the <script> element,
synchronously. If the wasm loading is fast, and finishes before the
HTML loading, the onRuntimeInitialized event may occur before
initPuzzles.  But initPuzzles sets up the event handler.

Fix this bug, and introduce a new comment containing an argument for
the correctness of the new approach.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
This commit is contained in:
Ian Jackson
2021-04-22 19:20:32 +01:00
committed by Simon Tatham
parent 56ef86f92b
commit 77866e1335
4 changed files with 49 additions and 2 deletions
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1
cmake/platforms/emscripten.cmake
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@ -43,6 +43,7 @@ endfunction()
function(set_platform_puzzle_target_properties NAME TARGET)
em_link_pre_js(${TARGET} ${CMAKE_SOURCE_DIR}/emccpre.js)
em_link_js_library(${TARGET} ${CMAKE_SOURCE_DIR}/emcclib.js)
em_link_post_js(${TARGET} ${CMAKE_SOURCE_DIR}/emccpost.js)
endfunction()
function(build_platform_extras)

1
emccpost.js Normal file
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@ -0,0 +1 @@
initPuzzle();

45
emccpre.js
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@ -35,6 +35,51 @@ var update_xmin, update_xmax, update_ymin, update_ymax;
// our own init stuff first), and that when main() returns nothing
// will get cleaned up so we remain able to call the puzzle's various
// callbacks.
//
//
// Page loading order:
//
// 1. The browser starts reading *.html (which comes from jspage.pl)
// 2. It finds the <script> tag. This is marked defer, so the
// browser will start fetching and parsing it, but not execute it
// until the page has loaded.
//
// Now the browser is loading *.html and *.js in parallel. The
// html is rendered as we go, and the js is deferred.
//
// 3. The HTML finishes loading. The browser is about to fire the
// `DOMContentLoaded` event (ie `onload`) but before that, it
// actually runs the deferred JS. THis consists of
//
// (i) emccpre.js (this file). This sets up various JS variables
// including the emscripten Module object.
//
// (ii) emscripten's JS. This starts the WASM loading.
//
// (iii) emccpost.js. This calls initPuzzle, which is defined here
// in this file. initPuzzle:
//
// (a) finds various DOM elements and bind them to variables,
// which depend on the HTML having loaded (it has).
//
// (b) makes various `cwrap` calls into the emscripten module to
// set up hooks; this depends on the emscripten JS having been
// loaded (it has).
//
// (c) Makes the call to emscripten's
// Module.onRuntimeInitialized, which sets the callback for when
// the WASM has finished loading and initialising. This has to
// come before the WASM finishes loading, or we'll miss the
// callback. We are executing synchronously here in the same JS
// file as started the WASM loading, so that is guaranteed.
//
// When this JS execution is complete, the browser fires the `onload`
// event. This is ignored. It continues loading the WASM.
//
// 4. The WASM loading and initialisation completes. The
// onRuntimeInitialised callback calls into emscripten-generated
// WASM to call the C `main`, to actually start the puzzle.
var Module = {
'noInitialRun': true,
'noExitRuntime': true

4
html/jspage.pl
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@ -73,7 +73,7 @@ EOF
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ASCII" />
<title>${puzzlename}, ${unfinishedtitlefragment}from Simon Tatham's Portable Puzzle Collection</title>
<script type="text/javascript" src="${jspath}${filename}.js"></script>
<script defer type="text/javascript" src="${jspath}${filename}.js"></script>
<style class="text/css">
/* Margins and centring on the top-level div for the game menu */
#gamemenu { margin-top: 0; margin-bottom: 0.5em; text-align: center }
@ -217,7 +217,7 @@ EOF
}
</style>
</head>
<body onLoad="initPuzzle();">
<body>
<h1 align=center>${puzzlename}</h1>
${unfinishedheading}
<h2 align=center>from Simon Tatham's Portable Puzzle Collection</h2>