[emscripten] Download the standalone capsule before running engine.

peter-b · peter-b · commit b6731f68a4c6 · 2015-08-10T18:06:08.000+01:00
Emscripten provides a "run dependency" mechanism for preventing the
program from being started until a set of dependencies are filled.
This is used by Emscripten for downloading the initial memory image
and any filesystem files provided via emcc's `--preload-file`
argument.

This patch makes the emscripten-js standalone engine use the same
dependency mechanism for downloading the standalone capsule (which is
expected to be a zip file containing the boot filesystem image for the
engine).  `em-preamble.js` is included at the start of the compiled
JavaScript file and is run before anything else.

The new code defines several new entries in the `Module` object:

* `livecodeStandalone`: the filename for the standalone zip file

* `livecodeStandalonePrefixURL`: prepended to `livecodeStandalone` to
  create a URL

* `livecodeStandaloneRequest`: The `XMLHttpRequest` instance for
  downloading the standalone file.
diff --git a/engine/emscripten-javascriptify.sh b/engine/emscripten-javascriptify.sh
@@ -16,7 +16,8 @@ input=$1
 output=$2
 exports=$3
 whitelist=$4
-shift 4
+preamble=$5
+shift 5
 
 for lib in $@ ; do
   libs+=\ --js-library\ "${lib}"
@@ -34,5 +35,6 @@ emcc -O2 -g ${CFLAGS} \
 	-s ALLOW_MEMORY_GROWTH=1 \
 	-s TOTAL_MEMORY=67108864 \
 	--preload-file boot \
+	--pre-js "${preamble}" \
 	${libs}
 
diff --git a/engine/engine-sources.gypi b/engine/engine-sources.gypi
@@ -734,6 +734,7 @@
 			'src/em-main.cpp',
 			'src/em-osspec-misc.cpp',
 			'src/em-osspec-network.cpp',
+			'src/em-preamble.js',
 			'src/em-resolution.cpp',
 			'src/em-stack.cpp',
 			'src/em-surface.h',
diff --git a/engine/engine.gyp b/engine/engine.gyp
@@ -772,6 +772,7 @@
 									'<(PRODUCT_DIR)/standalone-community.bc',
 									'src/em-exported.json',
 									'src/em-whitelist.json',
+									'src/em-preamble.js',
 									'src/em-util.js',
 									'src/em-async.js',
 									'src/em-event.js',
@@ -792,6 +793,7 @@
 									'<(PRODUCT_DIR)/standalone-community.html',
 									'src/em-exported.json',
 									'src/em-whitelist.json',
+									'src/em-preamble.js',
 									'src/em-util.js',
 									'src/em-async.js',
 									'src/em-event.js',
diff --git a/engine/src/em-preamble.js b/engine/src/em-preamble.js
@@ -0,0 +1,106 @@
+/*                                                              -*-Javascript-*-
+
+Copyright (C) 2015 Runtime Revolution Ltd.
+
+This file is part of LiveCode.
+
+LiveCode is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License v3 as published by the Free
+Software Foundation.
+
+LiveCode is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with LiveCode.  If not see <http://www.gnu.org/licenses/>.  */
+
+// The code in this file is run before the Emscripten engine starts.
+
+// Ensure that there is a Module object.
+
+var Module;
+if (!Module) {
+    Module = (typeof Module !== 'undefined' ? Module : null) || {};
+}
+
+// ----------------------------------------------------------------
+// Download standalone capsule
+// ----------------------------------------------------------------
+
+// Ensure the Module object has a preRun list
+if (!Module['preRun']) {
+	Module['preRun'] = [];
+}
+
+// Before the engine is allowed to start, we download the standalone
+// capsule, which is a zip file containing the root filesystem to be
+// used by the engine.
+//
+// There are several entries in the Module object that are used when
+// downloading the capsule:
+//
+// * Module['livecodeStandalone'] is the filename of the standalone
+//   zip file.  If it's not provided, the default value is
+//   'standalone.zip'.
+//
+// * Module['locateFile'] is a function that takes a filename and
+//   returns a corresponding URL.
+//
+// * Module['livecodeStandalonePrefixURL'] is prepended to the
+//   standalone filename if there's no locateFile function available.
+//
+// * Module['livecodeStandaloneRequest'] stores the XMLHttpRequest()
+//   object used to download the standalone file, for later use.
+
+// FIXME Should this be moved into the engine?
+
+Module['preRun'].push(function() {
+
+	// Block running the engine
+	Module['addRunDependency']('livecodeStandalone');
+
+	// Compute the URL from which to download the capsule
+	var standalone = 'standalone.zip';
+
+	if (Module['livecodeStandalone']) {
+		standalone = Module['livecodeStandalone'];
+	}
+
+	if (typeof Module['locateFile'] === 'function') {
+		standalone = Module['locateFile'](standalone);
+	} else if (Module['livecodeStandalonePrefixURL']) {
+		standalone = Module['livecodeStandalonePrefixURL'] + standalone;
+	}
+
+	// Download the capsule
+
+	// FIXME Can we cache the capsule locally?
+
+	if (!Module['livecodeStandaloneRequest']) {
+		var xhr = new XMLHttpRequest();
+
+		xhr.addEventListener('load', function(e) {
+			if (xhr.status !== 200 && xhr.status !== 0) {
+				throw 'Could not download LiveCode standalone';
+			}
+
+			if (!xhr.response ||
+			    typeof xhr.response !== 'object' ||
+			    !xhr.response.byteLength) {
+				throw 'Bad result when downloading LiveCode standalone';
+			}
+
+			// Unblock running the engine
+			Module['removeRunDependency']('livecodeStandalone');
+		});
+
+		xhr.open("GET", standalone);
+		xhr.responseType = "arraybuffer";
+		xhr.send();
+
+		// Save the request in the Module object for future reference.
+		Module['livecodeStandaloneRequest'] = xhr;
+	}
+});
