Quelle tracer.sys.mjs
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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/**
* This module implements the JavaScript tracer.
*
* It is being used by:
* - any code that want to manually toggle the tracer, typically when debugging code,
* - the tracer actor to start and stop tracing from DevTools UI,
* - the tracing state resource watcher in order to notify DevTools UI about the tracing state.
*
* It will default logging the tracers to the terminal/stdout.
* But if DevTools are opened, it may delegate the logging to the tracer actor.
* It will typically log the traces to the Web Console.
*
* `JavaScriptTracer.onEnterFrame` method is hot codepath and should be reviewed accordingly.
*/
const NEXT_INTERACTION_MESSAGE =
"Waiting for next user interaction before tracing (next mousedown or keydown event)";
const FRAME_EXIT_REASONS = {
// The function has been early terminated by the Debugger API
TERMINATED: "terminated",
// The function simply ends by returning a value
RETURN: "return",
// The function yields a new value
YIELD: "yield",
// The function await on a promise
AWAIT: "await",
// The function throws an exception
THROW: "throw",
};
const DOM_MUTATIONS = {
// Track all DOM Node being added
ADD: "add",
// Track all attributes being modified
ATTRIBUTES: "attributes",
// Track all DOM Node being removed
REMOVE: "remove",
};
const listeners = new Set();
// Detecting worker is different if this file is loaded via Common JS loader (isWorker global)
// or as a JSM (constructor name)
// eslint-disable-next-line no-shadow
const isWorker =
globalThis.isWorker ||
globalThis.constructor.name == "WorkerDebuggerGlobalScope";
// This module can be loaded from the worker thread, where we can't use ChromeUtils.
// So implement custom lazy getters (without XPCOMUtils ESM) from here.
// Worker codepath in DevTools will pass a custom Debugger instance.
const customLazy = {
get Debugger() {
// When this code runs in the worker thread, loaded via `loadSubScript`
// (ex: browser_worker_tracer.js and WorkerDebugger.tracer.js),
// this module runs within the WorkerDebuggerGlobalScope and have immediate access to Debugger class.
if (globalThis.Debugger) {
return globalThis.Debugger;
}
// When this code runs in the worker thread, loaded via `require`
// (ex: from tracer actor module),
// this module no longer has WorkerDebuggerGlobalScope as global,
// but has to use require() to pull Debugger.
if (isWorker) {
// require is defined for workers.
// eslint-disable-next-line no-undef
return require("Debugger");
}
const { addDebuggerToGlobal } = ChromeUtils.importESModule(
"resource://gre/modules/jsdebugger.sys.mjs"
);
// Avoid polluting all Modules global scope by using a Sandox as global.
const systemPrincipal = Services.scriptSecurityManager.getSystemPrincipal();
const debuggerSandbox = Cu.Sandbox(systemPrincipal);
addDebuggerToGlobal(debuggerSandbox);
delete customLazy.Debugger;
customLazy.Debugger = debuggerSandbox.Debugger;
return customLazy.Debugger;
},
get DistinctCompartmentDebugger() {
const { addDebuggerToGlobal } = ChromeUtils.importESModule(
"resource://gre/modules/jsdebugger.sys.mjs",
{ global: "contextual" }
);
const systemPrincipal = Services.scriptSecurityManager.getSystemPrincipal();
const debuggerSandbox = Cu.Sandbox(systemPrincipal, {
// As we may debug the JSM/ESM shared global, we should be using a Debugger
// from another system global.
freshCompartment: true,
});
addDebuggerToGlobal(debuggerSandbox);
delete customLazy.DistinctCompartmentDebugger;
customLazy.DistinctCompartmentDebugger = debuggerSandbox.Debugger;
return customLazy.DistinctCompartmentDebugger;
},
};
/**
* Start tracing against a given JS global.
* Only code run from that global will be logged.
*
* @param {Object} options
* Object with configurations:
* @param {Object} options.global
* The tracer only log traces related to the code executed within this global.
* When omitted, it will default to the options object's global.
* @param {Boolean} options.traceAllGlobals
* When set to true, this will trace all the globals running in the current thread.
* @param {String} options.prefix
* Optional string logged as a prefix to all traces.
* @param {Boolean} options.loggingMethod
* Optional setting to use something else than `dump()` to log traces to stdout.
* This is mostly used by tests.
* @param {Boolean} options.traceDOMEvents
* Optional setting to enable tracing all the DOM events being going through
* dom/events/EventListenerManager.cpp's `EventListenerManager`.
* @param {Array<string>} options.traceDOMMutations
* Optional setting to enable tracing all the DOM mutations.
* This array may contains three strings:
* - "add": trace all new DOM Node being added,
* - "attributes": trace all DOM attribute modifications,
* - "delete": trace all DOM Node being removed.
* @param {Boolean} options.traceValues
* Optional setting to enable tracing all function call values as well,
* as returned values (when we do log returned frames).
* @param {Boolean} options.traceOnNextInteraction
* Optional setting to enable when the tracing should only start when the
* use starts interacting with the page. i.e. on next keydown or mousedown.
* @param {Boolean} options.traceSteps
* Optional setting to enable tracing each frame within a function execution.
* (i.e. not only function call and function returns [when traceFunctionReturn is true])
* @param {Boolean} options.traceFunctionReturn
* Optional setting to enable when the tracing should notify about frame exit.
* i.e. when a function call returns or throws.
* @param {String} options.filterFrameSourceUrl
* Optional setting to restrict all traces to only a given source URL.
* This is a loose check, so any source whose URL includes the passed string will be traced.
* @param {Number} options.maxDepth
* Optional setting to ignore frames when depth is greater than the passed number.
* @param {Number} options.maxRecords
* Optional setting to stop the tracer after having recorded at least
* the passed number of top level frames.
* @param {Number} options.pauseOnStep
* Optional setting to delay each frame execution for a given amount of time in ms.
*/
class JavaScriptTracer {
constructor(options) {
this.onEnterFrame = this.onEnterFrame.bind(this);
// DevTools CommonJS Workers modules don't have access to AbortController
if (!isWorker) {
this.abortController = new AbortController();
}
if (options.traceAllGlobals) {
this.traceAllGlobals = true;
if (options.traceOnNextInteraction) {
throw new Error(
"Tracing all globals and waiting for next user interaction are not yet compatible"
);
}
if (this.traceDOMEvents) {
throw new Error(
"Tracing all globals and DOM Events are not yet compatible"
);
}
if (options.global) {
throw new Error(
"'global' option should be omitted when using 'traceAllGlobals'"
);
}
} else {
// By default, we would trace only JavaScript related to caller's global.
// As there is no way to compute the caller's global default to the global of the
// mandatory options argument.
this.tracedGlobal = options.global || Cu.getGlobalForObject(options);
}
// Instantiate a brand new Debugger API so that we can trace independently
// of all other DevTools operations. i.e. we can pause while tracing without any interference.
this.dbg = this.makeDebugger();
this.prefix = options.prefix ? `${options.prefix}: ` : "";
// List of all async frame which are poped per Spidermonkey API
// but are actually waiting for async operation.
// We should later enter them again when the async task they are being waiting for is completed.
this.pendingAwaitFrames = new Set();
this.loggingMethod = options.loggingMethod;
if (!this.loggingMethod) {
// On workers, `dump` can't be called with JavaScript on another object,
// so bind it.
this.loggingMethod = isWorker ? dump.bind(null) : dump;
}
this.traceDOMEvents = !!options.traceDOMEvents;
if (options.traceDOMMutations) {
if (!Array.isArray(options.traceDOMMutations)) {
throw new Error("'traceDOMMutations' attribute should be an array");
}
const acceptedValues = Object.values(DOM_MUTATIONS);
if (!options.traceDOMMutations.every(e => acceptedValues.includes(e))) {
throw new Error(
`'traceDOMMutations' only accept array of strings whose values can be: ${acceptedValues}`
);
}
this.traceDOMMutations = options.traceDOMMutations;
}
this.traceSteps = !!options.traceSteps;
this.traceValues = !!options.traceValues;
this.traceFunctionReturn = !!options.traceFunctionReturn;
this.maxDepth = options.maxDepth;
this.maxRecords = options.maxRecords;
this.records = 0;
if ("pauseOnStep" in options) {
if (typeof options.pauseOnStep != "number") {
throw new Error("'pauseOnStep' attribute should be a number");
}
this.pauseOnStep = options.pauseOnStep;
}
if ("filterFrameSourceUrl" in options) {
if (typeof options.filterFrameSourceUrl != "string") {
throw new Error("'filterFrameSourceUrl' attribute should be a string");
}
this.filterFrameSourceUrl = options.filterFrameSourceUrl;
}
// An increment used to identify function calls and their returned/exit frames
this.frameId = 0;
// This feature isn't supported on Workers as they aren't involving user events
if (options.traceOnNextInteraction && !isWorker) {
this.#waitForNextInteraction();
} else {
this.#startTracing();
}
}
// Is actively tracing?
// We typically start tracing from the constructor, unless the "trace on next user interaction" feature is used.
isTracing = false;
/**
* In case `traceOnNextInteraction` option is used, delay the actual start of tracing until a first user interaction.
*/
#waitForNextInteraction() {
// Use a dedicated Abort Controller as we are going to stop it as soon as we get the first user interaction,
// whereas other listeners would typically wait for tracer stop.
this.nextInteractionAbortController = new AbortController();
const listener = () => {
this.nextInteractionAbortController.abort();
// Avoid tracing if the users asked to stop tracing while we were waiting for the user interaction.
if (this.dbg) {
this.#startTracing();
}
};
const eventOptions = {
signal: this.nextInteractionAbortController.signal,
capture: true,
};
// Register the event listener on the Chrome Event Handler in order to receive the event first.
// When used for the parent process target, `tracedGlobal` is browser.xhtml's window, which doesn't have a chromeEventHandler.
const eventHandler =
this.tracedGlobal.docShell.chromeEventHandler || this.tracedGlobal;
eventHandler.addEventListener("mousedown", listener, eventOptions);
eventHandler.addEventListener("keydown", listener, eventOptions);
// Significate to the user that the tracer is registered, but not tracing just yet.
let shouldLogToStdout = listeners.size == 0;
for (const l of listeners) {
if (typeof l.onTracingPending == "function") {
shouldLogToStdout |= l.onTracingPending();
}
}
if (shouldLogToStdout) {
this.loggingMethod(this.prefix + NEXT_INTERACTION_MESSAGE + "\n");
}
}
/**
* Actually really start watching for executions.
*
* This may be delayed when traceOnNextInteraction options is used.
* Otherwise we start tracing as soon as the class instantiates.
*/
#startTracing() {
this.isTracing = true;
this.dbg.onEnterFrame = this.onEnterFrame;
if (this.traceDOMEvents) {
this.startTracingDOMEvents();
}
// This feature isn't supported on Workers as they aren't interacting with the DOM Tree
if (this.traceDOMMutations?.length > 0 && !isWorker) {
this.startTracingDOMMutations();
}
// In any case, we consider the tracing as started
this.notifyToggle(true);
}
startTracingDOMEvents() {
this.debuggerNotificationObserver = new DebuggerNotificationObserver();
this.eventListener = this.eventListener.bind(this);
this.debuggerNotificationObserver.addListener(this.eventListener);
this.debuggerNotificationObserver.connect(this.tracedGlobal);
// When we are tracing a document, also ensure connecting to all its children iframe globals.
// If we don't, Debugger API would fire onEnterFrame for their JavaScript code,
// but DOM Events wouldn't be notified by DebuggerNotificationObserver.
if (!isWorker && this.tracedGlobal instanceof Ci.nsIDOMWindow) {
const { browserId } = this.tracedGlobal.browsingContext;
// Keep track of any future global
this.dbg.onNewGlobalObject = g => {
try {
const win = g.unsafeDereference();
// only process globals relating to documents, and which are within the debugged tab
if (
win instanceof Ci.nsIDOMWindow &&
win.browsingContext.browserId == browserId
) {
this.dbg.addDebuggee(g);
this.debuggerNotificationObserver.connect(win);
}
} catch (e) {}
};
// Register all, already existing children
for (const browsingContext of this.tracedGlobal.browsingContext.getAllBrowsingContextsInSubtree()) {
try {
// Only consider children which run in the same process, and exposes their window object
if (browsingContext.window) {
this.dbg.addDebuggee(browsingContext.window);
this.debuggerNotificationObserver.connect(browsingContext.window);
}
} catch (e) {}
}
}
this.currentDOMEvent = null;
}
stopTracingDOMEvents() {
if (this.debuggerNotificationObserver) {
this.debuggerNotificationObserver.removeListener(this.eventListener);
this.debuggerNotificationObserver.disconnect(this.tracedGlobal);
this.debuggerNotificationObserver = null;
}
this.currentDOMEvent = null;
}
startTracingDOMMutations() {
this.tracedGlobal.document.devToolsWatchingDOMMutations = true;
const eventOptions = {
signal: this.abortController.signal,
capture: true,
};
// When used for the parent process target, `tracedGlobal` is browser.xhtml's window, which doesn't have a chromeEventHandler.
const eventHandler =
this.tracedGlobal.docShell.chromeEventHandler || this.tracedGlobal;
if (this.traceDOMMutations.includes(DOM_MUTATIONS.ADD)) {
eventHandler.addEventListener(
"devtoolschildinserted",
this.#onDOMMutation,
eventOptions
);
}
if (this.traceDOMMutations.includes(DOM_MUTATIONS.ATTRIBUTES)) {
eventHandler.addEventListener(
"devtoolsattrmodified",
this.#onDOMMutation,
eventOptions
);
}
if (this.traceDOMMutations.includes(DOM_MUTATIONS.REMOVE)) {
eventHandler.addEventListener(
"devtoolschildremoved",
this.#onDOMMutation,
eventOptions
);
}
}
stopTracingDOMMutations() {
this.tracedGlobal.document.devToolsWatchingDOMMutations = false;
// Note that the event listeners are all going to be unregistered via the AbortController.
}
/**
* Called for any DOM Mutation done in the traced document.
*
* @param {DOM Event} event
*/
#onDOMMutation = event => {
// Ignore elements inserted by DevTools, like the inspector's highlighters
if (event.target.isNativeAnonymous) {
return;
}
let type = "";
switch (event.type) {
case "devtoolschildinserted":
type = DOM_MUTATIONS.ADD;
break;
case "devtoolsattrmodified":
type = DOM_MUTATIONS.ATTRIBUTES;
break;
case "devtoolschildremoved":
type = DOM_MUTATIONS.REMOVE;
break;
default:
throw new Error("Unexpected DOM Mutation event type: " + event.type);
}
let shouldLogToStdout = true;
// The depth is the depth of the parent frame, consider the dom mutation as nested to it
const depth = this.depth + 1;
if (listeners.size > 0) {
shouldLogToStdout = false;
for (const listener of listeners) {
// If any listener return true, also log to stdout
if (typeof listener.onTracingDOMMutation == "function") {
shouldLogToStdout |= listener.onTracingDOMMutation({
depth,
prefix: this.prefix,
type,
element: event.target,
caller: Components.stack.caller,
});
}
}
}
if (shouldLogToStdout) {
const padding = "—".repeat(depth + 1);
this.loggingMethod(
this.prefix +
padding +
`[DOM Mutation | ${type}] ` +
objectToString(event.target) +
"\n"
);
}
};
/**
* Called by DebuggerNotificationObserver interface when a DOM event start being notified
* and after it has been notified.
*
* @param {DebuggerNotification} notification
* Info about the DOM event. See the related idl file.
*/
eventListener(notification) {
// For each event we get two notifications.
// One just before firing the listeners and another one just after.
//
// Update `this.currentDOMEvent` to be refering to the event name
// while the DOM event is being notified. It will be null the rest of the time.
//
// We don't need to maintain a stack of events as that's only consumed by onEnterFrame
// which only cares about the very lastest event being currently trigerring some code.
if (notification.phase == "pre") {
// We get notified about "real" DOM event when type is "domEvent",
// but also when some other DOM APIs are involved.
// notification's type will be "setTimeout" when the setTimeout method is called,
// or "setTimeoutCallback" when the callback passed to setTimeout is called.
// This also work against setInterval/clearTimeout/clearInterval and requestAnimationFrame.
if (notification.type == "domEvent") {
// `targetType` can help distinguish same-name DOM events fired against XHR, window or workers.
const { targetType } = notification;
let { type } = notification.event;
if (!type) {
// In the Worker thread, `notification.event` is an opaque wrapper.
// In other threads it is a Xray wrapper.
// Because of this difference, we have to fallback to use the Debugger.Object API.
type = this.dbg
.makeGlobalObjectReference(notification.global)
.makeDebuggeeValue(notification.event)
.getProperty("type").return;
}
this.currentDOMEvent = `${targetType}.${type}`;
} else {
this.currentDOMEvent = notification.type;
}
} else {
this.currentDOMEvent = null;
}
}
/**
* Stop observing execution.
*
* @param {String} reason
* Optional string to justify why the tracer stopped.
*/
stopTracing(reason = "") {
// Note that this may be called before `#startTracing()`, but still want to completely shut it down.
if (!this.dbg) {
return;
}
this.dbg.onEnterFrame = undefined;
this.dbg.removeAllDebuggees();
this.dbg.onNewGlobalObject = undefined;
this.dbg = null;
this.depth = 0;
// Cancel the traceOnNextInteraction event listeners.
if (this.nextInteractionAbortController) {
this.nextInteractionAbortController.abort();
this.nextInteractionAbortController = null;
}
if (this.traceDOMEvents) {
this.stopTracingDOMEvents();
}
if (this.traceDOMMutations?.length > 0 && !isWorker) {
this.stopTracingDOMMutations();
}
// Unregister all event listeners
if (this.abortController) {
this.abortController.abort();
}
this.tracedGlobal = null;
this.isTracing = false;
this.notifyToggle(false, reason);
}
/**
* Instantiate a Debugger API instance dedicated to each Tracer instance.
* It will notably be different from the instance used in DevTools.
* This allows to implement tracing independently of DevTools.
*/
makeDebugger() {
if (this.traceAllGlobals) {
const dbg = new customLazy.DistinctCompartmentDebugger();
dbg.addAllGlobalsAsDebuggees();
// addAllGlobalAsAdebuggees will also add the global for this module...
// which we have to prevent tracing!
// eslint-disable-next-line mozilla/reject-globalThis-modification
dbg.removeDebuggee(globalThis);
// Add any future global being created later
dbg.onNewGlobalObject = g => dbg.addDebuggee(g);
return dbg;
}
// When this code runs in the worker thread, Cu isn't available
// and we don't have system principal anyway in this context.
const { isSystemPrincipal } =
typeof Cu == "object" ? Cu.getObjectPrincipal(this.tracedGlobal) : {};
// When debugging the system modules, we have to use a special instance
// of Debugger loaded in a distinct system global.
const dbg = isSystemPrincipal
? new customLazy.DistinctCompartmentDebugger()
: new customLazy.Debugger();
// For now, we only trace calls for one particular global at a time.
// See the constructor for its definition.
dbg.addDebuggee(this.tracedGlobal);
return dbg;
}
/**
* Notify DevTools and/or the user via stdout that tracing
* has been enabled or disabled.
*
* @param {Boolean} state
* True if we just started tracing, false when it just stopped.
* @param {String} reason
* Optional string to justify why the tracer stopped.
*/
notifyToggle(state, reason) {
let shouldLogToStdout = listeners.size == 0;
for (const listener of listeners) {
if (typeof listener.onTracingToggled == "function") {
shouldLogToStdout |= listener.onTracingToggled(state, reason);
}
}
if (shouldLogToStdout) {
if (state) {
this.loggingMethod(this.prefix + "Start tracing JavaScript\n");
} else {
if (reason) {
reason = ` (reason: ${reason})`;
}
this.loggingMethod(
this.prefix + "Stop tracing JavaScript" + reason + "\n"
);
}
}
}
/**
* Called by the Debugger API (this.dbg) when a new frame is executed.
*
* @param {Debugger.Frame} frame
* A descriptor object for the JavaScript frame.
*/
onEnterFrame(frame) {
// Safe check, just in case we keep being notified, but the tracer has been stopped
if (!this.dbg) {
return;
}
try {
// If an optional filter is passed, ignore frames which aren't matching the filter string
if (
this.filterFrameSourceUrl &&
!frame.script.source.url?.includes(this.filterFrameSourceUrl)
) {
return;
}
// Because of async frame which are popped and entered again on completion of the awaited async task,
// we have to compute the depth from the frame. (and can't use a simple increment on enter/decrement on pop).
const depth = getFrameDepth(frame);
// Save the current depth for the DOM Mutation handler
this.depth = depth;
// Ignore the frame if we reached the depth limit (if one is provided)
if (this.maxDepth && depth >= this.maxDepth) {
return;
}
// When we encounter a frame which was previously popped because of pending on an async task,
// ignore it and only log the following ones.
if (this.pendingAwaitFrames.has(frame)) {
this.pendingAwaitFrames.delete(frame);
return;
}
// Auto-stop the tracer if we reached the number of max recorded top level frames
if (depth === 0 && this.maxRecords) {
if (this.records >= this.maxRecords) {
this.stopTracing("max-records");
return;
}
this.records++;
}
const frameId = this.frameId++;
let shouldLogToStdout = true;
// If there is at least one DevTools debugging this process,
// delegate logging to DevTools actors.
if (listeners.size > 0) {
shouldLogToStdout = false;
const formatedDisplayName = formatDisplayName(frame);
for (const listener of listeners) {
// If any listener return true, also log to stdout
if (typeof listener.onTracingFrame == "function") {
shouldLogToStdout |= listener.onTracingFrame({
frameId,
frame,
depth,
formatedDisplayName,
prefix: this.prefix,
currentDOMEvent: this.currentDOMEvent,
});
}
// Bail out early if any listener stopped tracing as the Frame object
// will be no longer usable by any other code.
if (!this.isTracing) {
return;
}
}
}
// DevTools may delegate the work to log to stdout,
// but if DevTools are closed, stdout is the only way to log the traces.
if (shouldLogToStdout) {
this.logFrameEnteredToStdout(frame, depth);
}
if (this.traceSteps) {
// Collect the location notified via onTracingFrame to also avoid redundancy between similar location
// between onEnterFrame and onStep notifications.
let { lineNumber: lastLine, columnNumber: lastColumn } =
frame.script.getOffsetMetadata(frame.offset);
frame.onStep = () => {
// Spidermonkey steps on many intermediate positions which don't make sense to the user.
// `isStepStart` is close to each statement start, which is meaningful to the user.
const { isStepStart, lineNumber, columnNumber } =
frame.script.getOffsetMetadata(frame.offset);
if (!isStepStart) {
return;
}
// onStep may be called on many instructions related to the same line and colunm.
// Avoid notifying duplicated steps if we stepped on the exact same location.
if (lastLine == lineNumber && lastColumn == columnNumber) {
return;
}
lastLine = lineNumber;
lastColumn = columnNumber;
shouldLogToStdout = true;
if (listeners.size > 0) {
shouldLogToStdout = false;
for (const listener of listeners) {
// If any listener return true, also log to stdout
if (typeof listener.onTracingFrameStep == "function") {
shouldLogToStdout |= listener.onTracingFrameStep({
frame,
depth,
prefix: this.prefix,
});
}
}
}
if (shouldLogToStdout) {
this.logFrameStepToStdout(frame, depth);
}
// Optionaly pause the frame execution by letting the other event loop to run in between.
if (typeof this.pauseOnStep == "number") {
syncPause(this.pauseOnStep);
}
};
}
frame.onPop = completion => {
this.depth--;
// Special case async frames. We are exiting the current frame because of waiting for an async task.
// (this is typically a `await foo()` from an async function)
// This frame should later be "entered" again.
if (completion?.await) {
this.pendingAwaitFrames.add(frame);
return;
}
if (!this.traceFunctionReturn) {
return;
}
let why = "";
let rv = undefined;
if (!completion) {
why = FRAME_EXIT_REASONS.TERMINATED;
} else if ("return" in completion) {
why = FRAME_EXIT_REASONS.RETURN;
rv = completion.return;
} else if ("yield" in completion) {
why = FRAME_EXIT_REASONS.YIELD;
rv = completion.yield;
} else if ("await" in completion) {
why = FRAME_EXIT_REASONS.AWAIT;
} else {
why = FRAME_EXIT_REASONS.THROW;
rv = completion.throw;
}
shouldLogToStdout = true;
if (listeners.size > 0) {
shouldLogToStdout = false;
const formatedDisplayName = formatDisplayName(frame);
for (const listener of listeners) {
// If any listener return true, also log to stdout
if (typeof listener.onTracingFrameExit == "function") {
shouldLogToStdout |= listener.onTracingFrameExit({
frameId,
frame,
depth,
formatedDisplayName,
prefix: this.prefix,
why,
rv,
});
}
}
}
if (shouldLogToStdout) {
this.logFrameExitedToStdout(frame, depth, why, rv);
}
};
// Optionaly pause the frame execution by letting the other event loop to run in between.
if (typeof this.pauseOnStep == "number") {
syncPause(this.pauseOnStep);
}
} catch (e) {
console.error("Exception while tracing javascript", e);
}
}
/**
* Display to stdout one given frame execution, which represents a function call.
*
* @param {Debugger.Frame} frame
* @param {Number} depth
*/
logFrameEnteredToStdout(frame, depth) {
const padding = "—".repeat(depth + 1);
// If we are tracing DOM events and we are in middle of an event,
// and are logging the topmost frame,
// then log a preliminary dedicated line to mention that event type.
if (this.currentDOMEvent && depth == 0) {
this.loggingMethod(
this.prefix + padding + "DOM | " + this.currentDOMEvent + "\n"
);
}
let message = `${padding}[${frame.implementation}]—> ${getTerminalHyperLink(
frame
)} - ${formatDisplayName(frame)}`;
// Log arguments, but only when this feature is enabled as it introduces
// some significant performance and visual overhead.
// Also prevent trying to log function call arguments if we aren't logging a frame
// with arguments (e.g. Debugger evaluation frames, when executing from the console)
if (this.traceValues && frame.arguments) {
message += "(";
for (let i = 0, l = frame.arguments.length; i < l; i++) {
const arg = frame.arguments[i];
// Debugger.Frame.arguments contains either a Debugger.Object or primitive object
if (arg?.unsafeDereference) {
// Special case classes as they can't be easily differentiated in pure JavaScript
if (arg.isClassConstructor) {
message += "class " + arg.name;
} else {
message += objectToString(arg.unsafeDereference());
}
} else {
message += primitiveToString(arg);
}
if (i < l - 1) {
message += ", ";
}
}
message += ")";
}
this.loggingMethod(this.prefix + message + "\n");
}
/**
* Display to stdout one given frame execution, which represents a step within a function execution.
*
* @param {Debugger.Frame} frame
* @param {Number} depth
*/
logFrameStepToStdout(frame, depth) {
const padding = "—".repeat(depth + 1);
const message = `${padding}— ${getTerminalHyperLink(frame)}`;
this.loggingMethod(this.prefix + message + "\n");
}
/**
* Display to stdout the exit of a given frame execution, which represents a function return.
*
* @param {Debugger.Frame} frame
* @param {String} why
* @param {Number} depth
*/
logFrameExitedToStdout(frame, depth, why, rv) {
const padding = "—".repeat(depth + 1);
let message = `${padding}[${frame.implementation}]<— ${getTerminalHyperLink(
frame
)} - ${formatDisplayName(frame)} ${why}`;
// Log returned values, but only when this feature is enabled as it introduces
// some significant performance and visual overhead.
if (this.traceValues) {
message += " ";
// Debugger.Frame.arguments contains either a Debugger.Object or primitive object
if (rv?.unsafeDereference) {
// Special case classes as they can't be easily differentiated in pure JavaScript
if (rv.isClassConstructor) {
message += "class " + rv.name;
} else {
message += objectToString(rv.unsafeDereference());
}
} else {
message += primitiveToString(rv);
}
}
this.loggingMethod(this.prefix + message + "\n");
}
}
/**
* Return a string description for any arbitrary JS value.
* Used when logging to stdout.
*
* @param {Object} obj
* Any JavaScript object to describe.
* @return String
* User meaningful descriptor for the object.
*/
function objectToString(obj) {
if (Element.isInstance(obj)) {
let message = `<${obj.tagName}`;
if (obj.id) {
message += ` #${obj.id}`;
}
if (obj.className) {
message += ` .${obj.className}`;
}
message += ">";
return message;
} else if (Array.isArray(obj)) {
return `Array(${obj.length})`;
} else if (Event.isInstance(obj)) {
return `Event(${obj.type}) target=${objectToString(obj.target)}`;
} else if (typeof obj === "function") {
return `function ${obj.name || "anonymous"}()`;
}
return obj;
}
function primitiveToString(value) {
const type = typeof value;
if (type === "string") {
// Use stringify to escape special characters and display in enclosing quotes.
return JSON.stringify(value);
} else if (value === 0 && 1 / value === -Infinity) {
// -0 is very special and need special threatment.
return "-0";
} else if (type === "bigint") {
return `BigInt(${value})`;
} else if (value && typeof value.toString === "function") {
// Use toString as it allows to stringify Symbols. Converting them to string throws.
return value.toString();
}
// For all other types/cases, rely on native convertion to string
return value;
}
/**
* Try to describe the current frame we are tracing
*
* This will typically log the name of the method being called.
*
* @param {Debugger.Frame} frame
* The frame which is currently being executed.
*/
function formatDisplayName(frame) {
if (frame.type === "call") {
const callee = frame.callee;
// Anonymous function will have undefined name and displayName.
return "λ " + (callee.name || callee.displayName || "anonymous");
}
return `(${frame.type})`;
}
let activeTracer = null;
/**
* Start tracing JavaScript.
* i.e. log the name of any function being called in JS and its location in source code.
*
* @params {Object} options (mandatory)
* See JavaScriptTracer.startTracing jsdoc.
*/
function startTracing(options) {
if (!options) {
throw new Error("startTracing excepts an options object as first argument");
}
if (!activeTracer) {
activeTracer = new JavaScriptTracer(options);
} else {
console.warn(
"Can't start JavaScript tracing, another tracer is still active and we only support one tracer at a time."
);
}
}
/**
* Stop tracing JavaScript.
*/
function stopTracing() {
if (activeTracer) {
activeTracer.stopTracing();
activeTracer = null;
} else {
console.warn("Can't stop JavaScript Tracing as we were not tracing.");
}
}
/**
* Listen for tracing updates.
*
* The listener object may expose the following methods:
* - onTracingToggled(state)
* Where state is a boolean to indicate if tracing has just been enabled of disabled.
* It may be immediatelly called if a tracer is already active.
*
* - onTracingFrame({ frame, depth, formatedDisplayName, prefix })
* Called each time we enter a new JS frame.
* - frame is a Debugger.Frame object
* - depth is a number and represents the depth of the frame in the call stack
* - formatedDisplayName is a string and is a human readable name for the current frame
* - prefix is a string to display as a prefix of any logged frame
*
* @param {Object} listener
*/
function addTracingListener(listener) {
listeners.add(listener);
if (
activeTracer?.isTracing &&
typeof listener.onTracingToggled == "function"
) {
listener.onTracingToggled(true);
}
}
/**
* Unregister a listener previous registered via addTracingListener
*/
function removeTracingListener(listener) {
listeners.delete(listener);
}
function getFrameDepth(frame) {
if (typeof frame.depth !== "number") {
let depth = 0;
let f = frame;
while ((f = f.older)) {
if (f.depth) {
depth = depth + f.depth + 1;
break;
}
depth++;
}
frame.depth = depth;
}
return frame.depth;
}
/**
* Generate a magic string that will be rendered in smart terminals as a URL
* for the given Frame object. This URL is special as it includes a line and column.
* This URL can be clicked and Firefox will automatically open the source matching
* the frame's URL in the currently opened Debugger.
* Firefox will interpret differently the URLs ending with `/:?\d*:\d+/`.
*
* @param {Debugger.Frame} frame
* The frame being traced.
* @return {String}
* The URL's magic string.
*/
function getTerminalHyperLink(frame) {
const { script } = frame;
const { lineNumber, columnNumber } = script.getOffsetMetadata(frame.offset);
// Use a special URL, including line and column numbers which Firefox
// interprets as to be opened in the already opened DevTool's debugger
const href = `${script.source.url}:${lineNumber}:${columnNumber}`;
// Use special characters in order to print working hyperlinks right from the terminal
// See https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda
return `\x1B]8;;${href}\x1B\\${href}\x1B]8;;\x1B\\`;
}
/**
* Helper function to synchronously pause the current frame execution
* for a given duration in ms.
*
* @param {Number} duration
*/
function syncPause(duration) {
let freeze = true;
const timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
timer.initWithCallback(
() => {
freeze = false;
},
duration,
Ci.nsITimer.TYPE_ONE_SHOT
);
Services.tm.spinEventLoopUntil("debugger-slow-motion", function () {
return !freeze;
});
}
export const JSTracer = {
startTracing,
stopTracing,
addTracingListener,
removeTracingListener,
NEXT_INTERACTION_MESSAGE,
DOM_MUTATIONS,
};
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2026-03-28
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