Bug 1719615 - [devtools] Enable 'observeWasm' only once the debugger is opened. r=yury,bomsy

Because WASM debugging triggers different machine code with debugging instruction,
the memory usage very significantly increase.
So avoid enabling it until the debugger is opened.

Differential Revision: https://phabricator.services.mozilla.com/D140069

devtools/client/debugger/src/client/firefox.js

@@ -43,13 +43,21 @@ export async function onConnect(_commands, _resourceCommand, _actions, store) {
     }
     await targetCommand.startListening();
   }
-  // `pauseWorkersUntilAttach` is one option set when the debugger panel is opened rather that from the toolbox.
-  // The reason is to support early breakpoints in workers, which will force the workers to pause
-  // and later on (when TargetMixin.attachThread is called) resume worker execution, after passing the breakpoints.
-  // We only observe workers when the debugger panel is opened (see the few lines before and listenForWorkers = true).
-  // So if we were passing `pauseWorkersUntilAttach=true` from the toolbox code, workers would freeze as we would not watch
-  // for their targets and not resume them.
-  const options = { pauseWorkersUntilAttach: true };
+
+  const options = {
+    // `pauseWorkersUntilAttach` is one option set when the debugger panel is opened rather that from the toolbox.
+    // The reason is to support early breakpoints in workers, which will force the workers to pause
+    // and later on (when TargetMixin.attachThread is called) resume worker execution, after passing the breakpoints.
+    // We only observe workers when the debugger panel is opened (see the few lines before and listenForWorkers = true).
+    // So if we were passing `pauseWorkersUntilAttach=true` from the toolbox code, workers would freeze as we would not watch
+    // for their targets and not resume them.
+    pauseWorkersUntilAttach: true,
+
+    // Bug 1719615 - Immediately turn on WASM debugging when the debugger opens.
+    // We avoid enabling that as soon as DevTools open as WASM generates different kind of machine code
+    // with debugging instruction which significantly increase the memory usage.
+    observeWasm: true,
+  };
   await commands.threadConfigurationCommand.updateConfiguration(options);
 
   // We should probably only pass descriptor informations from here

devtools/client/shared/thread-utils.js

@@ -43,7 +43,6 @@ exports.getThreadOptions = async function() {
     ),
     // This option is always true. See Bug 1654590 for removal.
     observeAsmJS: true,
-    observeWasm: true,
     breakpoints: sanitizeBreakpoints(await asyncStore.pendingBreakpoints),
     // XXX: `event-listener-breakpoints` is a copy of the event-listeners state
     // of the debugger panel. The `active` property is therefore linked to

devtools/server/actors/utils/make-debugger.js

@@ -61,8 +61,14 @@ module.exports = function makeDebugger({
   const dbg = new Debugger();
   EventEmitter.decorate(dbg);
 
+  // By default, we disable asm.js and WASM debugging because of performance reason.
+  // Enabling asm.js debugging (allowUnobservedAsmJS=false) will make asm.js fallback to JS compiler
+  // and be debugging as a regular JS script.
   dbg.allowUnobservedAsmJS = true;
+  // Enabling WASM debugging (allowUnobservedWasm=false) will make the engine compile WASM scripts
+  // into different machine code with debugging instructions. This significantly increase the memory usage of it.
   dbg.allowUnobservedWasm = true;
+
   dbg.uncaughtExceptionHook = reportDebuggerHookException;
 
   const onNewGlobalObject = function(global) {